Q. If you want to lift the water to damp against the force of gravity what is the energy used by motor.given that Mass=20Kg ; Height=20m;Gravity=9.8m/s; Time=20min Ans: Work done by the motor against the gravity, W = mgh = 20×9.8×20 = 3920 J Power delivered by the motor = W/t =3920/(20×60) = 3.27 J/s Q. when a ball is lifted to a height of height h when it is drooped just before touching the ground, what would the kinetic energy acting on the ball. Ans: mgh According to the conservation of energy Potential energy is converted into kinetic energy just above the ground. Just before touching the ground the kinetic energy of the body is equal to mgh Q. what are the conditions where the work can be zero? Ans: Work done is said to be zero if the force is acting at right angles to the displacement of the body or when there is no displacement of the body. Example: Work done by the force of gravity on box lying on a roof of a bus moving with a constant velocity on a straight
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