CBSE Class 11 Physics Work Energy and Power MCQs Set C

Question. A rubber ball is dropped from a height of 5m on a plane, where the acceleration due to gravity is not shown. On bouncing it rises to 1.8 m. The ball loses its velocity on bouncing by a factor of
(a) 16/25
(b) 2/5
(c) 3/5
(d) 9/25
Answer: b

Question. A body of mass (4m) is lying in x-y plane at rest. It suddenly explodes into three pieces. Two pieces, each of mass (m) move perpendicular to each other with equal speeds (v). The total kinetic energy generated due to explosion is :
(a) mv²
(b) (3/2) mv²
(c) 2 mv²
(d) 4 mv²
Answer: b

Question. A weight suspended from the free end of a vertically hanging spring produces an extension of 3 cm. The spring is cut into two parts so that the length of the longer part is 2/3 of the original length, If the same weight is now suspended from the longer part of the spring, the extension produced will be
(a) 0.1 cm
(b) 0.5 cm
(c) 1 cm
(d) 2 cm
Answer: d

Question. A particle of mass m moving eastward with a speed v collides with another particle of the same mass moving northwards with the same speed. If two particles coalesce on collision, the new particle of mass 2 m will move in the north-east direction with a velocity
(a) v / 2
(b) v 2
(c) v/ 2
(d) None of these
Answer: c

Question. A particle is moving in a region, where its potential energy U varies with co-ordinates x, y and z as U = a/2 (x2 + y2 + z2) , where a is a positive constant. The force acting on the particle is
(a) − a(x𝑖̂ + y𝑗̂ + z𝑘̂)
(b) − 2a(x𝑖̂ + y𝑗̂ + z𝑘̂)
(c) − a/2 ( x2 + y2 + z2
(d) (x𝑖̂ + y𝑗̂ + z𝑘̂)
Answer: a

Question. The potential energy U of a diatomic molecule varies with interatomic separation r as U = a/r12 − b/r6 , where a and b are positive constants. Find the interatomic separation for which atoms are in equilibrium
(a) Zero 
(b) (b/2a)1/6
(c) (2a/b)1/6
(d) (a/b)1/6
Answer: c

Question. An electron and a proton are moving under the influence of mutual forces. In calculating the change in the kinetic energy of the system during motion, one ignores the magnetic force of one on another. This is, because
(a) the two magnetic forces are equal and opposite, so they produce no net effect
(b) the magnetic forces do not work on each particle
(c) the magnetic forces do equal and opposite (but nonzero) work on each particle
(d) the magnetic forces are necessarily negligible
Answer: b

Question. The coefficient of friction between the tyres and the road is μ. A car is moving with momentum p. What will be the stopping distance due to friction alone ? The mass of the car is m.
(a) p²/2mg
(b) p²/2mmg
(c) p²/2m²mg
(d) p²/2mg
Answer: c

Question. A train of weight 10⁷ N is running on a level track with uniform speed of 36 km h^–1. The frictional force is 0.5 kg per quintal. If g = 10 m/s², then power of engine is 
(a) 500 kW
(b) 50 kW
(c) 5 kW
(d) 0.5 kW
Answer: a

Question. A ball of mass 500 gm moving with speed 2 m/s collide to a floor making an angle of 45° with vertical.
If coefficient of restitution is 1/√3 , then the angle made by the ball with normal, after collision is
(a) 60°
(b) 30°
(c) 45°
(d) 37°
Answer: b

Question. n small balls each of mass m impinge elastically each second on a surface with velocity v. The force experienced by the surface will be
(a) 1/2mnv
(b) 2 mnv
(c) mnv
(d) 2 mnv
Answer: c

Question. A ball moving with velocity 2 m/s collides head on with another stationary ball of double the mass. If the coefficient of restitution is 0.5, then their velocities (in m/s) after collision will be
(a) 0, 1
(b) 1, 1
(c) 1, 0.5
(d) 0, 2
Answer: a

Question. The potential energy of particle in a force field is U = A/r² – B/r, where A and B are positive constants and r is the distance of particle from the centre of the field. For stable equilibrium, the distance of the particle is
(a) B / 2A
(b) 2A / B
(c) A / B
(d) B / A
Answer: b

Question. A particle of mass 10 kg moving eastwards with a speed 5 ms^–1 collides with another particle of the same mass moving north-wards with the same speed 5 ms^–1. The two particles coalesce on collision. The new particle of mass 20 kg will move in the north-east direction with velocity
(a) 10 ms^–1
(b) 5 ms^–1
(c) (5 / √2)ms^-1
(d) none of these
Answer: c

Question. A 10 m long iron chain of linear mass density 0.8 kg m–1 is hanging freely from a rigid support. If g = 10 ms^–2, then the power required to left the chain upto the point of support in 10 second
(a) 10 W
(b) 20W
(c) 30 W
(d) 40 W
Answer: d

Question. If the kinetic energy of a body is increased by 300%, the momentum of the body is increased by
(a) 300%
(b) 200%
(c) 100%
(d) 50%
Answer: c

Question. A particle of mass 10 g moves along a circle of radius 6.4 cm with a constant tangential acceleration. What is the magnitude of this acceleration if the kinetic energy of the particle becomes equal to 8 × 10^–4 J by the end of the second revolution after the beginning of the motion ?
(a) 0.1 m/s²
(b) 0.15 m/s²
(c) 0.18 m/s²
(d) 0.2 m/s²
Answer: a

Question. Consider a drop of rain water having mass 1 g falling from a height of 1 km. It hits the ground with a speed of 50 m/s. Take ‘g’ constant with a value 10 m/s². The work done by the (i) gravitational force and the (ii) resistive force of air is
(a) (i) 1.25 J (ii) –8.25 J
(b) (i) 100 J (ii) 8.75 J
(c) (i) 10 J (ii) – 8.75 J
(d) (i) – 10 J (ii) –8.25 J
Answer: c

Question. A bomb at rest explodes into two fragments of masses 3 kg and 1 kg. The ratio of magnitude of velocities of the smaller and bigger fragment is
(a) 3
(b) 1
(c) 1/2
(d) 1/3
Answer: a

 Question. A bicyclist comes to a skidding stop in 10 m. During this process, the force on the bicycle due to the road is 200N and is directly opposed to the motion. The work done by the cycle on the road is
(a) + 2000 J
(b) – 200 J
(c) zero
(d) – 20,000 J
Answer: c

Question. A cricket ball of mass 150 g moving with a speed of 126 km/ h hits at the middle of the bat, held firmly at its position by the batsman. The ball moves straight back to the bowler after hitting the bat. Assuming that collision between ball and bat is completely elastic and the two remain in contact for 0.001s, the force that the batsman had to apply to hold the bat firmly at its place would be
(a) 10.5 N
(b) 21 N
(c) 1.05 × 10⁴ N
(d) 2.1 × 10⁴ N
Answer: c

Question. A ball is thrown vertically downwards from a height of 20 m with an initial velocity v0. It collides with the ground loses 50 percent of its energy in collision and rebounds to the same height. The initial velocity v0 is :
(Take g = 10 ms^–2)
(a) 20 ms^–1
(b) 28 ms^–1
(c) 10 ms^–1
(d) 14 ms^–1
Answer: a

Question. A knife having mass m is dropped from a height h above the surface of earth. If the knife penetrates upto distance x in the soil. The average force applied by the soil is
(a) mg(h/x + 1)
(b) mgh /x
(c) mg(h/x − 1)
(d) mgx/h
Answer: a

Question. A stationary body explodes into two pieces of masse 6 kg and 4 kg, which move in opposite directions. The ratio of their kinetic energies E₁/E₂ is
(a) 3/2
(b) 13/2
(c) 2/3
(d) 2/13
Answer: c

Question. A body is falling freely under the action of gravity alone in vaccum. Which of the following quantities remain constant during the fall?
(a) Kinetic energy
(b) Potential energy
(c) Total mechanical energy
(d) Total linear momentum
Answer: c

Question. A body falls freely under gravity. Its velocity is v when it has lost potential energy equal to U. What is the mass of the body ?
(a) U²/v²
(b) 2U²/v²
(c) 2U/v²
(d) U /v²
Answer: c

Question. A body of mass m moving with a constant velocity v hits another body of the same mass moving with the same speed v in the opposite direction and sticks to it. The speed of the compound body after collision is
(a) v√2
(b) 2v
(c) v/2
(d) Zero
Answer: d

Question. In a shotput event an athlete throws the shotput of mass 10 kg with an initial speed of 1 m s^–1 at 45° from a height 1.5 m above ground. Assuming air resistance to be negligible and acceleration due to gravity to be 10 m s^–2, the kinetic energy of the shotput when it just reaches the ground will be
(a) 2.5 J
(b) 5.0 J
(c) 52.5 J
(d) 155.0 J
Answer: d

Question. The kinetic energy of particle moving along a circle of radius R depends upon the distance covered S and is given by K = aS where a is a constant. Then the force acting on the particle is
(a) aS
(b) 2(aS)²/R
(c) aS²/R²
(d) 2aS/2
Answer: d

Question. If the mass of the body is halved and velocity gets doubled then final kinetic energy would be ………of initial.
(a) same
(b) four times
(c) double
(d) eight times
Answer: c

Question. A nucleus ruptures into two nuclear parts which have their velocity ratio equal to 2 :1. The ratio of their respective nuclear sizes (nuclear radii )is
(a) 1 : 2
(b) 1: 2
(c) 1 : 21/3
(d) 1 : 8
Answer: c

Question. A one-ton car moves with a constant velocity of 15 ms^–1 on a rough horizontal road. The total resistance to the motion of the car is 12% of the weight of the car. The power required to keep the car moving with the same constant velocity of 15ms^–1 is [Take g = 10 ms^–2]
(a) 9 kW
(b) 18 kW
(c) 24 kW
(d) 36 kW
Answer: b

Question. A person holding a rifle (mass of person and rifle together is 100 kg) stands on a smooth surface and fires 10 shots horizontally, in 5 s. Each bullet has a mass of 10 g with a muzzle velocity of 800 ms^–1. The final velocity acquired by the person and the average force exerted on the person are
(a) –1.6 ms^–1; 8 N
(b) –0.08 ms^–1; 16 N
(c) – 0.8 ms^–1; 8 N
(d) –1.6 ms^–1; 16 N
Answer: c

Question. A metal ball of mass 2 kg moving with a velocity of 36 km/h has a head on collision with a stationary ball of mass 3 kg. If after the collision, the two balls move together, the loss in kinetic energy due to collision is
(a) 140 J
(b) 100 J
(c) 60 J
(d) 40 J
Answer: c

Question. A ball is allowed to fall from a height of 10 m. If there is 40% loss of energy due to impact, then after one impact ball will go up to
(a) 10 m
(b) 8 m
(c) 4 m
(d) 6 m
Answer: d

Question. A particle moves in a straight line with retardation proportional to its displacement. Its loss of kinetic energy for any displacement x is proportional to
(a) x
(b) ex
(c) x²
(d) log_e x
Answer: c

Question. A block of mass m is lowered vertically by a distance d with constant acceleration g/4 by means of a light inextensible string. Work done on the block by gravity is
(a) mgd/4
(b) 3mgd/4
(c) −3mgd/4
(d) mgd
Answer: d

Question. Two similar springs P and Q have spring constants K_P and K_Q, such that K_P > K_Q. They are stretched, first by the same amount (case a,) then by the same force (case b). The work done by the springs W_P and W_Q are related as, in case (a) and case (b), respectively
(a) W_P = W_Q ; W_P = W_Q
(b) W_P > W_Q ; W_Q > W_P
(c) W_P < W_Q ; W_Q < W_P
(d) W_P = W_Q ; W_P > W_Q
Answer: b

Question. A stationary particle explodes into two particles of masses m₁ and m₂ which move in opposite directions with velocities v₁ and v₂. The ratio of their kinetic energies E₁/E₂ is
(a) m₁v₂/m₂v₁
(b) m₂/m₁
(c) m₁/m₂
(d) 1
Answer: b

Question. A body of mass m is lifted up from the earth surface to a height R above the earth’s surface without changing its kinetic energy. The work done in this process is equal to
(a) Zero by the resultant force acting on the body
(b) mgR/2 by the applied force acting on the body
(c) –mgR/2 by the gravitational force acting on the body
(d) All of these
Answer: d