Multiple choice question for engineering
Set 1
1. A boy carrying a box on his head is walking on a level load from one place to another on a straight road is doing no work. The statement is
a) Correct
b) Incorrect
c) Partly correct
d) Insufficient data
Answer
Answer: a [Reason:] The body applies force on the load in the upward direction. The displacement of the load is in the horizontal direction.
2. Kinetic energy with any reference must be
a) Zero
b) Positive
c) Negative
d) Either negative or positive
Answer
Answer: b [Reason:] KE = 1/2 mv2
It is always positive.
3. The kinetic energy of a body becomes four times of its initial value, then new momentum will
a) Become twice its initial value
b) Become thrice its initial value
c) Become four times its initial value
d) Remains constant
Answer
Answer: a [Reason:] p = √2mK
p‘ = √(2m×4K) = 2√2mK = 2p
If the kinetic energy of a body becomes four times of its initial value, then new momentum will become twice its initial value.
4. The decrease in potential energy of a ball of mass 20kg, which falls from a height of 50cm, is
a) 968J
b) 98J
c) 1980J
d) 450J
Answer
Answer: b [Reason:] U = mgh
U = 20×9.8×0.50 = 98J
5. If the water falls from a dam into a turbine wheel 19.6m below, then the velocity of water at the turbine is (g=9.8m/s)
a) 9.8m/s
b) 19.6m/s
c) 39.2m/s
d) 98m/s
Answer
Answer: b [Reason:] Loss in potential energy = Gain in kinetic energy
mgh = 1/2mv2
v = √2gh = √(2×9.8×19.6)=19.6m/s
6. Momentum is conserved in all collisions but not kinetic energy. True or false?
a) True
b) False
Answer
Answer: a [Reason:] Momentum is conserved in all collisions but not kinetic energy. Kinetic energy is conserved only in elastic collision.
7. Two particles are seen to collide and move jointly together after the collision. During such a collision, for the total system,
a) Neither the mechanical energy nor the linear momentum is conserved
b) Mechanical energy is conserved, but not the linear momentum
c) Both the mechanical energy and the linear momentum are conserved
d) Linear momentum is conserved, but not the mechanical energy
Answer
Answer: d [Reason:] The collision is inelastic. Only momentum is conserved and not mechanical energy. There is loss of kinetic energy in such a collision.
8. In elastic collision, 100% energy transfer takes place when
a) m1=2m2
b) m1 is greater than m2
c) m1 is lesser than m2
d) m1=m2
Answer
Answer: d [Reason:] During elastic collision between two equal masses, the velocities get exchanged. Hence energy transfer is maximum when m1=m2
9. Assertion: In and elastic collision of two billiard balls, the total kinetic energy is conserved during the short time of collision of balls [Reason:] Energy spent against friction does not follow the law of conservation of energy
a) Both assertion and reason are true and reason is the correct explanation of the assertion
b) Both assertion and reason are true but reason is not the correct explanation of the assertion
c) Both assertion and reason are false
d) Assertion is true but the reason is false
Answer
Answer: c [Reason:] The billiard balls in an elastic collision are in deformed state. Their total energy is partly kinetic and partly potential. So kinetic energy is less than the total energy spent against friction is dissipated as heat which is not available for doing work.
10. When a body moves with a constant speed along a circle
a) No work is done on it
b) No acceleration is produced in it
c) Its velocity remains constant
d) No force acts on it
Answer
Answer: a [Reason:] For a body in uniform circular motion, the centripetal force acts perpendicular to the circular path.
Set 2
1. Air in a cylinder is suddenly compressed by a piston which is then maintained at the same position. After sometime, the
a) Pressure will increase
b) Pressure remains the same
c) Pressure will decrease
d) Pressure may increase or decrease
Answer
Answer: c [Reason:] Sudden compression increases inside temperature. After sometime heat flows out. This decreases temperature of air, As V is constant, so pressure inside decreases.
2. Twp cylinders of equal size are filled with equal amount of ideal diatomic gas at room temperature. Both the cylinders are fitted with pistons. In cylinder A the piston is free to move, while in cylinder B the piston is fixed. When same amount of heat is added to cylinder A raises by 20 K. What will be the rise temperature of gas in cylinder B?
a) 28K
b) 20K
c) 15K
d) 10K
Answer
Answer: a [Reason:] For gas in cylinder A, Q = nCp ∆T1
For gas in cylinder B, Q = nCv ∆T2
∆T2=Cp/Cv ∆T1=7/5×20=28K
3. A gas does work during adiabatic expansion. The source of mechanical energy so produced is the internal energy of the gas itself. True or false?
a) True
b) False
Answer
Answer: a [Reason:] By the first law of thermodynamics,
∆Q=∆U+∆W.
But for an isothermal process, ∆Q is zero, so ∆W=-∆U. Thus the energy required for doing mechanical work during adiabatic expansion is the internal energy of the gas itself.
4. Out of solid, liquid and gas of the same mass and at the same temperature, which one has the greatest internal energy?
a) Solid
b) Liquid
c) Gas
d) All three have the same internal energy
Answer
Answer: c [Reason:] Gas has the greatest internal energy because the potential energy (Which is negative) of the molecules is very small.
5. Out of solid, liquid and gas of the same mass and at the same temperature, which one has the least internal energy?
a) Solid
b) Liquid
c) Gas
d) All three have the same internal energy
Answer
Answer: a [Reason:] The one which has a small potential energy has the greatest internal energy. On the other hand, the one which has large potential energy has the least internal energy; therefore the internal energy of the solid is least.
6. If Cp and Cv are the specific heats for a gas at constant pressure and at constant volume respectively, then the relation Cp-Cv=R is exact for
a) Ideal and real gases at all pressures
b) Ideal gas at all pressures and real gas at moderate pressure
c) Ideal gas and nearly true for real gases at high pressure
d) Ideal gas and nearly true for real gases at moderate pressure
Answer
Answer: d [Reason:] Cp-Cv=R
Holds for ideal gas and for real gases at moderate pressure.
7. In an adiabatic process, the quantity which remains constant is
a) Volume
b) Pressure
c) Temperature
d) Total heat of the system
Answer
Answer: d [Reason:] In an adiabatic process, no heat is exchanged between system and surroundings. Hence total heat of the system remains constant.
8. Assertion: In an isolated system, the entropy increases [Reason:] The processes in an isolated system are adiabatic
a) Both assertion and reason are true and reason is correct explanation of the assertion
b) Both assertion and reason are true but reason is not correct explanation of the assertion
c) Assertion is true but reason is false
d) Both assertion and reason are false
Answer
Answer: b [Reason:] Both the assertion and reason are true but the reason is not the correct explanation of the assertion. The entropy of an isolated system increases in accordance with the second law of thermodynamics.
9. The efficiency of a Carnot engine operating with reservoir temperatures kept at 100 °C and -23°C will be
a) (100+23)/100
b) (100-23)/100
c) (373+250)/373
d) (373-250)/373
Answer
Answer: d [Reason:] Ƞ=(T1-T2)/T1
Ƞ=1-300/400=1/4=25%
Hence efficiency of 26% with real engine is impossible.
10. A Carnot engine, whose sink is at 300 K, has an efficiency of 40%. By how much should the temperature of source be increased, so as to increase the efficiency by 50% of original efficiency?
a) 380 K
b) 275 K
c) 325 K
d) 250 K
Answer
Answer: d [Reason:] Ƞ=1-T2/T1
0.4=1- 300/T1
T1=300/0.6=500 K
Increase in temperature of the source= (T1)‘-T1=750-500=250K
Set 3
1. Force is a polar vector. True or false?
a) True
b) False
Answer
Answer: a [Reason:] Force is a polar vector because it has a point of application. It is a push or a pull that tends to change the state of rest or of uniform motion of a body.
2. A body is moving along a straight path. What will happen to the body in the absence of external field?
a) It will stop
b) It will move with the same speed in a different path
c) It will move with the same speed along the same straight path
d) It will move with a reduced speed along the same path
Answer
Answer: c [Reason:] Galileo stated that on a horizontal plane an object should move with a constant velocity in a straight line path. Therefore a body moving along a straight path will continue to move in the same direction with the same speed even in the absence of external forces.
3. A person is standing in a bus. When the bus starts moving forward suddenly
a) The person moves forward
b) The person remains stationary
c) The person is unaffected
d) The person moves backward
Answer
Answer: d [Reason:] A person standing in a bus moves backward when the bus moves forward. When the bus moves forward, the lower part of his body begins to move along with the bus while the upper part of his body continues to remain at rest due to inertia. That is why, a person falls backward when the bus starts.
4. When a moving bus suddenly stops, a person sitting
a) Stands up
b) Falls forward
c) Falls backward
d) Is unaffected
Answer
Answer: b [Reason:] As the bus stops, the lower part of the person’s body comes to rest along with the bus while the upper part of his body continues to remain in motion due to inertia and therefore the person falls forward.
5. If a body has more mass, it had more inertia. True or false?
a) True
b) False
Answer
Answer: a [Reason:] Mass of the body is the measure of its inertia. If a body has more mass, it is more difficult to change its state of rest or of uniform motion.
6. Which of the following is also known as law of inertia?
a) Newton’s second law of motion
b) Newton’s third law of motion
c) Aristotelian law of motion
d) Newton’s first law of motion
Answer
Answer: d [Reason:] According to Newton’s first law of motion, every body continues in its state of rest or uniform motion unless an external force acts upon it. This inability of a body to change its state of rest or of uniform motion along a straight path is called inertia of a body. Hence first law of motion is also called as law of inertia.
7. When we shake the branch of a tree, its fruits and dry leaves fall down. This is an example for
a) Inertia of motion
b) Inertia of rest
c) Inertia of direction
d) Newton’s third law of motion
Answer
Answer: b [Reason:] On shaking, the branch comes into motion while the dry fruits and leaves tend to remain at rest, inertia at rest, and so get separated. Thus the falling of leaves and fruits when the branch is shaken is an example of inertia of rest.
8. A ball thrown upward in a moving train does not come back into the thrower’s hand. True or false?
a) True
b) False
Answer
Answer: b [Reason:] The ball acquires the horizontal velocity of the train and maintains it during its upwards and downward motion. In this period the ball covers the same horizontal distance as the train, so it comes back into the thrower’s hands.
9. A dog is chasing a hare and the hare runs in zigzag path. What happens?
a) The dig becomes confused
b) The dog catches the hare easily
c) It becomes difficult for the dog to catch the hare
d) The hare dies
Answer
Answer: c [Reason:] When the hare runs in zigzag path, it becomes difficult for the dog to catch the hare. This is because the dog has more mass and hence has more inertia of direction than that of hare.
10. The driver of a car suddenly sees a broad wall in front of him. He should
a) Brake sharply
b) Turn sharply
c) Keep going
d) Jump out of the car
Answer
Answer: a [Reason:] Suppose FBis the force required in applying brakes to stop the truck in distance d, then
FB × d = 1/2mv2
Suppose FT is the force required in taking a turn of radius d then,
FT = 2FB
Clearly, it is better to apply brakes than to take circular turn.
Set 4
1. Internal force is responsible for change in velocity of an object. True or false?
a) True
b) False
Answer
Answer: b [Reason:] The outside force exerted on an object by any external agency is called an external force. Such a force changes the velocity of an object.
2. Internal force holds together the particles as a single object. True or false?
a) True
b) False
Answer
Answer: a [Reason:] The mutual forces exerted by the particles of a system on one another are called internal forces. These forces are responsible for holding together the particles as single object.
3. A hammer is resting on a plane surface. If force P is applied on the hammer along a line not passing through its centre of mass, them the hammer
a) Moves along a straight line path
b) Stays still
c) Is thrown away
d) Rotates about its centre of mass
Answer
Answer: d [Reason:] If single force acts on a body and the line of action of force passes through the centre of mass, the body will have only linear acceleration. When the force is applies along a line not passing through its centre of mass, then the hammer rotates about is centre of mass.
4. The point where whole weight may be assumed to act is called
a) Centre of mass
b) Centre of weight
c) Centre of gravity
d) Centre of acceleration
Answer
Answer: c [Reason:] The centre of mass of body is point where whole mass of the body may be assumed to be concentrated for describing its translator motion. On the other hand, the centre of gravity is a point at which the resultant of gravitational forces on all particles of the body acts.
5. The Sun’s attraction of both earth and moon are external forces acting on the centre of mass of the earth-moon system. True or false?
a) True
b) False
Answer
Answer: a [Reason:] The moon moves around the earth in a circular orbit and the earth moves around the sun in an elliptical orbit. Here the mutual force of gravitation between the earth and the moon are internal forces while the Sun’s attraction of both earth and moon are the external forces acting on the centre of mass of the earth-moon system.
6. Which of the following retain their relative positions even when they move under the action of an external force?
a) Rotation body
b) Stationary body
c) Rigid body
d) A body in equilibrium
Answer
Answer: c [Reason:] A body is said to be rigid if it does not undergo any change in its size and shape, however large the external force may be acting on it. A rigid body is one whose constituent particles retain their relative positions even when they move under the action of an external force.
7. On the application of a constant torque, a wheel is turned from rest through 400 radians in 10s. Find the angular acceleration.
a) 6 rad/s2
b) 4 rad/s2
c) 2 rad/s2
d) 8 rad/s2
Answer
Answer: d [Reason:] θ=400rad
ω_0=0 , t = 10s, α = ?
θ=ω0 t+1/2 αt2
400 = 0+1/2 α102
α = 8 rad/s2
8. A ball of mass 2kg and another of mass 4kg are dropped together from a 60feet tall building. After a fall of 30 feet each towards earth, their respective kinetic energies will be in the ration of
a) √2:1
b) 1:4
c) 1:2
d) 1:√2
Answer
Answer: c [Reason:] Both balls attain same velocity v after falling through 30 feet height.
E1/E2 =(1/2 m1 (v1)2)/(1/2 m2 (v2)2 )= (2v2)/(4v2 )=1/2=1:2
9. How much water a pump of 2kW can raise in one minute to a height of 10m? (g=10m/s2)
a) 1000litres
b) 1200litres
c) 10litres
d) 2000litres
Answer
Answer: b [Reason:] P = W/t=mgh/t
m = Pt/gh=(2000×60)/(10×10)=1200kg
10. A body of mass 2kg is placed on a rough horizontal plane. The coefficient of friction between body and the plane is 0.2. Then,
a) Body will move in forward direction if F=3N
b) Body will move in backward direction with acceleration of 0.5m/s2, if force F=3N
c) If F=3N, then body will be in rest condition
d) The body will move both in forward during a particular period and then remain at rest
Answer
Answer: d [Reason:] Limiting friction, f = μR = μmg = 0.2×2×9.8=3.92N
When F>3.92N the body will move in forward direction. When F=3N, the body will move remain at rest.
Set 5
1. When we steer a bicycle round a bend with both our hands on the handle bars, we apply a
a) Force
b) Torque
c) Couple
d) Energy
Answer
Answer: c [Reason:] A pair of equal and opposite forces acting on a body along two different lines of action constitutes a couple. A Couple has a turning effect, but no resultant force acts on a body. When we steer a bicycle round a bend with both our hands on the handle bars, we apple a couple
2. Couple cannot produce a translational motion. True or false?
a) True
b) False
Answer
Answer: a [Reason:] A pair of equal and opposite forces acting on a body along two different lines of action constitutes a couple. A couple has a turning effect, but no resultant force acts on a body. Thus it cannot produce translational motion.
3. The areal velocity of a planet is constant. This is stated by
a) Newton’s third law of motion
b) Kepler’s second law of planetary motion
c) Fourier’s law
d) Kirchhoff’s planetary law
Answer
Answer: b [Reason:] A planet evolves around the sun under the influence of gravitational force which acts towards the sun. This means that the areal velocity of a planet is constant. The is Kepler’s second law of planetary motion which states that the line joining the planet to the sun sweeps out equal areas in equal intervals of time.
4. The internal torque on the system due to internal force is
a) Increasing
b) Decreasing
c) Constant
d) Zero
Answer
Answer: d [Reason:] According to Newton’s third law, the internal torque on the system due to internal forces is zero because the forces between any two particles are equal and opposite and directed along the line joining the two particles. Hence the total torque is due to external forces only.
5. Which of the following can execute both translational and rotational motion under the influence of external field?
a) Rigid body
b) Ideal body
c) Rotating body
d) A body which is stationary
Answer
Answer: a [Reason:] A rigid body is one for which the distance between different particles do not change, even though they move. Under the influence of an external force, a rigid body can execute rotational and translational motion.
6. For a body in equilibrium, the linear acceleration of its centre of mass would be zero and also the angular acceleration of the rigid body about any axis would be zero. True or false?
a) True
b) False
Answer
Answer: a [Reason:] A rigid body is in equilibrium if both the linear and angular momentum of the rigid body remains constant with time. Hence for a body on equilibrium, the linear acceleration of its centre of mass would be zero and also the angular acceleration of the rigid body about any axis would be zero.
7. A spring 40mm ling is stretched by the application of force. If 10N force is required to stretch the spring through 1mm, then the work done in stretching the spring through 40mm is
a) 23J
b) 68J
c) 84J
d) 8J
Answer
Answer: d [Reason:] Spring constant, k = F/x= 10N/1mm= 10N/(10-3 m)= 104 N/m
Work done in stretching the spring through 40m,
W = 1/2 kx2=1/2×104×(40×10-3 )2 = 8J
8. Two bodies w=of masses m and 4m are moving with equal kinetic energies. The ratio of their linear momenta is
a) 1:2
b) 1:1
c) 4:1
d) 1:4
Answer
Answer: a [Reason:] p = √2mE
For same kinetic energy, p1/p2 =√(m1/m2 )=√(m/4m)=1:2
9. Water falls from a height of 60m at the rate of 15kg.s to operate a turbine. The losses due to friction forces are 10% of energy. How much power is generated by the turbine? g=m/s2
a) 7kW
b) 10.2kW
c) 12.3kW
d) 8.1kW
Answer
Answer: d [Reason:] P = W/t=mgh/t=(15×10×60)/1= 9000W = 9kW
Loss=10%
Useful power=90% of 9kW= (90×9)/100=8.1kW
10. A shell explodes into four unequal parts. Which one of the following is conserved?
a) Potential energy
b) Kinetic energy
c) Momentum
d) Both potential and kinetic energy
Answer
Answer: c [Reason:] In the absence of external force, linear momentum is conserved. If a shell explodes into four equal parts, momentum is conserved.
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