All Exams >
Grade 8 >
Science & Technology for Grade 8 >
All Questions
All questions of Work, Mechanical Advantage and Efficiency for Grade 8 Exam
A coolie was standing with 2 suitcases on his head and 2 bags in his hand. The luggage exerted a force of 159 N on him. What is the amount of work done by him?- a)79.5 J
- b)318 J
- c)0 J
- d)159 J
Correct answer is option 'C'. Can you explain this answer?
A coolie was standing with 2 suitcases on his head and 2 bags in his hand. The luggage exerted a force of 159 N on him. What is the amount of work done by him?
a)
79.5 J
b)
318 J
c)
0 J
d)
159 J
|
Ananya Das answered |
Work done is zero, since there is no displacement of the coolie.
Calculate the distance between the man and the box when the work done to move the box was 100 J with a force of 15 N.- a)6 m
- b)16.6 m
- c)26.6 m
- d)6.6 m
Correct answer is option 'D'. Can you explain this answer?
Calculate the distance between the man and the box when the work done to move the box was 100 J with a force of 15 N.
a)
6 m
b)
16.6 m
c)
26.6 m
d)
6.6 m
|
Raghav Bansal answered |
As given, W = 100 J, F = 15 N
We know, W = Fs
⇒ s = W/ F
⇒ s = 100 / 15
⇒ s = 6.6 m
We know, W = Fs
⇒ s = W/ F
⇒ s = 100 / 15
⇒ s = 6.6 m
Can you explain the answer of this question below:A force a body of mass 3 kg producing a displacement of 10 m in it. The work done by force is of 6 N acts on
- A:
30 J
- B:
60 J
- C:
18 J
- D:
36 J
The answer is a.
A force a body of mass 3 kg producing a displacement of 10 m in it. The work done by force is of 6 N acts on
30 J
60 J
18 J
36 J
|
Sonal Sinha answered |
Here,
work done(W)=fdcos60degree
W=6×10×1/2
w=60/2
w=30j(Ans).
work done(W)=fdcos60degree
W=6×10×1/2
w=60/2
w=30j(Ans).
To lift 700 kg of water, through a height of 10 m we use a pump. The work done in operating the pump is;
Take g = 10 m/s2.- a)7 kilo joules
- b)7 x 106 J
- c)700 J
- d)70000 J
Correct answer is option 'D'. Can you explain this answer?
To lift 700 kg of water, through a height of 10 m we use a pump. The work done in operating the pump is;
Take g = 10 m/s2.
Take g = 10 m/s2.
a)
7 kilo joules
b)
7 x 106 J
c)
700 J
d)
70000 J
|
|
Amit Sharma answered |
Work done = mgh
= 700 kg x 10m/s x 10m = 70000 joule
= 700 kg x 10m/s x 10m = 70000 joule
The winners of long jump each weighing 56 Kg, 40 Kg and 45 Kg stand on the podium. What is the work done by them on the podium while they receive the medals?- a)250 J
- b)0 J
- c)282 J
- d)141 J
Correct answer is option 'B'. Can you explain this answer?
The winners of long jump each weighing 56 Kg, 40 Kg and 45 Kg stand on the podium. What is the work done by them on the podium while they receive the medals?
a)
250 J
b)
0 J
c)
282 J
d)
141 J
|
|
Aayush Kumar answered |
The winners of long jump each weighing 56kg,40kg and 45 kg stand on podium.Work done by them is 0 Ioule because there is no displacement and if the displacement is 0 then work done will be also 0.
Work done by a body is- a)negative, positive or zero
- b)always positive
- c)always zero
- d)always negative
Correct answer is option 'A'. Can you explain this answer?
Work done by a body is
a)
negative, positive or zero
b)
always positive
c)
always zero
d)
always negative
|
|
Anita Menon answered |
Positive Work
- If a force displaces the object in its direction, then the work done is positive
So, W=Fd
The example of this kind of work done is motion of ball falling towards ground where displacement of ball is in the direction of force of gravity.
Negative work
- If the force and the displacement are in opposite directions, then the work is said to be negative. For example if a ball is thrown in upwards direction, its displacement would be in upwards direction but the force due to earth’s gravity is in the downward direction.
So here in this case gravity is doing negative work when you throw the ball upwards. Hence the work done by gravitational force is negative. Mathematically when displacement is opposite to the force work done is given by
- Negative work just means that the force and the displacement act in opposite directions.
Case of zero work done
- If the directions of force and the displacement are perpendicular to each other, the work done by the force on the object is zero.
For example, when we push hard against a wall, the force we are exerting on the wall does no work, because in this case the displacement of the wall is d = 0. However, in this process, our muscles are using our internal energy and as a result we get tired.
Scientifically, work is said to be done in which of the following cases- a)Manish pulls the trolley for a certain distance
- b)Mira studies for her exam
- c)Manoj pushes the wall with no change in the position of wall
- d)Mira pulls the wall with no change in the position of wall
Correct answer is option 'A'. Can you explain this answer?
Scientifically, work is said to be done in which of the following cases
a)
Manish pulls the trolley for a certain distance
b)
Mira studies for her exam
c)
Manoj pushes the wall with no change in the position of wall
d)
Mira pulls the wall with no change in the position of wall
|
|
Arjun Sharma answered |
The scientific definition of work reveals its relationship to energy—whenever work is done, energy is transferred. For work, in the scientific sense, to be done, a force must be exerted and there must be displacement in the direction of the force. Hence, manish pulls a trolley for a certain distance.
A body of mass 5 kg is moving with an acceleration of 5 ms-2 gets a displacement of 2m, the work done by the force is:- a)75 J
- b)25 J
- c)100 J
- d)50 J
Correct answer is option 'D'. Can you explain this answer?
A body of mass 5 kg is moving with an acceleration of 5 ms-2 gets a displacement of 2m, the work done by the force is:
a)
75 J
b)
25 J
c)
100 J
d)
50 J
|
Vikas Kapoor answered |
We know, F = ma
F = 5 * 5
F = 25
We, also know;
W = F. s
= 25 * 2
W = 50 J
F = 5 * 5
F = 25
We, also know;
W = F. s
= 25 * 2
W = 50 J
Calculate the work done in moving a body of mass 50 Kg through a height of 5 m.(g = 10m/s2)- a)250 J
- b)2500 J
- c)25 x 103 J
- d)2.5 x 105 J
Correct answer is option 'B'. Can you explain this answer?
Calculate the work done in moving a body of mass 50 Kg through a height of 5 m.(g = 10m/s2)
a)
250 J
b)
2500 J
c)
25 x 103 J
d)
2.5 x 105 J
|
Sonal Sinha answered |
W=fd
Force=mg =50×10=500N
W=fd
=500×5=2500j(Ans).
Force=mg =50×10=500N
W=fd
=500×5=2500j(Ans).
Can you explain the answer of this question below:A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
- A:
9 ×106J
- B:
5 × 108 Joule
- C:
10 × 105Joule
- D:
9 ×108J
The answer is d.
A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
9 ×106J
5 × 108 Joule
10 × 105Joule
9 ×108J
|
Sarita Reddy answered |
1 unit of energy is equal to 1 kilowatt hour (kWh).
1 unit = 1 kWh
1 kWh = 3.6 x 10^6J
Therefore, 250 units of energy = 250 x 3.6 x 10^6 = 9 x 10^8 J.
An electric bulb of 60 w is used for 6 h per day. Calculate the ‘units of energy consumed in one day by the bulb- a)0.60 ‘units’
- b)0.36 ‘units’
- c)0.45 ‘units
- d)0.24 ‘units’
Correct answer is option 'B'. Can you explain this answer?
An electric bulb of 60 w is used for 6 h per day. Calculate the ‘units of energy consumed in one day by the bulb
a)
0.60 ‘units’
b)
0.36 ‘units’
c)
0.45 ‘units
d)
0.24 ‘units’
|
Saikat Verma answered |
Understanding Energy Consumption
To calculate the energy consumed by a 60-watt bulb used for 6 hours a day, we first need to understand the concept of watts, which is a unit of power.
Step 1: Calculate Daily Energy Consumption
- The formula to calculate energy consumption in kilowatt-hours (kWh) is:
- Energy (kWh) = Power (kW) × Time (h)
- Convert watts to kilowatts:
- 60 watts = 0.06 kW
Step 2: Substitute Values
- Using the formula:
- Energy (kWh) = 0.06 kW × 6 h
Step 3: Perform the Calculation
- Multiply the values:
- Energy (kWh) = 0.36 kWh
Step 4: Convert to Units
- In the context of electricity, 1 unit is equivalent to 1 kWh.
- Therefore, the energy consumed by the bulb in one day is 0.36 units.
Conclusion
- The correct answer is option 'B' - 0.36 units.
- This means that using a 60-watt bulb for 6 hours will consume 0.36 units of energy per day.
This calculation helps in understanding how much energy different appliances use, which is essential for managing electricity costs effectively.
To calculate the energy consumed by a 60-watt bulb used for 6 hours a day, we first need to understand the concept of watts, which is a unit of power.
Step 1: Calculate Daily Energy Consumption
- The formula to calculate energy consumption in kilowatt-hours (kWh) is:
- Energy (kWh) = Power (kW) × Time (h)
- Convert watts to kilowatts:
- 60 watts = 0.06 kW
Step 2: Substitute Values
- Using the formula:
- Energy (kWh) = 0.06 kW × 6 h
Step 3: Perform the Calculation
- Multiply the values:
- Energy (kWh) = 0.36 kWh
Step 4: Convert to Units
- In the context of electricity, 1 unit is equivalent to 1 kWh.
- Therefore, the energy consumed by the bulb in one day is 0.36 units.
Conclusion
- The correct answer is option 'B' - 0.36 units.
- This means that using a 60-watt bulb for 6 hours will consume 0.36 units of energy per day.
This calculation helps in understanding how much energy different appliances use, which is essential for managing electricity costs effectively.
When a stone is thrown upward with a certain speed then its kinetic energy at the highest point is- a)Maximum
- b)Minimum
- c)Zero
- d)None of these
Correct answer is option 'C'. Can you explain this answer?
When a stone is thrown upward with a certain speed then its kinetic energy at the highest point is
a)
Maximum
b)
Minimum
c)
Zero
d)
None of these
|
|
Shilpa Choudhury answered |
At highest point, velocity is zero.
Hence, Kinetic energy is zero.
The type of energy possessed by a simple pendulum, when it is at the mean positions- a)K.E
- b)P.E
- c)K.E. + P. E.
- d)Sound energy
Correct answer is option 'A'. Can you explain this answer?
The type of energy possessed by a simple pendulum, when it is at the mean positions
a)
K.E
b)
P.E
c)
K.E. + P. E.
d)
Sound energy
|
Ritika Kumar answered |
The type of energy possessed by a simple pendulum, when it is at the mean position, is kinetic energy (K.E).
Explanation:
A simple pendulum consists of a mass (bob) attached to a string or rod that is fixed at one end. When the pendulum is at the mean position, it is at its lowest point and has the highest potential energy. As it swings away from the mean position, the potential energy is converted into kinetic energy, and vice versa.
Understanding the options:
To determine the type of energy possessed by a simple pendulum at the mean position, let's analyze each option:
a) K.E (Kinetic Energy): Kinetic energy is the energy possessed by an object due to its motion. In the case of a simple pendulum, when it is at the mean position, it momentarily comes to rest before changing direction. At this point, it has no potential energy but possesses kinetic energy due to its motion. Therefore, option 'A' is correct.
b) P.E (Potential Energy): Potential energy is the energy possessed by an object due to its position or state. When the pendulum is at the mean position, it is at its lowest point and has the highest potential energy. However, at this point, it does not possess potential energy but has kinetic energy instead.
c) K.E + P.E (Kinetic Energy + Potential Energy): This option suggests that the pendulum possesses both kinetic and potential energy simultaneously at the mean position. However, as explained earlier, the pendulum only has kinetic energy at this point.
d) Sound Energy: Sound energy is the energy produced by the vibrations of particles in a medium. When the simple pendulum is at the mean position, it is not producing any sound energy.
Conclusion:
Based on the analysis, the correct answer is option 'A' - Kinetic Energy (K.E). The simple pendulum possesses kinetic energy at the mean position.
Explanation:
A simple pendulum consists of a mass (bob) attached to a string or rod that is fixed at one end. When the pendulum is at the mean position, it is at its lowest point and has the highest potential energy. As it swings away from the mean position, the potential energy is converted into kinetic energy, and vice versa.
Understanding the options:
To determine the type of energy possessed by a simple pendulum at the mean position, let's analyze each option:
a) K.E (Kinetic Energy): Kinetic energy is the energy possessed by an object due to its motion. In the case of a simple pendulum, when it is at the mean position, it momentarily comes to rest before changing direction. At this point, it has no potential energy but possesses kinetic energy due to its motion. Therefore, option 'A' is correct.
b) P.E (Potential Energy): Potential energy is the energy possessed by an object due to its position or state. When the pendulum is at the mean position, it is at its lowest point and has the highest potential energy. However, at this point, it does not possess potential energy but has kinetic energy instead.
c) K.E + P.E (Kinetic Energy + Potential Energy): This option suggests that the pendulum possesses both kinetic and potential energy simultaneously at the mean position. However, as explained earlier, the pendulum only has kinetic energy at this point.
d) Sound Energy: Sound energy is the energy produced by the vibrations of particles in a medium. When the simple pendulum is at the mean position, it is not producing any sound energy.
Conclusion:
Based on the analysis, the correct answer is option 'A' - Kinetic Energy (K.E). The simple pendulum possesses kinetic energy at the mean position.
Each of the following statement describes a force acting. Which of these forces is causing work to be done?- a)The weight of a book at rest on a table
- b)The pull of a moving railway engine on its coaches
- c)The tension in an elastic band wrapped around a parcel
- d)The push of a person’s feet when standing on the floor
Correct answer is option 'B'. Can you explain this answer?
Each of the following statement describes a force acting. Which of these forces is causing work to be done?
a)
The weight of a book at rest on a table
b)
The pull of a moving railway engine on its coaches
c)
The tension in an elastic band wrapped around a parcel
d)
The push of a person’s feet when standing on the floor
|
|
Shilpa Choudhury answered |
For worktop be done, there should be a motion of the body in the direction of the force applied.
So, the correct answer is B) the pull of a moving railway engine on its coaches.
Two objects of masses 1 × 10–3 kg and 4 × 10–3 kg have equal momentum. What is the ratio of their kinetic energies- a)4 : 1
- b)2 : 1
- c)16 : 1
- d)2 :1
Correct answer is option 'A'. Can you explain this answer?
Two objects of masses 1 × 10–3 kg and 4 × 10–3 kg have equal momentum. What is the ratio of their kinetic energies
a)
4 : 1
b)
2 : 1
c)
16 : 1
d)
2 :1
|
|
Shilpa Choudhury answered |
K.E. = ½ mv2
p= mv => K.E. = ½ m (p/m)2 = p2/2m
∴ K.E1/K.E2 = (P12/2m1)/(p22/2m2) = m2/m1 = p12/p22
Here 'p1' = p2 (given)
∴ K.E1/K.E2 = 4×10-3 kg/ 1×10-3 kg = 4/1
The work done in lifting a 50 kg of bag from the ground to the head, height of coolie is 200 cm, by the coolie is (assume g = 10 m/s2)- a)500 J
- b)1000 J
- c)2000 J
- d)10,000 J
Correct answer is option 'B'. Can you explain this answer?
The work done in lifting a 50 kg of bag from the ground to the head, height of coolie is 200 cm, by the coolie is (assume g = 10 m/s2)
a)
500 J
b)
1000 J
c)
2000 J
d)
10,000 J
|
|
Arpita Sengupta answered |
Given:
Mass of the bag, m = 50 kg
Height of the coolie, h = 200 cm = 2 m
Acceleration due to gravity, g = 10 m/s^2
To Find:
The work done by the coolie in lifting the bag.
Solution:
The work done (W) is given by the formula:
W = mgh
where,
m = mass of the object (in kg)
g = acceleration due to gravity (in m/s^2)
h = height (in meters)
Substituting the given values:
m = 50 kg
g = 10 m/s^2
h = 2 m
W = (50 kg) * (10 m/s^2) * (2 m)
W = 1000 J
Hence, the work done by the coolie in lifting the bag is 1000 J.
Explanation:
- The work done in lifting an object is equal to the product of the force applied and the distance over which the force is applied.
- In this case, the coolie is applying a force to lift the bag against the force of gravity.
- The force applied by the coolie is equal to the weight of the bag, which is given by the formula: F = mg, where m is the mass of the bag and g is the acceleration due to gravity.
- The distance over which the force is applied is the height to which the bag is lifted.
- Therefore, the work done by the coolie is equal to the weight of the bag multiplied by the height to which it is lifted.
- Substituting the given values into the formula, we can calculate the work done as 1000 J.
- Hence, the correct answer is option B, 1000 J.
Mass of the bag, m = 50 kg
Height of the coolie, h = 200 cm = 2 m
Acceleration due to gravity, g = 10 m/s^2
To Find:
The work done by the coolie in lifting the bag.
Solution:
The work done (W) is given by the formula:
W = mgh
where,
m = mass of the object (in kg)
g = acceleration due to gravity (in m/s^2)
h = height (in meters)
Substituting the given values:
m = 50 kg
g = 10 m/s^2
h = 2 m
W = (50 kg) * (10 m/s^2) * (2 m)
W = 1000 J
Hence, the work done by the coolie in lifting the bag is 1000 J.
Explanation:
- The work done in lifting an object is equal to the product of the force applied and the distance over which the force is applied.
- In this case, the coolie is applying a force to lift the bag against the force of gravity.
- The force applied by the coolie is equal to the weight of the bag, which is given by the formula: F = mg, where m is the mass of the bag and g is the acceleration due to gravity.
- The distance over which the force is applied is the height to which the bag is lifted.
- Therefore, the work done by the coolie is equal to the weight of the bag multiplied by the height to which it is lifted.
- Substituting the given values into the formula, we can calculate the work done as 1000 J.
- Hence, the correct answer is option B, 1000 J.
If the speed of an object is doubled, then its kinetic energy is- a)Doubled
- b)Tripled
- c)Quadrupled
- d)Remains same
Correct answer is option 'C'. Can you explain this answer?
If the speed of an object is doubled, then its kinetic energy is
a)
Doubled
b)
Tripled
c)
Quadrupled
d)
Remains same
|
|
Ritu Menon answered |
Explanation:
Introduction:
Kinetic energy is the energy possessed by an object due to its motion. It depends on the mass and velocity of the object. The formula for kinetic energy is given by:
Kinetic energy (KE) = 0.5 * mass * velocity^2
Effect of doubling the speed:
When the speed of an object is doubled, it means that the new velocity becomes twice the original velocity. Let's assume the initial velocity is 'v' and the final velocity (after doubling) is '2v'.
Comparing the kinetic energy:
To determine the effect on kinetic energy, we can compare the kinetic energy before and after doubling the speed.
Initial kinetic energy:
Using the formula for kinetic energy, the initial kinetic energy can be calculated as:
KE1 = 0.5 * mass * v^2
Final kinetic energy:
After doubling the speed, the final kinetic energy can be calculated as:
KE2 = 0.5 * mass * (2v)^2 = 0.5 * mass * 4v^2
Comparing KE1 and KE2:
Let's compare the initial and final kinetic energies:
KE2 / KE1 = (0.5 * mass * 4v^2) / (0.5 * mass * v^2)
= (2 * mass * v^2) / (0.5 * mass * v^2)
= 4
Therefore, the final kinetic energy (KE2) is four times the initial kinetic energy (KE1) when the speed is doubled.
Conclusion:
Hence, the correct answer is option 'C' - Quadrupled. When the speed of an object is doubled, its kinetic energy increases by a factor of four.
Introduction:
Kinetic energy is the energy possessed by an object due to its motion. It depends on the mass and velocity of the object. The formula for kinetic energy is given by:
Kinetic energy (KE) = 0.5 * mass * velocity^2
Effect of doubling the speed:
When the speed of an object is doubled, it means that the new velocity becomes twice the original velocity. Let's assume the initial velocity is 'v' and the final velocity (after doubling) is '2v'.
Comparing the kinetic energy:
To determine the effect on kinetic energy, we can compare the kinetic energy before and after doubling the speed.
Initial kinetic energy:
Using the formula for kinetic energy, the initial kinetic energy can be calculated as:
KE1 = 0.5 * mass * v^2
Final kinetic energy:
After doubling the speed, the final kinetic energy can be calculated as:
KE2 = 0.5 * mass * (2v)^2 = 0.5 * mass * 4v^2
Comparing KE1 and KE2:
Let's compare the initial and final kinetic energies:
KE2 / KE1 = (0.5 * mass * 4v^2) / (0.5 * mass * v^2)
= (2 * mass * v^2) / (0.5 * mass * v^2)
= 4
Therefore, the final kinetic energy (KE2) is four times the initial kinetic energy (KE1) when the speed is doubled.
Conclusion:
Hence, the correct answer is option 'C' - Quadrupled. When the speed of an object is doubled, its kinetic energy increases by a factor of four.
Work done by a string when a stone is tied to it and whirled in a circle is- a)Positive
- b)Negative
- c)Zero
- d)None of these
Correct answer is option 'C'. Can you explain this answer?
Work done by a string when a stone is tied to it and whirled in a circle is
a)
Positive
b)
Negative
c)
Zero
d)
None of these
|
Gowri Ahuja answered |
Explanation:
When a stone is tied to a string and whirled in a circle, the work done by the string is zero. This is because work is defined as the product of force and displacement in the direction of the force. In this case, the force exerted by the string on the stone is always perpendicular to the displacement of the stone.
Work Done:
The work done by a force can be calculated using the formula:
Work = Force x Displacement x cosθ
Where:
- Force is the magnitude of the force applied.
- Displacement is the magnitude of the displacement.
- θ is the angle between the force and displacement vectors.
In this case, the force exerted by the string on the stone is always perpendicular to the displacement of the stone. Therefore, the angle θ between the force and displacement vectors is 90 degrees, and cosθ = 0.
Work Done by the String:
When cosθ = 0, the work done by the force is zero. This means that the work done by the string on the stone is zero. The string does not contribute any work to the motion of the stone.
Reasoning:
The reason behind the zero work done is that the force exerted by the string is always perpendicular to the displacement of the stone. As a result, the force and displacement vectors are always at right angles to each other, and the angle between them is 90 degrees. When the angle between the force and displacement vectors is 90 degrees, the work done by the force is zero.
Conclusion:
In conclusion, when a stone is tied to a string and whirled in a circle, the work done by the string is zero. This is because the force exerted by the string is always perpendicular to the displacement of the stone, resulting in a zero angle between the force and displacement vectors. Thus, the correct answer is option 'C' - Zero.
When a stone is tied to a string and whirled in a circle, the work done by the string is zero. This is because work is defined as the product of force and displacement in the direction of the force. In this case, the force exerted by the string on the stone is always perpendicular to the displacement of the stone.
Work Done:
The work done by a force can be calculated using the formula:
Work = Force x Displacement x cosθ
Where:
- Force is the magnitude of the force applied.
- Displacement is the magnitude of the displacement.
- θ is the angle between the force and displacement vectors.
In this case, the force exerted by the string on the stone is always perpendicular to the displacement of the stone. Therefore, the angle θ between the force and displacement vectors is 90 degrees, and cosθ = 0.
Work Done by the String:
When cosθ = 0, the work done by the force is zero. This means that the work done by the string on the stone is zero. The string does not contribute any work to the motion of the stone.
Reasoning:
The reason behind the zero work done is that the force exerted by the string is always perpendicular to the displacement of the stone. As a result, the force and displacement vectors are always at right angles to each other, and the angle between them is 90 degrees. When the angle between the force and displacement vectors is 90 degrees, the work done by the force is zero.
Conclusion:
In conclusion, when a stone is tied to a string and whirled in a circle, the work done by the string is zero. This is because the force exerted by the string is always perpendicular to the displacement of the stone, resulting in a zero angle between the force and displacement vectors. Thus, the correct answer is option 'C' - Zero.
The momentum of a bullet of mass 20 g fired from a gun is 10 kg. m/s. The kinetic energy of this bulletin kJ will be- a)5
- b)1.5
- c)2.5
- d)25
Correct answer is option 'C'. Can you explain this answer?
The momentum of a bullet of mass 20 g fired from a gun is 10 kg. m/s. The kinetic energy of this bulletin kJ will be
a)
5
b)
1.5
c)
2.5
d)
25
|
Saanvi Kaur answered |
Understanding Momentum and Kinetic Energy
To solve for the kinetic energy of the bullet, we first need to understand the relationship between momentum and kinetic energy.
Definitions
- Momentum (p): It is defined as the product of mass (m) and velocity (v) of an object.
- Formula: p = m * v
- Kinetic Energy (KE): It is defined as the energy that an object possesses due to its motion.
- Formula: KE = (1/2) * m * v^2
Given Information
- Mass of the bullet (m): 20 g = 0.020 kg (conversion to kg)
- Momentum (p): 10 kg·m/s
Finding Velocity
Using the momentum formula:
- p = m * v
- Rearranging gives us: v = p / m
- Substituting the values:
- v = 10 kg·m/s / 0.020 kg = 500 m/s
Calculating Kinetic Energy
Now that we have the velocity, we can find the kinetic energy:
- KE = (1/2) * m * v^2
- KE = (1/2) * 0.020 kg * (500 m/s)^2
- KE = 0.010 kg * 250000 m²/s²
- KE = 2500 J
Converting to kJ
Since 1 kJ = 1000 J:
- KE = 2500 J = 2.5 kJ
Conclusion
The kinetic energy of the bullet is 2.5 kJ, thus confirming that option 'C' is correct.
To solve for the kinetic energy of the bullet, we first need to understand the relationship between momentum and kinetic energy.
Definitions
- Momentum (p): It is defined as the product of mass (m) and velocity (v) of an object.
- Formula: p = m * v
- Kinetic Energy (KE): It is defined as the energy that an object possesses due to its motion.
- Formula: KE = (1/2) * m * v^2
Given Information
- Mass of the bullet (m): 20 g = 0.020 kg (conversion to kg)
- Momentum (p): 10 kg·m/s
Finding Velocity
Using the momentum formula:
- p = m * v
- Rearranging gives us: v = p / m
- Substituting the values:
- v = 10 kg·m/s / 0.020 kg = 500 m/s
Calculating Kinetic Energy
Now that we have the velocity, we can find the kinetic energy:
- KE = (1/2) * m * v^2
- KE = (1/2) * 0.020 kg * (500 m/s)^2
- KE = 0.010 kg * 250000 m²/s²
- KE = 2500 J
Converting to kJ
Since 1 kJ = 1000 J:
- KE = 2500 J = 2.5 kJ
Conclusion
The kinetic energy of the bullet is 2.5 kJ, thus confirming that option 'C' is correct.
If air resistance is negligible, the sum total of potential and Kinetic energies of a freely falling body- a)Increases
- b)Decreases
- c)Becomes zero
- d)Remains the same
Correct answer is option 'D'. Can you explain this answer?
If air resistance is negligible, the sum total of potential and Kinetic energies of a freely falling body
a)
Increases
b)
Decreases
c)
Becomes zero
d)
Remains the same
|
|
Srishti Sarkar answered |
Potential and Kinetic Energies of a Freely Falling Body
When air resistance is negligible, the sum total of potential and kinetic energies of a freely falling body remains the same.
Explanation:
1. Potential Energy:
Potential energy is the energy possessed by an object due to its position or state. In the case of a freely falling body, the potential energy is gravitational potential energy, which is given by the formula:
PE = mgh
Where:
- PE is the potential energy
- m is the mass of the object
- g is the acceleration due to gravity
- h is the height of the object above a reference point
As the object falls, its height decreases, resulting in a decrease in potential energy.
2. Kinetic Energy:
Kinetic energy is the energy possessed by an object due to its motion. In the case of a freely falling body, the kinetic energy is given by the formula:
KE = 0.5mv^2
Where:
- KE is the kinetic energy
- m is the mass of the object
- v is the velocity of the object
As the object falls, its velocity increases due to the acceleration due to gravity. Therefore, the kinetic energy increases.
3. Sum Total of Potential and Kinetic Energies:
The sum total of potential and kinetic energies is given by the equation:
Total Energy = Potential Energy + Kinetic Energy
As the object falls, the potential energy decreases, but the kinetic energy increases. However, the decrease in potential energy is exactly equal to the increase in kinetic energy. This means that the sum total of potential and kinetic energies remains constant throughout the fall.
Conclusion:
In the absence of air resistance, the sum total of potential and kinetic energies of a freely falling body remains the same. This is because the decrease in potential energy is exactly balanced by the increase in kinetic energy.
When air resistance is negligible, the sum total of potential and kinetic energies of a freely falling body remains the same.
Explanation:
1. Potential Energy:
Potential energy is the energy possessed by an object due to its position or state. In the case of a freely falling body, the potential energy is gravitational potential energy, which is given by the formula:
PE = mgh
Where:
- PE is the potential energy
- m is the mass of the object
- g is the acceleration due to gravity
- h is the height of the object above a reference point
As the object falls, its height decreases, resulting in a decrease in potential energy.
2. Kinetic Energy:
Kinetic energy is the energy possessed by an object due to its motion. In the case of a freely falling body, the kinetic energy is given by the formula:
KE = 0.5mv^2
Where:
- KE is the kinetic energy
- m is the mass of the object
- v is the velocity of the object
As the object falls, its velocity increases due to the acceleration due to gravity. Therefore, the kinetic energy increases.
3. Sum Total of Potential and Kinetic Energies:
The sum total of potential and kinetic energies is given by the equation:
Total Energy = Potential Energy + Kinetic Energy
As the object falls, the potential energy decreases, but the kinetic energy increases. However, the decrease in potential energy is exactly equal to the increase in kinetic energy. This means that the sum total of potential and kinetic energies remains constant throughout the fall.
Conclusion:
In the absence of air resistance, the sum total of potential and kinetic energies of a freely falling body remains the same. This is because the decrease in potential energy is exactly balanced by the increase in kinetic energy.
The P. E. of a body at a certain height is 200 J. The K.E. possessed by it when it just touches the surface of the earth is- a)> P.E.
- b)< p.="" />
- c)= P.E.
- d)can’t be known
Correct answer is option 'C'. Can you explain this answer?
The P. E. of a body at a certain height is 200 J. The K.E. possessed by it when it just touches the surface of the earth is
a)
> P.E.
b)
< p.="" />
c)
= P.E.
d)
can’t be known
|
|
Anjali Singh answered |
Explanation:
When a body is at a certain height, it possesses potential energy due to its position in the Earth's gravitational field. The potential energy (P.E.) possessed by the body is given as 200 J.
Step 1: Understanding Potential Energy and Kinetic Energy
Potential energy (P.E.) is the energy possessed by an object due to its position or state. It is the energy that can be converted into other forms of energy. In the case of a body at a height, the potential energy is due to its position in the Earth's gravitational field.
Kinetic energy (K.E.) is the energy possessed by an object due to its motion. It depends on the mass of the object and its velocity.
Step 2: Relationship between Potential Energy and Kinetic Energy
When the body just touches the surface of the Earth, it means that it has fallen from a height and its potential energy has been completely converted into kinetic energy. At this point, the body has no potential energy left.
According to the law of conservation of energy, energy cannot be created or destroyed, only transferred or converted from one form to another. Therefore, the potential energy possessed by the body at the height must be equal to the kinetic energy it possesses when it touches the surface of the Earth.
Step 3: Determining the Kinetic Energy
Since the potential energy of the body is given as 200 J, this must be equal to the kinetic energy when it touches the surface of the Earth.
Therefore, the K.E. possessed by the body when it just touches the surface of the Earth is also 200 J.
Step 4: Conclusion
Therefore, the correct answer is option C: 200 J. The K.E. possessed by the body when it just touches the surface of the Earth is equal to the potential energy it had at the height, which is 200 J.
When a body is at a certain height, it possesses potential energy due to its position in the Earth's gravitational field. The potential energy (P.E.) possessed by the body is given as 200 J.
Step 1: Understanding Potential Energy and Kinetic Energy
Potential energy (P.E.) is the energy possessed by an object due to its position or state. It is the energy that can be converted into other forms of energy. In the case of a body at a height, the potential energy is due to its position in the Earth's gravitational field.
Kinetic energy (K.E.) is the energy possessed by an object due to its motion. It depends on the mass of the object and its velocity.
Step 2: Relationship between Potential Energy and Kinetic Energy
When the body just touches the surface of the Earth, it means that it has fallen from a height and its potential energy has been completely converted into kinetic energy. At this point, the body has no potential energy left.
According to the law of conservation of energy, energy cannot be created or destroyed, only transferred or converted from one form to another. Therefore, the potential energy possessed by the body at the height must be equal to the kinetic energy it possesses when it touches the surface of the Earth.
Step 3: Determining the Kinetic Energy
Since the potential energy of the body is given as 200 J, this must be equal to the kinetic energy when it touches the surface of the Earth.
Therefore, the K.E. possessed by the body when it just touches the surface of the Earth is also 200 J.
Step 4: Conclusion
Therefore, the correct answer is option C: 200 J. The K.E. possessed by the body when it just touches the surface of the Earth is equal to the potential energy it had at the height, which is 200 J.
In which of the following situations will the potential energy of the spring be minimum?- a)Compressed
- b)Extended
- c)In its original shape
- d)None of these
Correct answer is option 'C'. Can you explain this answer?
In which of the following situations will the potential energy of the spring be minimum?
a)
Compressed
b)
Extended
c)
In its original shape
d)
None of these
|
|
Shilpa Choudhury answered |
When a spring is at its natural length (i.e. neither stretched nor compressed), the potential energy of a spring will be minimum.
A coolie carries a load of 50 N to a distance of 100 m. The work done by him is- a)10 N
- b)0
- c)5000 Nm
- d)½ J
Correct answer is option 'B'. Can you explain this answer?
A coolie carries a load of 50 N to a distance of 100 m. The work done by him is
a)
10 N
b)
0
c)
5000 Nm
d)
½ J
|
Meenal Menon answered |
Work is defined as the product of the force applied on an object and the distance over which the force is applied. Mathematically, work (W) is given by the formula:
W = F * d
where W is the work done, F is the force applied, and d is the distance over which the force is applied.
In this case, the coolie is carrying a load of 50 N over a distance of 100 m. This means that the force applied by the coolie is 50 N and the distance is 100 m. Plugging these values into the formula for work:
W = 50 N * 100 m
W = 5000 Nm
The unit for work is Joules (J), so 5000 Nm is equivalent to 5000 J.
Therefore, the correct answer is option B) 0.
Explanation:
The work done by the coolie is 0 because he is carrying the load horizontally, which means there is no displacement in the vertical direction. In other words, the force applied by the coolie is perpendicular to the direction of displacement.
When the force is perpendicular to the displacement, the work done is zero. This is because the angle between the force and the displacement is 90 degrees, and the cosine of 90 degrees is 0.
Therefore, in this case, the work done by the coolie is 0.
W = F * d
where W is the work done, F is the force applied, and d is the distance over which the force is applied.
In this case, the coolie is carrying a load of 50 N over a distance of 100 m. This means that the force applied by the coolie is 50 N and the distance is 100 m. Plugging these values into the formula for work:
W = 50 N * 100 m
W = 5000 Nm
The unit for work is Joules (J), so 5000 Nm is equivalent to 5000 J.
Therefore, the correct answer is option B) 0.
Explanation:
The work done by the coolie is 0 because he is carrying the load horizontally, which means there is no displacement in the vertical direction. In other words, the force applied by the coolie is perpendicular to the direction of displacement.
When the force is perpendicular to the displacement, the work done is zero. This is because the angle between the force and the displacement is 90 degrees, and the cosine of 90 degrees is 0.
Therefore, in this case, the work done by the coolie is 0.
A man of mass 100 kg jumps to a height of 50 cm. his potential energy at the highest point is(g = 10 m/s2)- a)50 J
- b)500 J
- c)1000 J
- d)0
Correct answer is option 'B'. Can you explain this answer?
A man of mass 100 kg jumps to a height of 50 cm. his potential energy at the highest point is
(g = 10 m/s2)
a)
50 J
b)
500 J
c)
1000 J
d)
0
|
|
Siddharth Banerjee answered |
Potential Energy Calculation:
To calculate the potential energy of the man at the highest point, we can use the formula for potential energy, which is given by:
Potential Energy (PE) = mass (m) * acceleration due to gravity (g) * height (h)
Given:
Mass (m) = 100 kg
Acceleration due to gravity (g) = 10 m/s^2
Height (h) = 50 cm = 0.5 m
Substituting the given values into the formula, we get:
PE = 100 kg * 10 m/s^2 * 0.5 m
= 500 J
Therefore, the potential energy of the man at the highest point is 500 J.
Explanation:
- The potential energy of an object is the energy possessed by it due to its position or height relative to the ground.
- In this case, the man jumps to a height of 50 cm, which means he gains potential energy as he moves upward against the force of gravity.
- The potential energy of an object is directly proportional to its mass, the acceleration due to gravity, and the height it is lifted to.
- The given mass of the man is 100 kg, the acceleration due to gravity is 10 m/s^2, and the height is 50 cm, which is equivalent to 0.5 m.
- By substituting these values into the potential energy formula, we can calculate the potential energy of the man at the highest point.
- After performing the calculation, we find that the potential energy is 500 J.
- Therefore, the correct answer is option 'B' which states that the potential energy at the highest point is 500 J.
To calculate the potential energy of the man at the highest point, we can use the formula for potential energy, which is given by:
Potential Energy (PE) = mass (m) * acceleration due to gravity (g) * height (h)
Given:
Mass (m) = 100 kg
Acceleration due to gravity (g) = 10 m/s^2
Height (h) = 50 cm = 0.5 m
Substituting the given values into the formula, we get:
PE = 100 kg * 10 m/s^2 * 0.5 m
= 500 J
Therefore, the potential energy of the man at the highest point is 500 J.
Explanation:
- The potential energy of an object is the energy possessed by it due to its position or height relative to the ground.
- In this case, the man jumps to a height of 50 cm, which means he gains potential energy as he moves upward against the force of gravity.
- The potential energy of an object is directly proportional to its mass, the acceleration due to gravity, and the height it is lifted to.
- The given mass of the man is 100 kg, the acceleration due to gravity is 10 m/s^2, and the height is 50 cm, which is equivalent to 0.5 m.
- By substituting these values into the potential energy formula, we can calculate the potential energy of the man at the highest point.
- After performing the calculation, we find that the potential energy is 500 J.
- Therefore, the correct answer is option 'B' which states that the potential energy at the highest point is 500 J.
Which of the following energy change involves frictional force?- a)Chemical energy to heat energy
- b)Kinetic energy to heat energy
- c)Potential energy to sound energy
- d)Chemical energy to kinetic energy
Correct answer is option 'B'. Can you explain this answer?
Which of the following energy change involves frictional force?
a)
Chemical energy to heat energy
b)
Kinetic energy to heat energy
c)
Potential energy to sound energy
d)
Chemical energy to kinetic energy
|
|
Mrudula answered |
Kinetic energy into heat energy
Find the energy possessed by a ball of mass 550g rolling on the surface with a speed of 25 m/sec.- a)130.896 J
- b)138.875 J
- c)171.875 J
- d)160.472 J
Correct answer is option 'C'. Can you explain this answer?
Find the energy possessed by a ball of mass 550g rolling on the surface with a speed of 25 m/sec.
a)
130.896 J
b)
138.875 J
c)
171.875 J
d)
160.472 J
|
|
Shilpa Choudhury answered |
Energy due to motion = K.E. = ½ mv2
= ½ × 550/1000 kg × (25m / sec)2
= 171.875 J
A batsman hits the ball of mass 250 g with his bat and the ball leaves the bat with a speed of 10 m/s Find the work done by the bat on the ball- a)10.25 J
- b)12.5 J
- c)1.25 J
- d)125 J
Correct answer is option 'B'. Can you explain this answer?
A batsman hits the ball of mass 250 g with his bat and the ball leaves the bat with a speed of 10 m/s Find the work done by the bat on the ball
a)
10.25 J
b)
12.5 J
c)
1.25 J
d)
125 J
|
|
Shilpa Choudhury answered |
K.E. = ½ mv2 = ½ × (250/1000) × (10)2 = 12.5 J
A girl weighing 400 N climbs a vertical ladder. If the value of g be 10 ms–2, the work done by her after climbing 2 m will be- a)200 J
- b)80 J
- c)2000 J
- d)800 J
Correct answer is option 'D'. Can you explain this answer?
A girl weighing 400 N climbs a vertical ladder. If the value of g be 10 ms–2, the work done by her after climbing 2 m will be
a)
200 J
b)
80 J
c)
2000 J
d)
800 J
|
|
Shilpa Choudhury answered |
Mg = 400 N, h = 2 m
∴ work done = P.E. = mgh = 800 Nm = 800J
Find the Kinetic energy of a body of mass 2 kg moving with velocity of 0.1 metre per second- a)0.1 J
- b)1.0 J
- c)0.01 J
- d)10.0 J
Correct answer is option 'C'. Can you explain this answer?
Find the Kinetic energy of a body of mass 2 kg moving with velocity of 0.1 metre per second
a)
0.1 J
b)
1.0 J
c)
0.01 J
d)
10.0 J
|
|
Shilpa Choudhury answered |
K.E. = 1/2 mv2 = 1/2 × 0.1 × 0.1 × 2
= 0.01 J
In case of negative work, the angle between the force and displacement is- a)0°
- b)45°
- c)90°
- d)180°
Correct answer is option 'D'. Can you explain this answer?
In case of negative work, the angle between the force and displacement is
a)
0°
b)
45°
c)
90°
d)
180°
|
|
Shilpa Choudhury answered |
W = F cosθ × s
For, θ = 180°, cosθ = –1
Force acts in opposite direction of the motion
of the body
How much energy does a 100 w electric bulb transfer in 1 minute?- a)60 J
- b)600 J
- c)6000 J
- d)3600 J
Correct answer is option 'C'. Can you explain this answer?
How much energy does a 100 w electric bulb transfer in 1 minute?
a)
60 J
b)
600 J
c)
6000 J
d)
3600 J
|
|
Shilpa Choudhury answered |
100 W means 100 J energy consumed in
one second. Thus, in1 minute or 60 seconds,
energy consumed is 100 J × 60
= 6000 J.
An object of mass 12 kgs is at a certain height above the ground. If the P.E of the object is 480 J,find the height at which the object is with respect to the ground. Given, g = 10 ms–2- a)12 m
- b)4 m
- c)8 m
- d)20 m
Correct answer is option 'B'. Can you explain this answer?
An object of mass 12 kgs is at a certain height above the ground. If the P.E of the object is 480 J,
find the height at which the object is with respect to the ground. Given, g = 10 ms–2
a)
12 m
b)
4 m
c)
8 m
d)
20 m
|
|
Shilpa Choudhury answered |
P.E. = mgh
480 J = 12 kg × 10 ms–2 × h
h = 480 J/120 kg ms-2
= 4 m
The object is at the height of 4 m above the ground.
Two unequal masses posses the same momentum, then the Kinetic energy of the heavier mass is ________ than the Kinetic energy of the lighter mass- a)More
- b)Less
- c)Equal
- d)Insufficient data
Correct answer is option 'B'. Can you explain this answer?
Two unequal masses posses the same momentum, then the Kinetic energy of the heavier mass is ________ than the Kinetic energy of the lighter mass
a)
More
b)
Less
c)
Equal
d)
Insufficient data
|
|
Shilpa Choudhury answered |
K.E. = ½ mv2 = ½ (mv)2/m = p2/2m (p = 2m = momentum)
Here pH=Pl of the two unequal masses me and mh
K.Eh/K.EL = ml/mh => K.Eh = me/wh (K.El)
∴ K.Eh < />l
A car is accelerated on a levelled road and attains a speed of 4 times its initial speed. In this process, the K.E. of the car- a)Does not change
- b)Becomes 4 times that of initial P.E
- c)Becomes 8 times that of initial P.E
- d)Becomes 16 times that of initial P.E
Correct answer is option 'D'. Can you explain this answer?
A car is accelerated on a levelled road and attains a speed of 4 times its initial speed. In this process, the K.E. of the car
a)
Does not change
b)
Becomes 4 times that of initial P.E
c)
Becomes 8 times that of initial P.E
d)
Becomes 16 times that of initial P.E
|
|
Shilpa Choudhury answered |
K.E. = 1/2mv2
so K.E. is proportional to v2 i.e., the kinetic energy of the car becomes 16 times that of initial kinetic energy
An iron sphere of mass 30 kg has the same diameter as an aluminium sphere whose mass is 10.5kg. The spheres are dropped simultaneously from a cliff. When they are 10 m from the ground, theyhave the same _______.- a)Acceleration
- b)Momentum
- c)Kinetic energy
- d)Potential energy
Correct answer is option 'A'. Can you explain this answer?
An iron sphere of mass 30 kg has the same diameter as an aluminium sphere whose mass is 10.5
kg. The spheres are dropped simultaneously from a cliff. When they are 10 m from the ground, they
have the same _______.
a)
Acceleration
b)
Momentum
c)
Kinetic energy
d)
Potential energy
|
|
Shilpa Choudhury answered |
Both the spheres are under the force of gravity (g = a)
A car is accelerated on a travelled road and acquires a velocity 4 times of its initial velocity. Duringthis process, the potential energy of the car- a)Does not change
- b)Becomes twice that of the initial P.E
- c)Becomes four times that of the initial P.E
- d)Becomes 16 times that of the initial P.E
Correct answer is option 'A'. Can you explain this answer?
A car is accelerated on a travelled road and acquires a velocity 4 times of its initial velocity. During
this process, the potential energy of the car
a)
Does not change
b)
Becomes twice that of the initial P.E
c)
Becomes four times that of the initial P.E
d)
Becomes 16 times that of the initial P.E
|
|
Shilpa Choudhury answered |
Potential energy is due to its position and not motion.
A 1 kg mass has a kinetic energy of 1 joule when its speed is- a)0.50 m/s
- b)1.24 m/s
- c)4.4 m/s
- d)1.4 m/s
Correct answer is option 'D'. Can you explain this answer?
A 1 kg mass has a kinetic energy of 1 joule when its speed is
a)
0.50 m/s
b)
1.24 m/s
c)
4.4 m/s
d)
1.4 m/s
|
|
Shilpa Choudhury answered |
K.E. = 1/2 mv2
V = √2K.E/m = √2×12/1 = √2 = 1.4 m/s
Chapter doubts & questions for Work, Mechanical Advantage and Efficiency - Science & Technology for Grade 8 2025 is part of Grade 8 exam preparation. The chapters have been prepared according to the Grade 8 exam syllabus. The Chapter doubts & questions, notes, tests & MCQs are made for Grade 8 2025 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests here.
Chapter doubts & questions of Work, Mechanical Advantage and Efficiency - Science & Technology for Grade 8 in English & Hindi are available as part of Grade 8 exam.
Download more important topics, notes, lectures and mock test series for Grade 8 Exam by signing up for free.
Science & Technology for Grade 8
6 videos|56 docs|7 tests
|
|
© EduRev
|
Education Revolution
|
|
Signup to see your scores
go up
within 7 days!
within 7 days!
Takes less than 10 seconds to signup