Case Based Questions: Exploring Forces | Science Curiosity Class 8 - New NCERT PDF Download

Q1: Read the source below and answer the questions that follow:

Ragini and Sonali were riding their bicycles with rubber tyres on a rough cemented road during recess. A strong wind blew against them, making it harder to pedal. They noticed that their legs had to work harder to keep the bicycles moving forward, and sometimes their speed slowed unexpectedly.

(a) Which force of air was opposing their motion? (1 mark)
(b) What effect of force is shown when speed decreases? (1 mark)
(c) If the wind started blowing from behind them, how would their speed be affected? (2 marks)

Ans:
(a) The force opposing their motion was air resistance, which is a form of frictional force exerted by air.
(b) The effect of the force is that it reduces the speed of the moving bicycles.
(c) If the wind blew in the same direction as their motion, it would apply a pushing force on the bicycles, thereby increasing their speed and making pedalling easier.

Q2: Read the source below and answer the questions that follow:

A student noticed a large cardboard box lying on the classroom floor. He tried to move it by pushing, pulling, and lifting it. He realised that sometimes the box moved easily, and sometimes it barely moved, depending on how much effort he applied.

(a) What common scientific term describes all these actions? (1 mark)
(b) Mention two effects of force seen in this situation. (2 marks)
(c) Can force exist without interaction between two objects? Give reasoning.(1 mark)

Ans:
(a) All these actions are described by the scientific term force, which means a push or pull.
(b) The effects of force here are:
1. The box could be set into motion when pushed or pulled.
2. The shape of the box may change slightly due to pressing or lifting.
(c) No, a force cannot exist without interaction. At least two objects must interact for a force to come into play (for example, the hand and the box).

Q3: Read the source below and answer the questions that follow:

In the playground, Amit kicked a rubber football with his leg while his friend tried to stop it using his hand. They noticed how the ball sped up, slowed down, or changed direction depending on the action applied. The students observed closely how forces can affect the motion of objects during play.

(a) Which type of force is applied in both actions? (1 mark)
(b) Which changes in the ball’s motion are shown here? (2 marks)
(c) Give one example from the case to show that force can change the direction of motion. (1 mark)

Ans:
(a) In both cases, the force applied is a muscular force.
(b) When Amit kicked the ball, the force increased the ball’s speed and changed its direction. When his friend stopped the ball, the force decreased its speed until it came to rest.
(c) The kick by Amit shows that force can change the direction in which the football was moving.

Q4: Read the source below and answer the questions that follow:

During a science activity, students pushed a lunch box with a flat metal base across different surfaces: glass, polished wood, and sand. They observed that the box stopped after travelling varying distances depending on the surface, showing how friction affects motion differently.

(a) Which force brought the box to rest? (1 mark)
(b) On which surface was the friction maximum? (1 mark)
(c) If the same experiment is done by pushing the box in water, which force will oppose its motion? (1 mark)

Ans:
(a) The box was brought to rest due to the force of friction between the surfaces.
(b) Friction was maximum on the sand surface, because it is the roughest among the three.
(c) In water, the box would be opposed by fluid friction (drag or resistance of water), which acts opposite to the direction of motion.

Q5: Read the source below and answer the questions that follow:

A student placed an iron nail close to a bar magnet. Without touching it, the nail moved toward the magnet. The student noticed that magnets can attract objects at a distance and wondered about the nature of such forces.

(a) Which type of force is acting here? (1 mark)
(b) Is this a contact or a non-contact force? (1 mark)
​(c) What will happen if two north poles of magnets are brought near each other? (1 mark)

Ans:
(a) The force acting here is the magnetic force.
(b) This is a non-contact force, as the magnet pulls the nail without touching it.
(c) If two north poles are brought near each other, they will repel each other, since like poles repel while unlike poles attract.

Q6: Read the source below and answer the questions that follow:

Two rubber balloons were rubbed with a woollen cloth to charge them and then hung close together using threads. The students observed that the balloons either moved towards or away from each other, noticing the effect of electric charges.

(a) What happened when the balloons were brought near each other? (1 mark)
(b) Which type of force is responsible for this behaviour? (1 mark)
(c) State one rule of electric charges observed in this experiment. (2 marks)

Ans:
(a) The balloons moved away from each other, i.e., they repelled each other.
(b) This behaviour was due to the electrostatic force produced by rubbing.
(c) The rule observed is: like charges repel each other, while unlike charges attract each other. Both balloons acquired the same kind of charge, hence they repelled.

Q7: Read the source below and answer the questions that follow:

Ravi threw a tennis ball straight up into the air on the school playground. The ball slowed down as it rose, stopped at the top, and then fell back to the ground. Students noticed how gravity continuously affected the ball’s motion.

(a) Which force pulled the ball back to the ground? (1 mark)
(b) During upward motion, in which direction does gravity act? (1 mark)
(c) Why does the ball return to the ground even if thrown very hard? (2 marks)

Ans:
(a) The ball was pulled back to the ground by gravitational force.
(b) During upward motion, gravity acts downwards, towards the Earth’s centre.
(c) The ball always comes back because gravity continuously pulls it downwards, and no matter how hard it is thrown, the upward force becomes weaker after some time, while gravity remains constant.

Q8: Read the source below and answer the questions that follow:

A pencil box was placed on a spring balance marked 0–10 N. The spring stretched downwards, showing a reading of 2 N. Students realised that weight depends on gravity and noticed how the reading would change if measured elsewhere, like on the Moon.

(a) Which force caused the spring to stretch? (1 mark)
(b) What is the SI unit of weight? (1 mark)
(c) If the pencil box is taken to the Moon, will its weight remain 2 N? Why/why not? (2 marks)

Ans:
(a) The spring stretched due to the gravitational force acting on the pencil box (its weight).
(b) The SI unit of weight is the newton (N).
(c) No, the weight would be less on the Moon because the Moon’s gravitational pull is weaker than Earth’s. The mass of the box remains the same, but its weight changes with gravity.

Q9: Read the source below and answer the questions that follow:

An empty plastic bottle with a tightly closed lid was pushed into a bucket of water. It immediately bounced back to the surface. Students observed how the upward force acted and wondered why some objects float while others sink.

(a) Which upward force acted on the bottle? (1 mark)
(b) What is the term used for the tendency of objects to float in liquids? (1 mark)
(c) If the weight of the bottle was greater than the upward force, what would happen? (2 marks)

Ans:
(a) The upward force acting on the bottle was the buoyant force (upthrust).
(b) The tendency of objects to float in liquids is called buoyancy.
(c) If the bottle’s weight was greater than the buoyant force, it would sink to the bottom of the bucket.

Q10: Read the source below and answer the questions that follow:

Three blocks made of wood, aluminium, and iron, all of the same size, were placed in water. Students observed that the wooden block floated, the aluminium dipped halfway, and the iron sank completely. They discussed how density affects whether objects float or sink.

(a) Why do these blocks float or sink differently in water? (2 marks)
(b) Which two opposing forces act on these blocks in water? (1 mark)
(c) On what property of liquid does the buoyant force depend? (1 mark)

Ans:
(a) The blocks behave differently because they have different densities. Wood is less dense than water and floats, aluminium has a density close to water and partly sinks, while iron is denser than water and sinks completely.
(b) The two opposing forces are: downward gravitational force (weight of the block) and upward buoyant force (upthrust of water).
(c) The buoyant force depends on the density of the liquid.