# Work Energy and Power Problems and Solutions

### Physics Work, Power, Energy and Collision Conceptual Questions:

Ques. The quantity that is not conserved in an inelastic collision is
(a) Momentum
(b) Kinetic energy
(c) Total energy
(d) All of these

Ans. (b)

Ques. If the water falls from a dam into a turbine wheel 19.6 m below, then the velocity of water at the turbine is (g = 9.8 m/s2)
(a) 9.8 m/s
(b) 19.6 m/s
(c) 39.2 m/s
(d) 98.0 m/s

Ans. (b)

Ques. A ball is dropped from height 10 m. Ball is embedded in sand 1 m and stops, then
(a) Only momentum remains conserved
(b) Only kinetic energy remains conserved
(c) Both momentum and K.E. are conserved
(d) Neither K.E. nor momentum is conserved

Ans. (a)

Ques. A ball is released from certain height. It loses 50% of its kinetic energy on striking the ground. It will attain a height again equal to
(a) One fourth the initial height
(b) Half the initial height
(c) Three fourth initial height
(d) None of these

Ans. (b)

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Ques. A body of mass 10 kg is dropped to the ground from a height of 10 metres. The work done by the gravitational force is (g = 9.8 m/sec2)
(a) â€“ 490 Joules
(b) + 490 Joules
(c) â€“ 980 Joules
(d) + 980 Joules

Ans. (d)

Ques. A motor boat is travelling with a speed of 3.0 m/sec. If the force on it due to water flow is 500 N, the power of the boat is
(a) 150 kW
(b) 15 kW
(c) 1.5 kW
(d) 150 W

Ans. (c)

Ques. A mass of 10 gm moving with a velocity of 100 cm/s strikes a pendulum bob of mass 10 gm. The two masses stick together. The maximum height reached by the system now is (g = 10 m/s2)
(a) Zero
(b) 5 cm
(c) 2.5 cm
(d) 1.25 cm

Ans. (d)

Ques. Masses of two substances are 1 g and 9 g respectively. If their kinetic energies are same, then the ratio of their momentum will be
(a) 1 : 9
(b) 9 : 1
(c) 3 : 1
(d) 1 : 3

Ans. (d)

Ques. Two bodies having same mass 40 kg are moving in opposite directions, one with a velocity of 10m/s and the other with 7 m/s. If they collide and move as one body, the velocity of the combination is
(a) 10 m/s
(b) 7 m/s
(c) 3 m/s
(d) 1.5 m/s

Ans. (d)

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Ques. Work done in raising a box depends on
(a) How fast it is raised
(b) The strength of the man
(c) The height by which it is raised
(d) None of these

Ans. (c)

Ques. A steel ball of radius 2 cm is at rest on a friction-less surface. Another ball of radius 4cm moving at a velocity of 81 cm/sec collides elastically with first ball. After collision the smaller ball moves with speed of
(a) 81 cm/sec
(b) 63 cm/sec
(c) 144 cm/sec
(d) None of these

Ans. (c)

Ques. If the stone is thrown up vertically and return to ground, its potential energy is maximum
(a) During the upward journey
(b) At the maximum height
(c) During the return journey
(d) At the bottom

Ans. (b)

Ques. A mass of 0.5kg moving with a speed of 1.5 m/s on a horizontal smooth surface, collides with a nearly weightless spring of force constant k = 50 N/m. The maximum compression of the spring would be
(a) 0.15 m
(b) 0.12 m
(c) 1.5 m
(d) 0.5 m

Ans. (a)

Ques. The kinetic energy of a body of mass 3 kg and momentum 2 Ns is
(a) 1 J
(b) 2/3 J
(c) 3/2 J
(d) 4 J

Ans. (b)

Ques. A body of mass 2 kg is thrown up vertically with K.E. of 490 joules. If the acceleration due to gravity is 9.8m/s2, then the height at which the K.E. of the body becomes half its original value is given by
(a) 50 m
(b) 12.5 m
(c) 25 m
(d) 10 m

Ans. (b)

Ques. A position dependent force F = 7 â€“ 2x + 3x2 newton acts on a small body of mass 2 kg and displaces it from x = 0 to x = 5 m. The work done in joules is
(a) 70
(b) 270
(c) 35
(d) 135

Ans. (d)

Ques. A rubber ball is dropped from a height of 5 m on a planet where the acceleration due to gravity is not known. 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

Ans. (b)

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Ques. Two perfectly elastic particles P and Q of equal mass travelling along the line joining them with velocities 15 m/sec and 10 m/sec. After collision, their velocities respectively (in m/sec) will be
(a) 0, 25
(b) 5, 20
(c) 10, 15
(d) 20, 5

Ans. (c)

Ques. It is easier to draw up a wooden block along an inclined plane than to haul it vertically, principally because
(a) The friction is reduced
(b) The mass becomes smaller
(c) Only a part of the weight has to be overcome
(d) â€˜gâ€™ becomes smaller

Ans. (c)

Ques. A cylinder of mass 10kg is sliding on a plane with an initial velocity of 10m/s. If coefficient of friction between surface and cylinder is 0.5, then before stopping it will describe
(a) 12.5 m
(b) 5 m
(c) 7.5 m
(d) 10 m

Ans. (d)

Ques. A 10 H.P. motor pumps out water from a well of depth 20m and fills a water tank of volume 22380 litres at a height of 10m from the ground. the running time of the motor to fill the empty water tank is (g = 10 msâ€“2)
(a) 5 minutes
(b) 10 minutes
(c) 15 minutes
(d) 20 minutes

Ans. (c)

Ques. A body of mass 4 kg moving with velocity 12 m/s collides with another body of mass 6 kg at rest. If two bodies stick together after collision, then the loss of kinetic energy of system is
(a) Zero
(b) 288 J
(c) 172.8 J
(d) 144 J

Ans. (c)

Ques. An inelastic ball is dropped from a height of 100 m. Due to earth, 20% of its energy is lost. To what height the ball will rise
(a) 80 m
(b) 40 m
(c) 60 m
(d) 20 m

Ans. (a)