All questions of Mechanical Engineering (SSC JE) Tests for Mechanical Engineering Exam

C

Manometers are devices in which columns of a suitable liquid are used to measure the difference in pressure between two points or between a certain point and the atmosphere. Manometer is needed for measuring large gauge pressures. It is basically the modified form of the piezometric tube.

Given parameters:
Young's modulus (E) = 125 GPa
Poisson's ratio (ν) = 0.25

Modulus of rigidity (G) is given by the formula:
G = E/ (2(1+ν))

Calculation:
G = 125/(2(1+0.25))
G = 50 GPa

Therefore, the modulus of rigidity of the material is 50 GPa.

Explanation:
Modulus of rigidity is a measure of a material's resistance to shearing deformation. It is the ratio of the shear stress to the shear strain. It is also known as shear modulus.

Young's modulus is a measure of a material's stiffness or resistance to deformation in response to an applied force. Poisson's ratio is a measure of the ratio of lateral strain to longitudinal strain when a material is subjected to an applied force.

Using the formula for modulus of rigidity, we can calculate it using Young's modulus and Poisson's ratio. In this case, we were given the values of Young's modulus and Poisson's ratio. Substituting these values in the formula, we get the modulus of rigidity as 50 GPa.

Conclusion:
The correct option for the given question is (B) 50 GPa.

The fillet welds are subjected to tensile stress. The minimum cross-section of the fillet is at the throat. Therefore the failure due to tensile stress occurs at the throat section. Thus the weakest area of the weld is the throat.

Viscosity of Water with Respect to Air

Viscosity is defined as the measure of a fluid's resistance to flow. It is a property of fluids that describes the internal frictional forces that cause a fluid to resist flow. The viscosity of a fluid is dependent on the fluid's molecular structure and temperature.

Water and air are both fluids with different molecular structures, and hence, they have different viscosities. The viscosity of water with respect to air is about 55 times.

Explanation:

The viscosity of air at standard temperature and pressure (STP) is approximately 0.0000181 Pa.s, while the viscosity of water at STP is approximately 0.001 Pa.s. This means that water is approximately 55 times more viscous than air.

This difference in viscosity plays a crucial role in many fluid mechanics applications. For example, a fluid such as water will flow more slowly through a pipe than air, even if the pressure difference driving the flow is the same. The higher viscosity of water means that more energy is required to move it through a pipe, and this can result in higher pumping costs.

In summary, the viscosity of water is much higher than the viscosity of air, and this difference has significant implications for fluid mechanics applications.

 law of thermodynamics is the basis for measurement of temperature and setting its scale. In simple word, Zeroth law of thermodynamics says that “When two bodies are separately in thermal equilibrium with the third body, then the two are also in thermal equilibrium with each other.”

Complete Combustion in Oxy-acetylene Gas Welding

Complete combustion is an essential process in oxy-acetylene gas welding where the fuel gas, Acetylene, burns in the presence of oxygen gas to generate heat. During the process, the acetylene gas is decomposed into carbon and hydrogen, releasing a large amount of heat energy. The heat generated by the exothermic reaction is utilized to melt and join metallic components.

Volume of Oxygen Required for Complete Combustion

The volume of oxygen required for complete combustion in oxy-acetylene gas welding depends on the stoichiometric ratio of acetylene and oxygen. The stoichiometric ratio is the minimum ratio of oxygen required to burn a unit volume of acetylene gas completely.

The stoichiometric ratio for acetylene and oxygen is 1:2.5. It means that for every unit volume of acetylene gas, 2.5 units of oxygen gas are required to burn it completely. Therefore, the correct option for the volume of oxygen required per unit of acetylene is 2.5.

Conclusion

In summary, complete combustion is a critical process in oxy-acetylene gas welding, where the volume of oxygen required for complete combustion depends on the stoichiometric ratio of acetylene and oxygen. The stoichiometric ratio for acetylene and oxygen is 1:2.5, which means that for every unit volume of acetylene gas, 2.5 units of oxygen gas are required to burn it completely.

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dQ/T =0 ; for reversible process

Swiss physician Hans Christian Jacobaeus performed the first Laparoscopic surgery on humans. 1916. Austrian surgeon Hermann Schloffer performed boring the first splenectomy operation.

Cupola furnace..

Iso- some (constant) where the temperature is constant the process is known as isothermal process.

Pre-ignition (or preignition) in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. ... Many engines have suffered such failure where improper fuel delivery is present.

Ratio of Static Friction to Dynamic Friction

Static friction is the force that resists the motion of an object when it is at rest, while dynamic friction is the force that resists the motion of an object when it is in motion. The ratio of static friction to dynamic friction depends on various factors, such as the surface roughness, the weight of the object, and the force applied to the object. However, there is a general rule that can be applied in most cases.

The Ratio Is Greater Than One

The ratio of static friction to dynamic friction is greater than one. This means that it takes more force to overcome static friction and start an object moving than it does to keep it moving once it is in motion. This is because static friction is usually stronger than dynamic friction.

Example

For example, imagine trying to push a heavy box across a rough floor. At first, the box will not move because of static friction. You will need to apply a certain amount of force to overcome this static friction and start the box moving. Once the box is in motion, you will need to apply less force to keep it moving, because dynamic friction is weaker than static friction.

Conclusion

In conclusion, the ratio of static friction to dynamic friction is always greater than one. This means that it is harder to overcome static friction and start an object moving than it is to keep it moving once it is in motion. Understanding this ratio is important in many applications, such as designing machines and calculating the forces required to move objects.

Function of condenser is to condensate the steam which is expanded in turbine and also maintain the low pressure in the last stages of turbine for better efficiency.So Condenser is designed for low pressure and low temperature.

Option C is the correct answer

Enthalapy units and work units are same W = F x S = MLT^-2 x L=ML^2T^-2

The air preheater in a boiler is an important component that helps in improving the efficiency of the boiler system. It is responsible for heating the combustion air before it enters the furnace, which results in better fuel combustion and heat transfer.

The correct location of the air preheater in a boiler is between the forced draft fan and the furnace. This placement allows the preheated air to be supplied directly to the furnace for combustion. Let's understand why this is the ideal location:

1. Forced Draft Fan (FDF):
The forced draft fan is responsible for supplying the air required for combustion into the furnace. It creates a positive pressure inside the furnace and ensures the proper airflow. The air preheater is placed before the FDF to preheat the air before it enters the fan.

2. Air Preheater (APH):
The air preheater is a heat exchanger that transfers heat from the flue gases to the incoming combustion air. It consists of a series of tubes or plates through which the flue gases flow. As the hot flue gases pass over these tubes, they transfer their heat to the air flowing through them, thereby preheating it.

3. Furnace:
The furnace is the central combustion chamber where the fuel is burned. The preheated air from the air preheater is supplied to the furnace through the forced draft fan. This preheated air helps in achieving better fuel combustion by providing the required oxygen and temperature for efficient combustion.

Benefits of placing the air preheater between the forced draft fan and furnace:

- Improved Fuel Combustion: Preheating the combustion air results in better fuel combustion as it provides the necessary temperature and oxygen for efficient combustion. This leads to reduced fuel consumption and increased boiler efficiency.

- Enhanced Heat Transfer: The air preheater increases the overall heat transfer in the boiler system by recovering heat from the flue gases. This recovered heat is then utilized to preheat the incoming combustion air, thereby reducing the energy required to heat the air.

- Reduced Flue Gas Temperature: By extracting heat from the flue gases, the air preheater helps in lowering the temperature of the flue gases exiting the boiler. This reduces the heat loss through the chimney and increases the overall efficiency of the boiler system.

- Environmental Benefits: The preheating of combustion air reduces the emission of pollutants such as nitrogen oxides (NOx) and carbon monoxide (CO) by improving the combustion efficiency. This contributes to a cleaner and more environmentally friendly operation.

In conclusion, placing the air preheater between the forced draft fan and furnace in a boiler system is essential for achieving better fuel combustion, improved heat transfer, and increased overall efficiency.

Cop of refrigerator is given as desired effect upon work done

Flow rate means discharge. as u know venturimeter is a discharge measuring device so  correct option is d

In eutectoid reaction, the austenite transforms into a phase mixture of ferrite (containing 0.76% C) and cementite. This phase mixture is known as pearlite. The eutectoid reaction occurs at a constant temperature. This is known as eutectoid temperature and is 723DigreeC

Higher the metacentric height ,higher will be the stability of the body

Explanation:

A Hookes joint is a type of universal joint that is used to connect two non-coplanar and nonparallel shafts. It is also known as Cardan joint or universal coupling.

Let's understand the term non-coplanar and nonparallel shafts. Coplanar means lying in the same plane, and parallel means lying in the same direction and never meeting. In the case of shafts, coplanar shafts lie in the same plane, and parallel shafts lie in the same direction. Non-coplanar and nonparallel shafts do not lie in the same plane or direction. They have a misalignment between them.

A Hookes joint is used to transmit torque between two shafts that are not in the same plane or direction. It is used to connect the drive shaft to the driven shaft in automotive applications.

The Hookes joint consists of two yokes and a cross-shaped member called the spider, which is fitted between them. Each yoke has two arms, and the spider fits between them. The spider has four arms at right angles to each other, and each arm is connected to a yoke. The yokes are connected to the shafts, and the spider connects the two yokes.

When the drive shaft rotates, it causes the yoke to rotate, and the spider transmits the rotation to the driven shaft. The Hookes joint can accommodate a certain amount of misalignment between the two shafts. It can also operate at different angles between the two shafts.

Therefore, a Hookes joint is used to connect two non-coplanar and nonparallel shafts that have a misalignment between them. It can transmit torque between them and accommodate different angles between the shafts.

Bulk modulus and Young's modulus are both measures of a material's elasticity, but they measure different aspects of elasticity.

Bulk modulus is a measure of a material's resistance to compression, while Young's modulus is a measure of a material's resistance to deformation in tension or compression.

Poisson's ratio is a measure of a material's tendency to compress laterally when it is stretched or pulled in one direction.

The relationship between these three measures can be expressed as follows:

Bulk modulus / Young's modulus = 3(1 - 2Poisson's ratio) / (1 + Poisson's ratio)

If Poisson's ratio is 0.25, then:

Bulk modulus / Young's modulus = 3(1 - 2(0.25)) / (1 + 0.25)

= 3(0.5) / 1.25

= 1.2

Therefore, the ratio of bulk modulus to Young's modulus for a Poisson's ratio of 0.25 is 2/3 (option B).

C

Centrifugal tension in belts

Introduction:
In belt drives, there are two types of tensions- initial tension and operating tension. The initial tension is provided to keep the belt in a state of tension when there is no load on the belt. The operating tension is the tension that the belt experiences when power is transmitted. Centrifugal tension is a type of operating tension.

Explanation:
Centrifugal tension is a phenomenon that occurs due to the rotation of the belt. When a belt rotates, the particles on the outer edge of the belt tend to move away from the center of rotation due to centrifugal force. This results in an increase in tension on the outer edge of the belt. This increase in tension is called centrifugal tension.

Effect of centrifugal tension:
Centrifugal tension can be harmful to the belt drive system. This is because it increases the tension in the belt which can lead to excessive wear and tear of the belt. Additionally, centrifugal tension reduces the power transmitted by the belt. This is because the increased tension causes the belt to slip on the pulley, reducing the amount of power transmitted.

Conclusion:
Centrifugal tension is a type of operating tension in belt drives that occurs due to the rotation of the belt. While it can be useful in maintaining some tension when no power is transmitted, it can be harmful when power is transmitted. Centrifugal tension increases the tension in the belt which can lead to excessive wear and tear and reduces the power transmitted by the belt.

In a Porter governor, the balls are attached to the extension of lower links.Porter governor is a modification of Watt's governor, with a central load attached to the sleeve.A Hartnell governor is a spring loaded governor.

Explanation:

A kinematic chain is a set of links connected in such a way that one link is fixed and the other links move relative to it. The number of pairs (P) forming a kinematic chain can be calculated using the formula:

P = l - 1

where P is the number of pairs and l is the number of links.

The relation between the number of pairs and the number of links can be derived from the above formula as:

l = P + 1

Therefore, the relation between the number of pairs (P) forming a kinematic chain and the number of links (l) is:

l = P + 1

Substituting the value of P from the given options, we get:

a) l = 2P + 2
b) l = 2P + 3
c) l = 2P + 4
d) l = 2P + 5

The correct option is (c) because it satisfies the above relation. Therefore, the relation between the number of pairs (P) forming a kinematic chain and the number of links (l) is:

l = 2P + 4

The correct answer is option 'D': The cetane number of alcohol fuels is very low which prevents their ignition by combustion.

Explanation:

Alcohol fuels, such as methanol and ethanol, have been considered as potential alternatives to traditional fossil fuels due to their renewable nature and lower carbon emissions. However, they are not suitable as fuels for diesel engines mainly because of their low cetane number.

1. Cetane number: The cetane number is a measure of the ignition quality of a fuel. It indicates the ease and speed at which the fuel will ignite in a diesel engine when subjected to compression. A higher cetane number indicates better ignition properties.

2. Diesel combustion process: In a diesel engine, the fuel is injected into the combustion chamber, where it mixes with the compressed air. The heat of compression causes the fuel to self-ignite and burn, leading to the power stroke. This process is known as compression ignition.

3. Alcohol fuels and cetane number: Alcohol fuels, such as methanol and ethanol, have low cetane numbers compared to diesel fuel. Methanol has a cetane number of around 4-7, while ethanol has a cetane number of around 8-15. In contrast, diesel fuel typically has a cetane number of 40-55.

4. Ignition delay: The low cetane number of alcohol fuels leads to a longer ignition delay. Ignition delay refers to the time between the start of fuel injection and the start of combustion. A longer ignition delay can result in poor combustion, incomplete fuel burn, and reduced engine performance.

5. Compression ignition: Alcohol fuels with low cetane numbers require higher compression ratios or higher temperatures to achieve self-ignition. Diesel engines are designed to operate with a specific compression ratio, and altering this ratio for alcohol fuels may not be feasible or efficient.

6. Other factors: In addition to the low cetane number, alcohol fuels also have lower energy content compared to diesel fuel, which can further reduce their efficiency as engine fuels. They also have different physical properties, such as higher volatility and lower lubricity, which may require modifications to engine components and fuel systems.

In conclusion, the low cetane number of alcohol fuels, such as methanol and ethanol, prevents their ignition by combustion in diesel engines. This limitation, along with other factors, makes alcohol fuels unsuitable as direct replacements for diesel fuel in diesel engines.

The correct answer is option 'B', the process of removing non-condensable is called scavenging process. The scavenging process is an important step in the operation of steam and other vapor cycle systems to ensure efficient and effective heat transfer.

Scavenging Process:

The scavenging process involves the removal of non-condensable gases from the vapor cycle system. Non-condensable gases are substances that do not easily condense into a liquid state at the operating temperature and pressure of the system. These gases can accumulate in the system and hinder the heat transfer process, leading to decreased system efficiency and performance.

Reasons for Non-Condensable Gases:

Non-condensable gases can enter the vapor cycle system from various sources, including air leaks, incomplete purging during system startup, or the presence of volatile substances. These gases can include air, nitrogen, carbon dioxide, and other trace gases.

Effects of Non-Condensable Gases:

The presence of non-condensable gases in the vapor cycle system can lead to several adverse effects:

1. Reduced Heat Transfer: Non-condensable gases create a barrier between the heat source (such as a boiler or condenser) and the working fluid (such as steam or refrigerant). This barrier hinders the transfer of heat, reducing the overall efficiency of the system.

2. Increased Pressure: Non-condensable gases can accumulate in certain parts of the system, leading to increased pressure. This can cause operational issues and potentially damage system components.

3. Corrosion: Non-condensable gases can react with the working fluid or the materials within the system, leading to corrosion and degradation of system components over time.

Methods of Scavenging:

There are various methods employed to remove non-condensable gases from the vapor cycle system:

1. Purging: Purging involves removing the non-condensable gases from the system by introducing a purge gas (typically steam or an inert gas) and venting the mixture of gases. This process is commonly used during system startup and shutdown.

2. Deaeration: Deaeration is a process that involves removing dissolved gases from the working fluid. This is typically done using deaerator tanks or trays, where the working fluid is heated and exposed to a vacuum to facilitate gas removal.

3. Condenser Air Extraction: In steam power plants, air extraction systems are used to remove non-condensable gases from the condenser. These systems extract air from the condenser and discharge it to the atmosphere.

Importance of Scavenging:

The scavenging process is crucial to maintain the efficiency and performance of vapor cycle systems. By removing non-condensable gases, heat transfer is enhanced, pressure is controlled, and the risk of corrosion is minimized. Regular scavenging helps to ensure optimal system operation and prevents potential issues associated with the presence of non-condensable gases.

In conclusion, the process of removing non-condensable gases from steam and other vapor cycle systems is called the scavenging process. This process is essential for maintaining the efficiency, performance, and longevity of the system by enhancing heat transfer, controlling pressure, and preventing corrosion.

The joule symbol: J) is a derived unit of energy in the International System of Units. It is equal to the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of its motion through a distance of one metre (1 newton metre or N⋅m).