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

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.

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.

I think 'b' is correct answer

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.

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

Cupola furnace..

 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.”

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.

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

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.

C

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.

A

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

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

Cetane and alpha-methyl naphthalene are the reference fuels taken for cetane rating. option (d) is correct.

Option C is the correct answer

Solution:

Given,

Number of teeth on external gear, Z₁ = 60

Number of teeth on pinion, Z₂ = 20

Module, m = 6 mm

We know that the centre distance between two gears is given by the formula,

C = (Z₁ + Z₂) × m / 2

Substituting the given values in the above equation, we get

C = (60 + 20) × 6 / 2

C = 240 mm

Therefore, the centre distance between the external gear and pinion is 240 mm.

Hence, option (c) is the correct answer.

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

Sorry I haven't answer

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.

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).

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.