Category: Class 11 Physics Objective Questions

Physics Multiple Choice Questions on “Thermodynamics – Refrigerators and Heat Pumps”.

1. What is the coefficient of performance of a refrigerator? Let Q₁ be the heat released to hot reservoir, Q₂ be the heat extracted from a cold reservoir & W be the work done on the refrigerator.
a) Q₁/W
b) Q₁/Q₂
c) Q₂/W
d) Q₂/Q₁
Answer: c
Clarification: Coefficient of performance of a refrigerator is defined as the ratio of heat extracted from the cold reservoir to the work done on the refrigerator. It is denoted by α = Q₂/W.

2. Select the correct order of steps in which a working substance(gas) goes through in a refrigerator.
a) Expansion of gas, absorption of heat, heating of vapour, release of heat
b) Absorption of heat, expansion of gas, heating of vapour, release of heat
c) Heating of vapour, expansion of gas, absorption of heat, release of heat
d) Heating of vapour, absorption of heat, expansion of gas, release of heat
Answer: a
Clarification: In a refrigerator, the working substance(gas) first gets expanded from high to low pressure which cools it into a vapour-liquid mixture. Then heat is absorbed from the region to be cooled. Then the external work done on the system heats up the vapour. Finally, the vapour’s heat is released to the surroundings.

3. Efficiency of a refrigerator can be greater than 1. True or False?
a) True
b) False
Answer: b
Clarification: Efficiency by definition can never be greater than 1. Else the law of energy conservation will be violated.

4. The coefficient of a refrigerator can be greater than 1. True or False?
a) True
b) False
Answer: a
Clarification: The coefficient of a refrigerator is defined as α = Q₂/W,
where Q₂ is heat extracted from a cold reservoir and W is work done.
α = Q₂/( Q₁ – Q₂).
Therefore α can be less than or greater than one.

5. 20J of heat energy is extracted from a cold reservoir when 30J of work is done on a refrigerator. What is the coefficient of the refrigerator?
a) 2/3
b) 5/3
c) 3/5
d) 1/3
Answer: a
Clarification: The coefficient of a refrigerator is defined as α = Q₂/W,
where Q₂ is heat extracted from a cold reservoir and W is work done.
α = 20/30 = 2/3.

Physics Multiple Choice Questions on “Forced Oscillations and Resonance”.

1. What is the frequency with which forced periodic oscillations oscillate?
a) Frequency of forced oscillator
b) Their natural frequency
c) No specific frequency
d) Sum of frequency of forced oscillator & their natural frequency
Answer: a
Clarification: In the case of forced oscillations, the oscillations made by the body do not depend on their natural frequency, instead they oscillate with the frequency of the forced oscillator.

2. Resonance occurs when frequency of forced oscillations is close to natural frequency. True or False?
a) True
b) False
Answer: a
Clarification: Resonance is the phenomenon of oscillation with increased amplitude when frequency of forced oscillator is close to natural frequency. The reason for this increase in amplitude is that the vibrations of the external oscillator match with the vibrations of particles being oscillated and thus there is a net increase in vibrations.

3. Higher the value of damping constant, higher will be the amplitude at resonance. True or False?
a) True
b) False
Answer: b
Clarification: Higher the value of damping lesser will be the effect of forced oscillations in increasing the amplitude close to natural frequency. Therefore, higher the value of damping constant, lesser will be the amplitude at resonance.

Physics Multiple Choice Questions on “Motion in a Straight Line – Relative Velocity”.

1. A body is moving with respect to a stationary frame, its motion can be called _____
a) Absolute
b) Relative
c) Circular
d) Parabolic
Answer: a
Clarification: The motion with respect to a stationary frame is called absolute motion. Relative motion is with respect to a moving frame of reference.

2. A small block is placed over another block which is moving with a velocity of 5m/s. What is the absolute velocity of the small block?
a) 5m/s
b) 10m/s
c) 0m/s
d) 14m/s
Answer: a
Clarification: The absolute velocity of the small block is the same as that of the block moving with 5m/s. This is because absolute velocity is studied with respect to the ground frame. The relative velocity of the small block with respect to the other block is zero.

3. What is the correct formula for relative velocity of a body A with respect to B?
a) Vector V_R = Vector V_A – Vector V_B
b) Vector V_R = Vector V_A + Vector V_B
c) Vector V_R = Vector V_A x Vector V_B
d) Vector V_R = Vector V_B – Vector V_A
Answer: a
Clarification: The relative velocity of one body with respect to another body can be found out by subtracting their respective velocities in respective order. This rule applies not only to velocity, to all other vector quantities.

4. The relative velocity of a body A with respect to a body B is 5 m/s. The absolute velocity of body B is 10 m/s. Both the bodies are moving in the same direction. What is the absolute velocity of body A?
a) 10m/s
b) 15m/s
c) -5m/s
d) 0m/s
Answer: b
Clarification: Here we will use the formula for relative velocity, Vector V_R = Vector V_A – Vector V_B. Since both the bodies are moving in the same direction, the velocity vectors are of the same sign. V_R = 5, V_B = 10, therefore, V_A = 15m/s.

5. If two bodies are moving in opposite directions with non-zero velocities, which of the following statements is true?
a) Relative velocity > Absolute velocity
b) Relative velocity < Absolute velocity
c) Relative velocity = Absolute velocity
d) Relative velocity <= Absolute velocity
Answer: a
Clarification: The formula for relative velocity is, Vector V_R = Vector V_A – Vector V_B. When both the velocities are opposite in direction, the equation becomes V_R = V_A – (-V_B). Hence the magnitudes add up making the relative velocity greater than the absolute velocity of any of the two bodies.

6. A car is moving with 20m/s velocity, another car is moving with a velocity of 50 m/s. What is the relative velocity of first car with respect to the second?
a) 30 m/s
b) -30 m/s
c) 20 m/s
d) 25 m/s
Answer: b
Clarification: The formula for relative velocity is V_R = Vector V_A – Vector V_B. Assuming the cars move in the same direction, the relative velocity = 20-50 = -30 m/s. The relative velocity of the second with respect to the first car is 30 m/s.

7. A truck is moving with 40 m/s velocity, a train is moving with a velocity of 80 m/s. How fast is the rain moving with respect to the truck?
a) 40 m/s faster
b) -40 m/s faster
c) 40 m/s slower
d) 60 m/s slower
Answer: a
Clarification: Here we need to find the relative velocity of the train with respect to the truck is V_R = V_A-V_B, V_B = 40, V_A = 80. On solving we get, V_R = 40 m/s. Hence the train will move faster by 40 m/s.

8. A point A is placed at a distance of 7 m from the origin, another point B is placed at a distance of 10 m from the origin. What is the relative position of B with respect to A?
a) 3 m from A
b) 4 m from A
c) -3 m from A
d) 5 m from A
Answer: a
Clarification: Relative displacement of B with respect to A = Displacement of B -Displacement of A. On solving we get, relative displacement = 3 m. Hence B is placed at a distance of 3 m from A.

9. An observer is sitting on a car moving with some constant velocity. The observer sees things around him, in the ____
a) Relative frame of reference
b) Absolute frame of reference
c) Valid frame of reference
d) Ground frame of reference
Answer: a
Clarification: The observer is not stationary with respect to ground. The observer is stationary with respect to the frame of the moving car, i.e., to the relative frame of reference. The observer will see everything around him/her with respect to the relative frame of reference.

10. A body A is moving in North direction, while another body B is moving towards South. Velocity of A is greater than that of B. If North is taken as positive, which of the following relative velocities is positive?
a) Velocity of A with respect to B
b) Velocity of B with respect to A
c) Velocity of A with respect to ground
d) Velocity of B with respect to ground
Answer: a
Clarification: The formula for relative velocity is, Vector V_R = Vector V_A – Vector V_B. The velocity of A is greater in magnitude hence the relative velocity of A will be positive. The direction of the relative velocity will be Northwards.

11. What does relative motion signify?
a) The motion of a body with respect to other body
b) Uniformly accelerated
c) Non-uniformly accelerated
d) Motion along a curve
Answer: b
Clarification: The three equations of motion are valid for uniformly accelerated motion. The equations do not work in situations where the acceleration is non-uniform. In that case it is better to work with the differential forms of velocity and acceleration.

12. What method is used to find relative value for any vector quantity?
a) Vector sum
b) Vector difference
c) Vector multiplication
d) Vector division
Answer: b
Clarification: To find the relative value of any quantity, vector difference is used. The relative value is defined as the subtraction of the remaining vector from the vector whose relative value is to be calculated with respect to the remaining vector.

Physics Multiple Choice Questions on “Non-Uniform Circular Motion”.

1. A stone tie with a string held at one end is being rotated at a constant angular velocity in the air. The tangential velocity will remain constant if _______
a) The plane of motion of the stone is parallel to the ground
b) The plane of motion of the stone is perpendicular to the ground
c) The plane of motion of the stone is at an angle of 45 degrees to the ground
d) The tangential velocity is independent of the plane of motion
Answer: a
Clarification: When the plane motion of the stone is parallel to the ground, the tangential velocity experiences acceleration in two directions, one towards the centre and other towards the ground. Both these directions are perpendicular to the direction of the tangential velocity. Hence the tangential velocity will not change.

2. A ball tied at the end of a perfect string tied tightly (assume fixed) to a wooden bar at the other end is rotating with constant angular velocity. Its tangential velocity will _______
a) Increase with time
b) Decrease with time
c) Will remain constant
d) Will decrease exponentially
Answer: b
Clarification: As the other end of the string is fixed to the wooden bar, the motion will cause the string to wrap itself around the bar. This will result in decrease in the effective radius of the circular motion. Tangential velocity v = ωr, the tangential velocity will decrease with time.

3. A rocket takes off from the earth and continues to move in a circular orbit with the thrusters on. What can be said about the angular velocity of the rocket?
a) It increases
b) It decreases
c) It remains constant
d) It changes abruptly
Answer: a
Clarification: When the rocket is in the orbit with the thrusters on, there is a tangential force that the rocket experiences. This force will result in increasing the tangential velocity. Since the angular velocity is directly proportional to the tangential velocity, the angular velocity will also increase.

4. The radius of a body moving in a circle with constant angular velocity is given by r = 4t², with respect to time. What is the magnitude of the tangential velocity at t = 2s, if the angular velocity is 7 rad/s?
a) 112
b) 113
c) 56
d) 28
Answer: a
Clarification: The tangential velocity is given as v = ωr. At t = 2s, r = 16 units. The angular velocity is 7 rad/s. Hence, the tangential velocity, v = 7 x 16 = 112 units/s.

5. A car moving around a tree has the distance from the tree defined as r = 5t² + 7. What is the magnitude of the centripetal acceleration at t = 2s, if the angular velocity is 2 rad/s.
a) 102
b) 108
c) 59
d) 54
Answer: b
Clarification: The centripetal acceleration is given as a_c = rω². Here, we have r = 5t² + 7 = 27 units at t = 2s. Angular velocity is 2 rad/s. On substituting all the values, we get, the acceleration as 108 units/s².

6. The tangential velocity in a circular motion change as v = 2t² – 7 with the radius being equal to 3 m. What is the angular velocity at t = 1 s in rad/s?
a) -5/3
b) -2/3
c) 3
d) -3/5
Answer: a
Clarification: The angular velocity is given as, ω = v/r. Hence, the function for angular velocity is ω = (2t² – 7)/3. On substituting, the suitable values, we will get the angular velocity as -5/3 rad/s.

7. The tangential velocity of a body in a non-uniform circular motion varies as v = 7t² – 2v with the radius being equal to 21 m. What is the angular acceleration at t = 2 s in rad/s²?
a) 4/3
b) 5/3
c) 4
d) 7/5
Answer: a
Clarification: The angular velocity is given as, ω = v/r. Hence, the function for angular velocity is ω = (7t² – 2)/21. On differentiating this expression, we will get the angular acceleration as t²/3 On substituting, the suitable values, we will get the angular acceleration as 4/3 rad/s².

8. Which of the following is a suitable word to describe the motion of a rotating ball tied to rope tied tightly to a fixed support?
a) Spiral motion
b) Circular motion
c) Elliptic motion
d) Uniform motion
Answer: a
Clarification: A ball, tied to a rope, rotating around a fixed support will cause the rope to wind itself around the support. Hence, the motion will be circular with decreasing radius. This motion will cause the path look like a spiral. Hence the motion is spiral in nature.

9. A block of mass 5 Kg, exhibits circular motion with the mass decreasing at the rate of 0.5 Kg/s. At what time the centripetal force will be zero on the block?
a) At 10 s
b) At 5 s
c) At 1 s
d) At 7 s
Answer: a
Clarification: The centripetal force on a body is directly proportional to its mass with a proportionality constant of 1. Hence the centripetal force will be zero when the mass is zero. The function for the mass at t seconds is m = 5 – 0.5t. On equating this to zero we get, t = 10 s. this is the required time.

10. A box of mass 10 Kg, moves in circular motion with the mass function as m = 7t. The function for the radius is r = 2t². By what factor does the centripetal force exceed the square of the tangential velocity at t = 2 s?
a) 7/4
b) 7/5
c) 7/2
d) 7
Answer: a
Clarification: The centripetal force on a body is given as F_c = mv²/r. Hence, the required factor is the ratio of the mass to the radius at t = 2s. Hence, required factor = m/r = 7/2t. On substituting the values, we get the factor as 7/4.

5. A radioactive particle at rest, having mass 10g, breaks into 2 fragments (1&2) in the mass ratio 2:3 respectively. If the first one moves with a velocity of 10m/s what will be the speed of the second particle?
a) 4.33 m/s
b) -4.33 m/s
c) -6.67 m/s
d) 6.67 m/s
Answer: c
Clarification: let the mass of part 1 be 2x, & the mass of part 2 be 3x since the masses are in the ratio 2:3. 2x + 3x = 10g.
∴ x = 2g. Therefore, masses of part 1 & 2 are 4g & 6g respectively.
The total momentum will be conserved because there is no external force. Let the velocity of 2nd part be ‘v’.
∴ 0 = 4*10 + 6*v ∴ v = -40/6 = -6.67 m/s.

6. There are two external forces acting on a system of particles. Select the correct statement.
a) Linear momentum is necessarily not conserved
b) Linear momentum may be conserved
c) Linear momentum is zero
d) Centre of mass will move with increasing speed
Answer: b
Clarification: The two forces acting on the body may or may not have a net vector sum of zero, depending on which linear momentum may or may not be conserved. Thus, the option ‘linear momentum may be conserved’ is correct.

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