Q20. The work performed by non-conservative forces on a particle is equal to which quantity?
[UPPSC Assistant Engineer (Mechanical), exam held 13 Dec 2020]
(A) Change in internal energy only
(B) Change in mechanical energy
(C) Change in potential energy only
(D) Change in kinetic energy only
Answer: (B) Change in mechanical energy
Explanation: Mechanical energy is the sum of kinetic and potential energies. Non-conservative work, such as frictional work, changes this total: Wnc = Δ(K + U). Individual changes in kinetic or potential energy alone cannot generally represent the whole non-conservative contribution.
Q19. Which of the following physical quantities is a vector?
[WBCS Prelims, 2020]
(A) Heat
(B) Angular momentum
(C) Time
(D) Work
Answer: (B) Angular momentum
Explanation: Angular momentum has both magnitude and direction, defined for a particle by the cross product of position vector and linear momentum. Heat, elapsed time and work are scalars, specified by magnitude without a spatial direction. Therefore only angular momentum is a vector.
Q18. What is Newton’s first law of motion commonly called?
[CGPSC Civil Services Prelims, 2019]
(A) Law of gravity
(B) Law of inertia
(C) Law of conservation of momentum
(D) Law of vertical motion
Answer: (B) Law of inertia
Explanation: Newton’s first law says a body stays at rest or continues in uniform straight-line motion unless acted upon by an unbalanced external force. It expresses inertia, the resistance of matter to changing velocity. That is why it is called the law of inertia.
Q17. An aeroplane moves horizontally at constant speed and releases a bomb. Ignoring air drag, how does the pilot see the bomb move?
[WBCS Prelims, 2019]
(A) Along a curve behind the plane
(B) Along a curve ahead of the plane
(C) Vertically downward
(D) At rest relative to the plane
Answer: (C) Vertically downward
Explanation: At release, the bomb already shares the aircraft’s horizontal velocity. Ignoring air resistance, both retain that same horizontal velocity, while gravity accelerates the bomb downward. Relative to the pilot, horizontal separation remains zero, so the bomb appears to fall vertically.
Q16. A stone tied to a string moves in a horizontal circular path. If the string suddenly breaks, in which direction does the stone initially travel?
[WBCS Prelims, 2018]
(A) Along the tangent at the release point
(B) Directly outward along the radius
(C) Towards the circle’s centre
(D) It immediately stops
Answer: (A) Along the tangent at the release point
Explanation: The string supplies the inward centripetal force needed to curve the trajectory. When it breaks, that force vanishes, but the stone retains its instantaneous velocity. This velocity is tangent to the circle, so the stone initially moves along the tangent.
Q15. The rate of change of velocity with respect to time is called what?
[RRB Group D, 15 Oct 2018 (Shift 2)]
(A) Momentum
(B) Displacement
(C) Speed
(D) Acceleration
Answer: (D) Acceleration
Explanation: Acceleration describes how quickly velocity changes. It may arise from a change in speed, direction, or both. Mathematically, average acceleration equals change in velocity divided by elapsed time. Momentum depends on mass, while displacement and speed are different measures of motion.
Q14. Which type of frictional force is self-adjusting up to a maximum limiting value?
[RPSC Second Grade Science, 1 Jul 2017]
(A) Static friction
(B) Limiting friction
(C) Dynamic friction
(D) Rolling friction
Answer: (A) Static friction
Explanation: Static friction matches the applied tangential force while a body remains at rest, rising from zero up to its limiting value. Once that maximum is exceeded, slipping starts. Kinetic friction then acts during motion and does not self-adjust in the same way.
Q13. Two unequal, oppositely directed forces act along non-collinear lines on a rigid body. What motion can they produce?
[WBCS Prelims, 2017]
(A) Rotation only
(B) Translation only
(C) Neither rotation nor translation
(D) Both rotation and translation
Answer: (D) Both rotation and translation
Explanation: Because the opposite forces are unequal, their vector sum is nonzero, causing translational acceleration. Since their lines of action are different, a net moment can also exist, causing angular acceleration. Thus the combination may produce simultaneous translation and rotation.
Q12. Which one is a non-conservative force?
[WBCS Prelims, 2016]
(A) Gravitational force
(B) Frictional force
(C) Electrostatic force
(D) Magnetostatic force
Answer: (B) Frictional force
Explanation: Work done by friction depends on the path length and converts mechanical energy into heat. It cannot generally be described by a position-only potential energy. Gravity and electrostatic forces are conservative in ordinary static situations, making friction the correct non-conservative choice.
Q11. A ball is moved from Earth’s equator to a pole. Which statement about it is true?
[WBCS Prelims, 2015]
(A) Its mass increases
(B) Both its mass and weight change
(C) Its weight increases
(D) Its weight decreases
Answer: (C) Its weight increases
Explanation: Mass does not depend on geographical location. Earth’s rotation reduces apparent gravity most strongly at the equator, and its equatorial bulge places equatorial locations farther from the centre. Therefore gravitational acceleration, and hence the ball’s weight, is generally greater at the poles.
