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.
