Q40. Which form of energy causes the blades of a windmill to rotate?
[RRB Group D, 16 Dec 2025]
(A) Light energy
(B) Kinetic energy of wind
(C) Elastic energy
(D) Sound energy
Answer: (B) Kinetic energy of wind
Explanation: Moving air possesses kinetic energy. As it passes over the windmill blades, aerodynamic forces exert torque and turn the rotor. This converts part of the air’s kinetic energy into rotational mechanical energy, which may subsequently drive machinery or an electrical generator.
Q39. A heavy box is difficult both to set moving and to stop once moving. Which property explains these observations?
[SSC CGL, 14 Sep 2025 (Shift 1)]
(A) Power
(B) Conservation of energy
(C) Inertia
(D) Action–reaction
Answer: (C) Inertia
Explanation: An object resists changes in its velocity because of inertia. A heavy box has substantial mass and therefore greater translational inertia. It takes force to accelerate the box from rest and another force to decelerate it after it is moving.
Q38. Which statement is NOT a correct consequence of Newton’s second law?
[UPSC NDA-II, 14 Sep 2025]
(A) Force is proportional to acceleration for constant mass
(B) Acceleration points in the direction of net force
(C) Net force equals the rate of change of momentum
(D) Force is proportional to momentum itself
Answer: (D) Force is proportional to momentum itself
Explanation: Newton’s second law concerns the rate at which momentum changes, not the amount of momentum already possessed. A body can have large momentum while experiencing zero resultant force during uniform motion. With constant mass, the law reduces to F = ma.
Q37. Why does a cricket fielder move the hands backward while catching a fast-moving ball?
[UPSC NDA-II, 14 Sep 2025]
(A) To reduce the magnitude of deceleration
(B) To increase the deceleration
(C) To increase the ball’s final velocity
(D) To make the impulse vanish
Answer: (A) To reduce the magnitude of deceleration
Explanation: The ball’s momentum falls to zero when caught, giving a fixed momentum change. Drawing the hands backward increases the stopping interval, reducing the average contact force and deceleration. This makes the catch safer while still bringing the ball to rest.
Q36. For a rigid body rotating about a fixed axis, how do the angular velocities of its different particles compare?
[SSC CGL, 12 Sep 2025 (Shift 1)]
(A) They increase with distance from the axis
(B) They are equal for all particles
(C) They decrease with distance from the axis
(D) They all remain zero
Answer: (B) They are equal for all particles
Explanation: In rigid rotation, every point sweeps out the same angle in the same time. Consequently, angular velocity is identical throughout the rigid body. Linear speed is different: v = ωr, so particles farther from the rotation axis move faster along their circular paths.
Q35. Two objects of different masses are dropped simultaneously in free fall, neglecting air resistance. Which quantity is equal for both?
[UPSC NDA-I, 13 Apr 2025]
(A) Momentum
(B) Acceleration
(C) Gravitational potential energy
(D) Kinetic energy
Answer: (B) Acceleration
Explanation: In ideal free fall near Earth’s surface, every object experiences gravitational acceleration g, regardless of mass. Momentum and kinetic energy depend on mass, as does potential energy at a specified height. Therefore equal acceleration is the common quantity for the falling bodies.
Q34. A motorcycle moves at 5 m/s initially and at 7 m/s after three seconds. Assuming uniform acceleration, calculate its displacement.
[UPSC CDS-I, 13 Apr 2025]
(A) 21 m
(B) 18 m
(C) 36 m
(D) 6 m
Answer: (B) 18 m
Explanation: For constant acceleration, average velocity equals the mean of initial and final velocities: (5 + 7)/2 = 6 m/s. Multiplying this by three seconds yields 18 metres. The displacement is positive because the motorcycle continues in the same direction.
Q33. If the length of a simple pendulum is made four times its original length and the bob’s mass is doubled, what is the new period compared with the original period?
[UPSC NDA-I, 13 Apr 2025]
(A) Three times
(B) Two-fifths
(C) Twice
(D) Three-halves
Answer: (C) Twice
Explanation: The small-oscillation period of a simple pendulum is T = 2π√(L/g). It does not depend on the bob’s mass. Quadrupling length multiplies the square root, and hence the period, by two. Doubling mass produces no additional change.
Q32. Which one of these quantities is NOT a unit of energy?
[SSC CGL, 17 Sep 2024]
(A) Joule
(B) Kilowatt-hour
(C) Calorie
(D) Newton
Answer: (D) Newton
Explanation: Joule, kilowatt-hour and calorie all measure energy or work, although they differ in scale. A newton is the SI unit of force, equivalent to kg m/s². Work requires force multiplied by displacement, so one joule equals one newton-metre.
Q31. Which of the following effects is NOT generally produced by applying a force to an object?
[SSC CHSL, 10 Jul 2024]
(A) Making a stationary object move
(B) Changing its direction of motion
(C) Changing its shape
(D) Increasing its mass
Answer: (D) Increasing its mass
Explanation: An applied force may start or stop motion, change a body’s velocity direction, or deform its shape. In ordinary classical mechanics, force does not change the body’s mass. Mass is a property of the matter rather than a direct effect of force.
