ISRO 2018 Mechanical Engineering Questions, Answers and Explanation
1. If the stream function of a two dimensional flow is Ψ = 5xy, then the velocity at a point (3,4) is
a) 25 m/s
b) 20 m/s
c) 7 m/s
d) 5 m/s
Answer
a) 25 m/s
Explanation[Fluid Mechanics]
u = ∂ Ψ/∂ y = 5x
v =- ∂ Ψ/∂ x = -5y
At (3,4) u = 5x3 = 15 m/s
At (3,4), v = -5x4 = -20 m/s
Resultant velocity = √(152+(-20)2)= 25 m/s
2. A glass tube of 8 mm diameter is immersed in water. Surface tension of water is 0.0075 kg/m and rise of water in the tube due to capillary effect is
a) 7.5 mm
b) 3.75 mm
c) 11.25 mm
d) none of these
Answer
b) 3.75 mm
Explanation[Fluid Mechanics]
Surface tension of the water tends to pull the water above the level and the weight of water tends to pull it back to its level. For water the angle between the surface tension force and plane of water is 90o.
Force due to surface tension = Weight of water above the level
Force due to surface tension = Surface tension x circumference (as surface tension acts only at the interface) = 0.0075 x π x 0.008 N
Let the rise of the water be h m
Weight of water above the level = Density of water xArea of cross section x height = 1000xπ d2/4 h N
Equating both0.0075 x π x 0.008 = 1000xπ x 0.008 x 0.008 x h/4
0.0075 = 8h/4
h = 0.0075/2 m = 7.5/2 mm = 3.75 mm
3. An impulse turbine produces 125 hp under a head of 25 metres. By what percentage should the speed be increased for a head of 100 metres.
a) 25 %
b) 50 %
c) 75 %
d) 100 %
Answer
d) 100%
Explanation [Fluid Mechanics]
If a turbine is working under different heads, the behavior of turbine can be easily known from the unit quantities
Nu = N1/√(H1) = N2/√(H2)
N1/√(H1) = N2/√(H2)
N2/N1 = √ (100/25)=2
ie, the speed has to be increased by 100%.
4. The specific speed of centrifugal pump, delivering 1000 litres of water per second against a head of 16 metre at 800 rpm
a) 50 rpm
b) 100 rpm
c) 150 rpm
d) 200 rpm
Answer
b) 100 rpm
Explanation[Fluid Mechanics]
An impeller with a low specific speed has a thin profile (the shrouds are close together) and a large outside diameter relative to the eye diameter. An impeller with a high specific speed has a fat profile (the shrouds are far apart) and has an eye diameter that is closer in size to the impeller outer diameter.
Specific speed Ns = N √(Q)/H3/4
N = 800 rpm
Q = 1000 l/s = 1m3/s
H = 16m
Ns = 800 x 1/(16)3/4
= 800/8 = 100 rpm
5. A pipe of 0.1 m2 suddenly changes to 0.5 m2 area. The quantity of water flowing in the pipe is 0.5 m3/s. Head loss due to sudden enlargement is nearly
a) 0.025 m
b) 0.05 m
c) 0.8 m
d) 1.2 m
Answer
Explanation
Head loss due to sudden expansion is given by HL = (v1-v2)2/2g
Q = Ρ Av = Ρ A1v1 = Ρ A2v2
v1 = Q/ΡA1 = 0.5/(1000x0.1)=5x10-3 m/s
v2 = Q/ΡA2 = 0.5/(1000x0.5)=1x10-3 m/s
HL = (4x10-3)2/(2x9.81) ~ 0.8 x 10-6 m
6. Efficiency of a centrifugal pump is maximum when the blades are
a) straight
b) forward curved
c) backward curved
d) forward and backward curved
Answer
c) backward curved
Explanation [Fluid Mechanics]
For fluids in in-compressible regime of operation such as water backward curved vanes are used for maximum efficiency.
For radial and forward bent vanes the power requirement increases monotonically as the discharge increases. But for backward curved the power requirement peaks at maximum efficiency.
7. The impact strength of a material is an index of its
a) fatigue strength
b) tensile strength
c) hardness
d) toughness
Answer
d) toughness
Explanation [Strength of Materials]
Answer
d) BCC - FCC - HCP
Explanation [Material Science]
BCC - Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Barium (Ba), Vanadium (V), Chromium (Cr), Iron (Fe).
The bcc arrangement does not allow the atoms to pack together as closely as the fcc or hcp arrangements. Metals which have a bcc structure are usually harder and less malleable than close-packed metals such as gold. When the metal is deformed, the planes of atoms must slip over each other, and this is more difficult in the bcc structure.
FCC - Aluminium (Al), Copper(Cu), Gold (Au), Iridium (Ir), Lead(Pb), Nickel (Ni), Platinum (Pt), Silver (Ag).
HCP - Beryllium(Be), Cadmium (Cd), Magnesium (Mg), Titanium (Ti), Zinc (Zn), Zirconium (Zr)
9. In a machining operation cutting speed is reduced by 50%. Assuming n = 0.5, C = 300 in Taylor's equation. The increase in tool life is
a) 2
b) 4
c) 8
d) 16
Answer
b) 4
Explanation [Manufacturing Technology]
When both V and T are plotted in log-scale, linear relationship appears With the slope, n and intercept, c, the equation is VTn = C
where, n is called, Taylor’s tool life exponent. The values of both ‘n’ and ‘c’ depend mainly upon the tool-work materials and the cutting environment (cutting fluid application).
V2/V1 = 50%, T2/T1 = ?
V2/V1 . (T2/T1)n = 1
0.5 . (T2/T1)0.5 = 1
(T2/T1)0.5 = 2
T2/T1 = 4
10. In a linear arc welding process, the heat input per unit length is inversely proportional to
a) welding current
b) welding speed
c) welding voltage
d) duty cycle of power source
Answer
b) welding speed
Explanation [Manufacturing Technology]
Welding power gives the heat input per unit length. Welding current and welding voltage is directly proportional to welding power and hence directly proportional to the heat input per unit length.
Duty cycle is the ratio of arcing time to the weld cycle time multiplied by 100. Welding cycle time is either 5 minutes as per European standards or 10 minutes as per American standard and accordingly power sources are designed. If arcing time is continuously 5 minutes then as per European standard it is 100% duty cycle and 50% as per American standard. At 100% duty cycle minimum current is to be drawn i.e. with the reduction of duty cycle current drawn can be of higher level. Duty cycle and associated currents are important as it ensures that power source remains safe and its windings are not getting damaged due to increase in temperature beyond specified limit. The maximum current which can be drawn from a power source depends upon its size of winding wire, type of insulation and cooling system of the power source. Hence with other parameters constant duty cycle is directly proportional to welding power or heat input per unit length
With other parameters constant higher the welding speed lesser the heat input per unit length. So it is inversely proportional
39. An expansion process as per law PV = constant is known as
a) parabolic expansion
b) hyperbolic expansion
c) isentropic expansion
d) free expansion
Answer
b) hyperbolic expansion
Explanation
In a P-V diagram, PV = constant represents an equation of hyperbola
Free expansion is an irreversible process in which a gas expands into an insulated evacuated chamber. It is also called Joule expansion. Real gases experience a temperature change during free expansion. For an ideal gas, the temperature doesn't change
Isentropic (or adiabatic) Compression/Expansion Processes. If compression or expansion of gas takes place with no flow of heat energy either into or out of the gas - the process is said to be isentropic or adiabatic. Temperature can change in this process.
Parabolic expansion occurs when there are any square terms in either P or V.
40. Speed of an aeroplane is measured by
a) pitot tube
b) hot wire anemometer
c) venturimeter
d) rotameter
Answer
a) pitot tube
Explanation [Fluid Mechanics]
Pitot tube measures the velocity flowing liquid or air by measuring the pressure difference between stagnation and static points in flow using Bernoulli's equation.
Hot Wire Anemometer - When an electrically heated wire is placed in a flowing gas stream, heat is transferred from the wire to the gas and hence the temperature of the wire reduces, and due to this, the resistance of the wire also changes. This change in resistance of the wire becomes a measure of flow rate.
Venturimeter - A venturimeter is a device used for measuring the rate of flow of a fluid flowing through a pipe.
Two cross section, first at the inlet and the second one is present at the throat. The difference in the pressure heads of these two sections is used to calculate the rate of flow through venturimeter. As the water enters at the inlet section i.e. in the converging part it converges and reaches to the throat.
The throat has the uniform cross section area and least cross section area in the venturimeter. As the water enters in the throat its velocity gets increases and due to increase in the velocity the pressure drops to the minimum.
Now there is a pressure difference of the fluid at the two sections. At the section 1(i.e. at the inlet) the pressure of the fluid is maximum and the velocity is minimum. And at the section 2 (at the throat) the velocity of the fluid is maximum and the pressure is minimum.
The pressure difference at the two section can be seen in the manometer attached at both the section.
This pressure difference is used to calculate the rate flow of a fluid flowing through a pipe.
Rotameter - A rotameter is a device that measures the volumetric flow rate of fluid in a closed tube.
A rotameter consists of a tapered tube, typically made of glass with a 'float' (a shaped weight, made either of anodized aluminum or a ceramic), inside that is pushed up by the drag force of the flow and pulled down by gravity. The drag force for a given fluid and float cross section is a function of flow speed squared only, see drag equation.
A higher volumetric flow rate through a given area increases flow speed and drag force, so the float will be pushed upwards. However, as the inside of the rotameter is cone shaped (widens), the area around the float through which the medium flows increases, the flow speed and drag force decrease until there is mechanical equilibrium with the float's weight.
Floats are made in many different shapes, with spheres and ellipsoids being the most common. The float may be diagonally grooved and partially colored so that it rotates axially as the fluid passes. This shows if the float is stuck since it will only rotate if it is free. Readings are usually taken at the top of the widest part of the float; the center for an ellipsoid, or the top for a cylinder. Some manufacturers use a different standard.
The "float" must not float in the fluid: it has to have a higher density than the fluid, otherwise it will float to the top even if there is no flow.
The mechanical nature of the measuring principle provides a flow measurement device that does not require any electrical power. If the tube is made of metal, the float position is transferred to an external indicator via a magnetic coupling. This capability has considerably expanded the range of applications for the variable area flowmeter, since the measurement can observed remotely from the process or used for automatic control.
41. Euler's dimensionless number relates the following
a) inertial force and gravity
b) viscous force and inertial force
c) pressure force and inertial force
d) viscous force and pressure force
Answer
c) pressure force and inertial force
Explanation [Fluid Mechanics]
inertial and gravity - Froude's number - The Froude's number is a measurement of bulk flow characteristics such as waves, sand bed-forms, flow/depth interactions at a cross section or between boulders.
viscous force and inertial force - Reynolds number - The Reynolds Number can be used to determine if flow is laminar, transient or turbulent.
pressure force and inertial force - Euler number - It expresses the relationship between a local pressure drop caused by a restriction and the kinetic energy per volume of the flow, and is used to characterize energy losses in the flow, where a perfect friction-less flow corresponds to an Euler number of 0.
viscous force and pressure force - No such number
42. A simple compressible substance inside a cylinder undergoes a change of state quasistatically, isothermally and at a constant pressure of 1.5MPa. If the enthalpy change for the system is 4000 kJ at 150oC, what is the corresponding change in its entropy?
a) 9.45 kJ/K
b) 26.67 kJ/K
c) 14.17 kJ/K
d) 0
Answer
a) 9.45 kJ/K
Explanation [Thermal Engineering]
dH = TdS + Vdp
At constant pressure, dH = T dS
Here dH = 4000 kJ, T = 150oC = 423 K
dS = dH/T = 4000/423 = 9.45 kJ/K
76. Which of the following is an amorphous material?
a) mica
b) lead
c) rubber
d) glass
Answer
c) rubber and d) glass
Explanation [Material Science]
In condensed matter physics and materials science, an amorphous (from the Greek a, without, morphé, shape, form) or non-crystalline solid is a solid that lacks the long-range order that is characteristic of a crystal.
Mica - The mica group of sheet silicate (phyllosilicate) minerals includes several closely related materials having nearly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition. The nearly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms. So it is not amorphous
Rubber - It is amorphous. But under stretched conditions it is partly amorphous and partly crystalline
Glass - Glass is a non-crystalline amorphous solid
77. In a party, each person shook hands with every other person present. The total number of hand shakes was 45. The number of people in the party are
a) 12
b) 10
c) 8
d) 6
Answer
b) 10
Explanation [Engineering Mathematics]
First person can shake the hand with n-1 persons.
The second person can shake hand with n-2 persons, because the second person has already shaken hands with the first person.
Proceeding this way the last person has no one to shake hands with. That persons handshake has been accounted previously.
So if there are n persons there are (n-1)+(n-2)+(n-3)+.....+1 hand shakes = (n-1)n/2 = nC2 = Sum of first n-1 integers.
Here (n-1)n/2 = 45
(n-1)n = 90 = 9 x 10
So n = 10
7. The impact strength of a material is an index of its
a) fatigue strength
b) tensile strength
c) hardness
d) toughness
Answer
d) toughness
Explanation [Strength of Materials]
Yield strength is the lowest stress that produces a permanent deformation in a material. In some materials, like aluminium alloys, the point of yielding is difficult to identify, thus it is usually defined as the stress required to cause 0.2% plastic strain.
Compressive strength is a limit state of compressive stress that leads to failure in a material in the manner of ductile failure (infinite theoretical yield) or brittle failure (rupture as the result of crack propagation, or sliding along a weak plane — see shear strength).
Tensile strength or ultimate tensile strength is a limit state of tensile stress that leads to tensile failure in the manner of ductile failure (yield as the first stage of that failure, some hardening in the second stage and breakage after a possible "neck" formation) or brittle failure (sudden breaking in two or more pieces at a low stress state).
Fatigue strength is a measure of the strength of a material or a component under cyclic loading. Fatigue strength is quoted as stress amplitude or stress range , usually at zero mean stress, along with the number of cycles to failure under that condition of stress.
Impact strength is the capability of the material to withstand a suddenly applied load and is expressed in terms of energy. In order for a material or object to have a high impact strength the stresses must be distributed evenly throughout the object. It also must have a large volume with a low modulus of elasticity and a high material yield strength. In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing. One definition of material toughness is the amount of energy per unit volume that a material can absorb before rupturing. It is also defined as a material's resistance to fracture when stressed. The toughness of a material can be measured using a small specimen of that material. A typical testing machine uses a pendulum to strike a notched specimen of defined cross-section and deform it. The height from which the pendulum fell, minus the height to which it rose after deforming the specimen, multiplied by the weight of the pendulum is a measure of the energy absorbed by the specimen as it was deformed during the impact with the pendulum.
8. Which of the following order of crystal structure will match with metals Iron-Nickel-Titanium in that order at room temperature
a) BCC - HCP - FCC
b) FCC - BCC - HCP
c) HCP - FCC - BCC
d) BCC - FCC - HCP
8. Which of the following order of crystal structure will match with metals Iron-Nickel-Titanium in that order at room temperature
a) BCC - HCP - FCC
b) FCC - BCC - HCP
c) HCP - FCC - BCC
d) BCC - FCC - HCP
Answer
d) BCC - FCC - HCP
Explanation [Material Science]
BCC - Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Barium (Ba), Vanadium (V), Chromium (Cr), Iron (Fe).
The bcc arrangement does not allow the atoms to pack together as closely as the fcc or hcp arrangements. Metals which have a bcc structure are usually harder and less malleable than close-packed metals such as gold. When the metal is deformed, the planes of atoms must slip over each other, and this is more difficult in the bcc structure.
FCC - Aluminium (Al), Copper(Cu), Gold (Au), Iridium (Ir), Lead(Pb), Nickel (Ni), Platinum (Pt), Silver (Ag).
HCP - Beryllium(Be), Cadmium (Cd), Magnesium (Mg), Titanium (Ti), Zinc (Zn), Zirconium (Zr)
9. In a machining operation cutting speed is reduced by 50%. Assuming n = 0.5, C = 300 in Taylor's equation. The increase in tool life is
a) 2
b) 4
c) 8
d) 16
Answer
b) 4
Explanation [Manufacturing Technology]
When both V and T are plotted in log-scale, linear relationship appears With the slope, n and intercept, c, the equation is VTn = C
where, n is called, Taylor’s tool life exponent. The values of both ‘n’ and ‘c’ depend mainly upon the tool-work materials and the cutting environment (cutting fluid application).
V2/V1 = 50%, T2/T1 = ?
V2/V1 . (T2/T1)n = 1
0.5 . (T2/T1)0.5 = 1
(T2/T1)0.5 = 2
T2/T1 = 4
10. In a linear arc welding process, the heat input per unit length is inversely proportional to
a) welding current
b) welding speed
c) welding voltage
d) duty cycle of power source
Answer
b) welding speed
Explanation [Manufacturing Technology]
Welding power gives the heat input per unit length. Welding current and welding voltage is directly proportional to welding power and hence directly proportional to the heat input per unit length.
Duty cycle is the ratio of arcing time to the weld cycle time multiplied by 100. Welding cycle time is either 5 minutes as per European standards or 10 minutes as per American standard and accordingly power sources are designed. If arcing time is continuously 5 minutes then as per European standard it is 100% duty cycle and 50% as per American standard. At 100% duty cycle minimum current is to be drawn i.e. with the reduction of duty cycle current drawn can be of higher level. Duty cycle and associated currents are important as it ensures that power source remains safe and its windings are not getting damaged due to increase in temperature beyond specified limit. The maximum current which can be drawn from a power source depends upon its size of winding wire, type of insulation and cooling system of the power source. Hence with other parameters constant duty cycle is directly proportional to welding power or heat input per unit length
With other parameters constant higher the welding speed lesser the heat input per unit length. So it is inversely proportional
39. An expansion process as per law PV = constant is known as
a) parabolic expansion
b) hyperbolic expansion
c) isentropic expansion
d) free expansion
Answer
b) hyperbolic expansion
Explanation
In a P-V diagram, PV = constant represents an equation of hyperbola
Free expansion is an irreversible process in which a gas expands into an insulated evacuated chamber. It is also called Joule expansion. Real gases experience a temperature change during free expansion. For an ideal gas, the temperature doesn't change
Isentropic (or adiabatic) Compression/Expansion Processes. If compression or expansion of gas takes place with no flow of heat energy either into or out of the gas - the process is said to be isentropic or adiabatic. Temperature can change in this process.
Parabolic expansion occurs when there are any square terms in either P or V.
40. Speed of an aeroplane is measured by
a) pitot tube
b) hot wire anemometer
c) venturimeter
d) rotameter
Answer
a) pitot tube
Explanation [Fluid Mechanics]
Pitot tube measures the velocity flowing liquid or air by measuring the pressure difference between stagnation and static points in flow using Bernoulli's equation.
Hot Wire Anemometer - When an electrically heated wire is placed in a flowing gas stream, heat is transferred from the wire to the gas and hence the temperature of the wire reduces, and due to this, the resistance of the wire also changes. This change in resistance of the wire becomes a measure of flow rate.
Venturimeter - A venturimeter is a device used for measuring the rate of flow of a fluid flowing through a pipe.
Two cross section, first at the inlet and the second one is present at the throat. The difference in the pressure heads of these two sections is used to calculate the rate of flow through venturimeter. As the water enters at the inlet section i.e. in the converging part it converges and reaches to the throat.
The throat has the uniform cross section area and least cross section area in the venturimeter. As the water enters in the throat its velocity gets increases and due to increase in the velocity the pressure drops to the minimum.
Now there is a pressure difference of the fluid at the two sections. At the section 1(i.e. at the inlet) the pressure of the fluid is maximum and the velocity is minimum. And at the section 2 (at the throat) the velocity of the fluid is maximum and the pressure is minimum.
The pressure difference at the two section can be seen in the manometer attached at both the section.
This pressure difference is used to calculate the rate flow of a fluid flowing through a pipe.
Rotameter - A rotameter is a device that measures the volumetric flow rate of fluid in a closed tube.
A rotameter consists of a tapered tube, typically made of glass with a 'float' (a shaped weight, made either of anodized aluminum or a ceramic), inside that is pushed up by the drag force of the flow and pulled down by gravity. The drag force for a given fluid and float cross section is a function of flow speed squared only, see drag equation.
A higher volumetric flow rate through a given area increases flow speed and drag force, so the float will be pushed upwards. However, as the inside of the rotameter is cone shaped (widens), the area around the float through which the medium flows increases, the flow speed and drag force decrease until there is mechanical equilibrium with the float's weight.
Floats are made in many different shapes, with spheres and ellipsoids being the most common. The float may be diagonally grooved and partially colored so that it rotates axially as the fluid passes. This shows if the float is stuck since it will only rotate if it is free. Readings are usually taken at the top of the widest part of the float; the center for an ellipsoid, or the top for a cylinder. Some manufacturers use a different standard.
The "float" must not float in the fluid: it has to have a higher density than the fluid, otherwise it will float to the top even if there is no flow.
The mechanical nature of the measuring principle provides a flow measurement device that does not require any electrical power. If the tube is made of metal, the float position is transferred to an external indicator via a magnetic coupling. This capability has considerably expanded the range of applications for the variable area flowmeter, since the measurement can observed remotely from the process or used for automatic control.
41. Euler's dimensionless number relates the following
a) inertial force and gravity
b) viscous force and inertial force
c) pressure force and inertial force
d) viscous force and pressure force
Answer
c) pressure force and inertial force
Explanation [Fluid Mechanics]
inertial and gravity - Froude's number - The Froude's number is a measurement of bulk flow characteristics such as waves, sand bed-forms, flow/depth interactions at a cross section or between boulders.
viscous force and inertial force - Reynolds number - The Reynolds Number can be used to determine if flow is laminar, transient or turbulent.
pressure force and inertial force - Euler number - It expresses the relationship between a local pressure drop caused by a restriction and the kinetic energy per volume of the flow, and is used to characterize energy losses in the flow, where a perfect friction-less flow corresponds to an Euler number of 0.
viscous force and pressure force - No such number
42. A simple compressible substance inside a cylinder undergoes a change of state quasistatically, isothermally and at a constant pressure of 1.5MPa. If the enthalpy change for the system is 4000 kJ at 150oC, what is the corresponding change in its entropy?
a) 9.45 kJ/K
b) 26.67 kJ/K
c) 14.17 kJ/K
d) 0
Answer
a) 9.45 kJ/K
Explanation [Thermal Engineering]
dH = TdS + Vdp
At constant pressure, dH = T dS
Here dH = 4000 kJ, T = 150oC = 423 K
dS = dH/T = 4000/423 = 9.45 kJ/K
76. Which of the following is an amorphous material?
a) mica
b) lead
c) rubber
d) glass
Answer
c) rubber and d) glass
Explanation [Material Science]
In condensed matter physics and materials science, an amorphous (from the Greek a, without, morphé, shape, form) or non-crystalline solid is a solid that lacks the long-range order that is characteristic of a crystal.
Mica - The mica group of sheet silicate (phyllosilicate) minerals includes several closely related materials having nearly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition. The nearly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms. So it is not amorphous
Rubber - It is amorphous. But under stretched conditions it is partly amorphous and partly crystalline
Glass - Glass is a non-crystalline amorphous solid
77. In a party, each person shook hands with every other person present. The total number of hand shakes was 45. The number of people in the party are
a) 12
b) 10
c) 8
d) 6
Answer
b) 10
Explanation [Engineering Mathematics]
First person can shake the hand with n-1 persons.
The second person can shake hand with n-2 persons, because the second person has already shaken hands with the first person.
Proceeding this way the last person has no one to shake hands with. That persons handshake has been accounted previously.
So if there are n persons there are (n-1)+(n-2)+(n-3)+.....+1 hand shakes = (n-1)n/2 = nC2 = Sum of first n-1 integers.
Here (n-1)n/2 = 45
(n-1)n = 90 = 9 x 10
So n = 10