Q60. On a level circular cycling track of radius 10 m, riders P, Q and R travel at 2, 3 and 5 m/s respectively. If tyre-road friction is 0.1 and g = 10 m/s², which rider slips?
[UPSC NDA-II, 13 Sep 2026]
(A) P only
(B) Q only
(C) R only
(D) All three riders
Answer: (C) R only
Explanation: Friction provides the centripetal force on a level road. The maximum safe speed is √(μrg) = √10, approximately 3.16 m/s. P and Q remain below this limit, while R requires greater friction than the road can supply and slips.
Q59. A tennis ball of mass 350 g is projected vertically upward at 4 m/s. What are its maximum gravitational potential energy M and maximum height H? Take g = 10 m/s².
[UPSC NDA-II, 13 Sep 2026]
(A) M = 5.6 J; H = 1.6 m
(B) M = 2.8 J; H = 1.6 m
(C) M = 5.6 J; H = 0.8 m
(D) M = 2.8 J; H = 0.8 m
Answer: (D) M = 2.8 J; H = 0.8 m
Explanation: At the top, the initial kinetic energy becomes gravitational potential energy. Thus M = ½ × 0.35 × 4² = 2.8 joules. Equating mgh to 2.8 gives H = 4²/(2 × 10) = 0.8 metre, neglecting air resistance.
Q58. Consider three statements: I. Weight is constant everywhere on Earth while inertial mass may vary. II. Inertial mass and gravitational mass are proportional with the same constant everywhere. III. Inertial mass and weight are the same quantity. Which is correct?
[UPSC NDA-II, 13 Sep 2026]
(A) I only
(B) II only
(C) I and II
(D) II and III
Answer: (B) II only
Explanation: Mass is an intrinsic property, while weight equals mg and changes with local gravitational acceleration. Inertial and gravitational masses are proportional, as established by the equivalence principle. Therefore I and III are false; only statement II gives the correct physical relationship.
Q57. Rain falls vertically with speed v. One observer travels east at speed v and another north at speed v. What angle separates the apparent directions of the rain observed by them?
[UPSC NDA-II, 13 Sep 2026]
(A) 0°
(B) 30°
(C) 45°
(D) 60°
Answer: (D) 60°
Explanation: Each observer subtracts their own horizontal velocity from the vertical rain velocity. The resulting relative velocity vectors have equal magnitudes √2v, while their dot product equals v². Therefore the cosine of the angle is one-half, giving an angle of 60°.
Q56. What minimum horizontal speed must a pendulum bob have at its lowest point to complete a vertical circle when its string is 50 cm long and g = 10 m/s²?
[UPSC NDA-II, 13 Sep 2026]
(A) 5 m/s
(B) 7.5 m/s
(C) 10 m/s
(D) 12.5 m/s
Answer: (A) 5 m/s
Explanation: At the highest point, minimum tension is zero, requiring top speed √(gL). From the bottom to the top, the bob rises by 2L. Energy conservation gives bottom speed √(5gL), equal to √25 or 5 m/s.
Q55. In carrom, a 15 g striker hits a stationary 5 g coin. The coin acquires 0.36 m/s in 3 milliseconds. What is the average force on the coin?
[UPSC NDA-II, 13 Sep 2026]
(A) 0.72 N
(B) 0.60 N
(C) 0.54 N
(D) 0.30 N
Answer: (B) 0.60 N
Explanation: Average force equals the coin’s change in momentum divided by contact time. Its momentum gain is 0.005 × 0.36 = 0.0018 kg m/s. Dividing by 0.003 second yields 0.60 N. The striker’s own mass is unnecessary.
Q54. Two astronauts float at rest in deep space. Astronaut X throws a tool towards astronaut Y. For the system consisting of both astronauts and the tool, what happens to its centre of mass?
[UPSC NDA-II, 13 Sep 2026]
(A) It moves towards Y
(B) It follows astronaut X
(C) It follows the tool
(D) It remains fixed
Answer: (D) It remains fixed
Explanation: No external force acts on the combined system. The tool gains momentum towards Y, while astronaut X recoils with equal opposite momentum. The total momentum, initially zero, remains zero. Hence the velocity and position of the system’s centre of mass remain unchanged.
Q53. A uniform rope hangs over a smooth pulley, and a small extra mass makes one side heavier. Which statement is correct after the rope begins moving?
[UPSC NDA-II, 13 Sep 2026]
(A) Every rope element has the same acceleration vector
(B) All elements have the same speed but not the same acceleration vector
(C) Tension is identical throughout the rope
(D) The rope’s centre of mass stays stationary
Answer: (B) All elements have the same speed but not the same acceleration vector
Explanation: Because the rope is inextensible, its elements have equal speed magnitudes. Their acceleration directions differ on opposite hanging sides, and the curved portion has centripetal acceleration. Since the rope has mass, tension may vary; the system’s centre of mass also moves.
Q52. A car increases its velocity from 10 m/s to 30 m/s in 5 seconds. What is its acceleration?
[RRB Group D, 5 Aug 2026]
(A) 2 m/s²
(B) 10 m/s²
(C) 6 m/s²
(D) 4 m/s²
Answer: (D) 4 m/s²
Explanation: Acceleration is the rate at which velocity changes. Here the change is 30 − 10 = 20 m/s, spread uniformly over five seconds. Dividing 20 by 5 gives 4 m/s². The positive sign indicates that the car is speeding up.
Q51. Find the linear momentum of a body of mass 10 kg moving at 50 m/s.
[RRB NTPC Undergraduate, 18 Jun 2026]
(A) 40 kg m/s
(B) 60 kg m/s
(C) 5 kg m/s
(D) 500 kg m/s
Answer: (D) 500 kg m/s
Explanation: Linear momentum is the product of mass and velocity. Using p = mv, a ten-kilogram body moving at fifty metres per second has momentum of 10 × 50 = 500 kg m/s. Momentum is directional, so it points along the body’s velocity.
Q50. A planet takes eight Earth years to orbit the Sun once. Compared with Earth’s orbit, what is the semimajor axis of this planet’s orbit?
[UPSC NDA-I, 12 Apr 2026]
(A) The same
(B) Twice as large
(C) Three times as large
(D) Four times as large
Answer: (D) Four times as large
Explanation: Kepler’s third law states that orbital period squared is proportional to semimajor axis cubed for planets orbiting the same star. With period ratio eight, the axis ratio is 8 to the power two-thirds, which equals four.
Q49. For a particle moving under forces, the total work done depends on its initial and final speeds. Which statement follows?
[UPSC NDA-I, 12 Apr 2026]
(A) The force must be conservative
(B) The net work equals the change in kinetic energy
(C) The force must remain constant
(D) Its velocity must always increase
Answer: (B) The net work equals the change in kinetic energy
Explanation: The work–energy theorem gives net work as one-half the mass multiplied by final speed squared minus initial speed squared. This applies even when forces vary or include friction. It does not imply that each individual force is conservative or constant.
Q48. Which condition ensures stable equilibrium for a floating body?
[UPSC NDA-I, 12 Apr 2026]
(A) Its centre of gravity lies above its centre of buoyancy
(B) Its centre of gravity lies below its centre of buoyancy
(C) Its centre of gravity coincides with its centre of buoyancy
(D) Its centre of gravity lies below its metacentre
Answer: (D) Its centre of gravity lies below its metacentre
Explanation: When a floating object tilts slightly, the buoyancy line shifts. If the metacentre is above the centre of gravity, the buoyant force and weight generate a restoring couple. This metacentric condition determines stability, not the buoyancy-centre position alone.
Q47. Why is a person landing on soft sand less likely to suffer injury than someone landing on a hard surface?
[UPSC NDA-I, 12 Apr 2026]
(A) Sand exerts no reaction force
(B) Sand reduces the change in momentum
(C) Sand lengthens the impact time
(D) Sand increases the person’s momentum
Answer: (C) Sand lengthens the impact time
Explanation: The person must lose nearly the same downward momentum whether landing on sand or concrete. Soft sand deforms and extends the stopping time. Since average impact force equals change in momentum divided by time, the longer interval produces a smaller force.
Q46. Which statement correctly expresses Newton’s second law of motion?
[UPSC CDS-I, 12 Apr 2026]
(A) Rate of change of angular momentum equals applied force
(B) Rate of change of displacement is proportional to applied force
(C) Rate of change of acceleration is proportional to applied force
(D) Rate of change of linear momentum equals the applied force
Answer: (D) Rate of change of linear momentum equals the applied force
Explanation: Newton’s second law relates the resultant external force to the time rate of change of linear momentum. For constant mass, differentiating momentum mv gives F = ma. Angular momentum instead changes under torque, so the first choice confuses two different physical relationships.
Q45. What is the property of a body that makes it resist changes to its state of rest or uniform motion in a straight line?
[RRB NTPC Graduate, 25 Mar 2026]
(A) Acceleration
(B) Momentum
(C) Force
(D) Inertia
Answer: (D) Inertia
Explanation: Inertia is the tendency of matter to preserve its existing motion unless a net external force acts. Newton’s first law describes this behaviour. Mass measures translational inertia: an object with greater mass generally needs a greater force for the same acceleration.
Q44. Which expression represents the SI unit of linear momentum?
[RRB NTPC Graduate, 22 Mar 2026]
(A) g m s⁻¹
(B) kg m s⁻²
(C) kg m s
(D) kg m s⁻¹
Answer: (D) kg m s⁻¹
Explanation: Momentum equals mass multiplied by velocity. In SI, mass uses kilograms and velocity uses metres per second, making kg m s⁻¹ the correct unit. The unit kg m s⁻² belongs to force, the newton, rather than momentum.
Q43. If the separation between two masses is doubled while the masses stay constant, what happens to their gravitational force?
[RRB Group D, 27 Dec 2025]
(A) It doubles
(B) It becomes four times larger
(C) It becomes half
(D) It becomes one-fourth
Answer: (D) It becomes one-fourth
Explanation: Newton’s universal gravitation law states that force varies inversely with the square of separation. Doubling the distance changes its square by a factor of four, so the mutual gravitational attraction becomes one-quarter of its previous value, assuming masses are unchanged.
Q42. An unbalanced net force acting on a moving object can directly change which quantity?
[RRB Group D, 26 Dec 2025]
(A) Density
(B) Mass
(C) Time
(D) Velocity
Answer: (D) Velocity
Explanation: A net external force produces acceleration under Newton’s second law. Acceleration represents a change in velocity, either its size, its direction, or both. A force does not automatically change an object’s mass or density, and time is not the motion variable affected.
Q41. When the masses of both attracting bodies are doubled but their separation is unchanged, what is the gravitational force?
[RRB Group D, 23 Dec 2025]
(A) Twice the original
(B) Four times the original
(C) Eight times the original
(D) Half the original
Answer: (B) Four times the original
Explanation: According to universal gravitation, force is proportional to the product of the two masses. Doubling each mass makes this product four times larger: (2m₁)(2m₂) = 4m₁m₂. Since the distance is unchanged, the attraction increases fourfold.
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.
Q30. A planet moves around the Sun in a circular orbit of radius r. What work does gravity perform as it moves through half the orbit?
[WBCS Prelims, 2024]
(A) Zero
(B) F × 2πr
(C) F × πr
(D) F × 2r
Answer: (A) Zero
Explanation: For circular motion, gravitational force points radially towards the Sun, while the instantaneous displacement is tangential to the orbit. Force and displacement remain perpendicular, so their dot product and the work done are zero throughout the half revolution.
Q29. If fluid density is ρ, velocity is v, lift coefficient is Cₗ and projected area is A, what is the lift force?
[MPPSC Assistant Engineer (Civil), 8 Oct 2023]
(A) ½ρv²Cₗ
(B) ½ρv²CₗA
(C) 2ρv²CₗA
(D) ρv²CₗA
Answer: (B) ½ρv²CₗA
Explanation: Aerodynamic lift is expressed through dynamic pressure, one-half density times speed squared, multiplied by projected area and the dimensionless lift coefficient. Thus L = ½ρv²CₗA. Omitting area would give incorrect physical dimensions for a force.
Q28. When a stone is thrown upward, what energy conversion takes place as it rises?
[SSC MTS, 12 Sep 2023 (Shift 3)]
(A) Kinetic energy changes into mechanical energy
(B) Potential energy changes into gravitational energy
(C) Mechanical energy changes into kinetic energy
(D) Kinetic energy changes into gravitational potential energy
Answer: (D) Kinetic energy changes into gravitational potential energy
Explanation: During ascent, gravity reduces the stone’s speed while its height increases. Its kinetic energy gradually converts to gravitational potential energy. At the highest point, its instantaneous speed is zero, so the initial kinetic energy has become potential energy, neglecting air resistance.
Q27. For straight-line motion with uniform acceleration, what does the signed area under a velocity–time graph represent?
[UPSC CDS-I, 16 Apr 2023]
(A) Average velocity
(B) Net displacement
(C) Distance travelled in every case
(D) Average speed
Answer: (B) Net displacement
Explanation: Velocity multiplied by a short time interval gives displacement in that interval. Adding these contributions produces the signed area under the graph, equal to net displacement. Negative portions subtract from positive ones, so total distance need not equal the signed area.
Q26. A car initially moves at 12 m/s and is braked with a constant deceleration of 6 m/s². How far does it travel before stopping?
[UPSC CDS-I, 16 Apr 2023]
(A) 12 m
(B) 24 m
(C) 36 m
(D) 48 m
Answer: (A) 12 m
Explanation: Use the kinematic relation v² = u² + 2as. At rest v = 0, while u = 12 m/s and a = −6 m/s². Thus 0 = 144 − 12s, so the stopping distance is 12 metres.
Q25. A particle travels at speed v around a circle of radius r. What is the magnitude of its acceleration?
[WBCS Prelims, 2022]
(A) 0
(B) v/r
(C) v/r²
(D) v²/r
Answer: (D) v²/r
Explanation: Even when speed remains constant, the velocity vector changes direction around a circle. This requires centripetal acceleration towards the centre with magnitude v²/r. A zero acceleration would correspond to constant velocity in a straight line, not circular motion.
Q24. Consider two claims: I. Rolling friction is greater than sliding friction. II. Friction exerted by fluids is called drag. Which statement is correct?
[SSC MTS, 22 Jul 2022 (Shift 1)]
(A) Only I
(B) Neither I nor II
(C) Both I and II
(D) Only II
Answer: (D) Only II
Explanation: Rolling resistance is typically lower than sliding friction, so the first statement is incorrect. Air and water oppose relative movement of immersed objects through fluid resistance, commonly called drag. Thus the second statement is correct, and the correct choice is II only.
Q23. A heavy object and a light object possess equal kinetic energy. Which has greater linear momentum?
[WBCS Prelims, 2021]
(A) The heavier object
(B) The lighter object
(C) Both have the same momentum
(D) Both necessarily have zero momentum
Answer: (A) The heavier object
Explanation: Kinetic energy and momentum obey K = p²/(2m), which gives p = √(2mK). If kinetic energy is fixed, momentum magnitude grows as the square root of mass. Therefore the heavier body carries a greater momentum magnitude.
Q22. A person standing on a playground swing rises from a sitting position to standing. What happens to the period of oscillation?
[WBCS Prelims, 2021]
(A) It increases
(B) It stays unchanged
(C) It decreases
(D) It becomes zero
Answer: (C) It decreases
Explanation: Standing raises the combined centre of mass closer to the suspension point. This effectively shortens the pendulum length. Since the period varies as the square root of length, the period decreases, provided oscillations remain small enough for the pendulum approximation.
Q21. A uniform spring with force constant k is cut into two equal lengths. What is the force constant of each half?
[WBCS Prelims, 2021]
(A) k
(B) k/2
(C) 2k
(D) 4k
Answer: (C) 2k
Explanation: A uniform spring’s stiffness is inversely proportional to its length because shorter springs stretch less under the same force. Halving the original length therefore doubles the spring constant. Each half has stiffness 2k, even though the original complete spring had stiffness k.
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.
Q10. Which liquid has the least viscosity among the following?
[RPSC Second Grade Science, 2015]
(A) Water
(B) Mercury
(C) Coal tar
(D) Glycerol
Answer: (A) Water
Explanation: Viscosity measures internal resistance to flow. At ordinary room temperature, water has lower dynamic viscosity than mercury, glycerol and coal tar. It therefore offers the least resistance to shearing motion among the listed liquids, making it the least viscous choice.
Q9. Sound waves travelling through air are of what type?
[WBCS Prelims, 2014]
(A) Transverse waves
(B) Longitudinal waves
(C) Electromagnetic waves
(D) Polarised waves
Answer: (B) Longitudinal waves
Explanation: Sound travels through air as successive compressions and rarefactions. Air particles vibrate back and forth parallel to the direction in which the disturbance propagates. This defines a longitudinal mechanical wave; unlike electromagnetic waves, sound requires a material medium for transmission.
Q8. Ball bearings are used in bicycles and automobiles chiefly because they do which of the following?
[UPSC Civil Services Prelims, 2013]
(A) Increase the actual contact area between wheel and axle
(B) Increase the effective contact area between wheel and axle
(C) Reduce the effective contact area between wheel and axle
(D) Have no effect on contact between moving parts
Answer: (C) Reduce the effective contact area between wheel and axle
Explanation: Ball bearings replace considerable sliding contact with rolling contact. Rolling resistance is usually much lower, and the effective contact area is small. This reduces frictional losses between rotating components such as a wheel and its axle, improving smoothness and efficiency.
Q7. For equal values of kinetic energy, which particle has the greatest magnitude of momentum?
[WBCS Prelims, 2013]
(A) Electron
(B) Proton
(C) Deuteron
(D) Alpha particle
Answer: (D) Alpha particle
Explanation: Using K = p²/(2m), momentum magnitude is p = √(2mK). At a fixed kinetic energy, momentum increases with mass. An alpha particle is heavier than an electron, proton or deuteron, so it has the greatest momentum under the stated condition.
Q6. Four device–energy conversion pairs are listed:
I. Generator: mechanical to electrical;
II. Motor: electrical to mechanical;
III. Diesel engine: light to electrical;
IV. Solar cell: chemical to mechanical.
Which pairs are correctly matched?
[RPSC RAS Prelims, 26 Oct 2013]
(A) I and II
(B) I and III
(C) II and IV
(D) III and IV
Answer: (A) I and II
Explanation: An electrical generator converts mechanical work into electrical energy, while an electric motor performs the reverse conversion. A diesel engine converts fuel chemical energy into useful mechanical energy, and a solar cell converts light into electricity. Only statements I and II are correct.
Q5. An object moves at constant velocity along a straight horizontal path. Which quantity does it lack?
[WBCS Prelims, 2012]
(A) Velocity
(B) Momentum
(C) Kinetic energy
(D) Acceleration
Answer: (D) Acceleration
Explanation: In straight-line motion at constant velocity, neither speed nor direction changes, so acceleration is zero. The body still possesses velocity and, provided it has mass, linear momentum and kinetic energy. A resultant force is unnecessary to maintain uniform motion on an ideal frictionless path.
Q4. Which one is a scalar physical quantity?
[WBCS Prelims, 2011]
(A) Velocity
(B) Force
(C) Momentum
(D) Energy
Answer: (D) Energy
Explanation: Energy has magnitude but no inherent spatial direction and is therefore a scalar. Velocity, force and momentum require both magnitude and direction to describe them fully. They obey vector addition, whereas energies are combined algebraically according to their signs and physical forms.
Q3. An empty truck and a loaded truck move at equal speeds. More force is needed to stop the loaded truck in the same manner. Which law best explains this?
[WBCS Prelims, 2007]
(A) Newton’s first law
(B) Newton’s second law
(C) Newton’s third law
(D) Law of gravitation
Answer: (B) Newton’s second law
Explanation: Newton’s second law gives F = ma for constant mass. If both vehicles must undergo the same deceleration, the loaded truck’s greater mass requires a larger braking force. The example illustrates how the force necessary for a given acceleration depends on mass.
Q2. During simple harmonic motion, how often does kinetic energy become zero?
[WBCS Prelims, 2004]
(A) It never becomes zero
(B) Twice in each complete period
(C) Only when passing through the mean position
(D) It stays constant throughout
Answer: (B) Twice in each complete period
Explanation: At the two extreme positions of simple harmonic motion, the oscillator momentarily stops before reversing direction. Its velocity and kinetic energy therefore become zero at both turning points during each full cycle. Kinetic energy is maximum at the equilibrium position.
Q1. Which physical quantity is measured by an ordinary beam balance?
[WBCS Prelims, 2002]
(A) Volume
(B) Mass
(C) Weight
(D) Density
Answer: (B) Mass
Explanation: A beam balance compares an unknown body with standard masses through their gravitational effects on opposite pans. Because both sides experience the same local gravitational acceleration, that factor cancels in equilibrium. The instrument thus measures mass rather than weight directly.