Q5 of 28 Page 159

State the principle of conservation of energy.

The law states that energy can neither be created nor be destroyed, rather it can only be transferred from one form to another.


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3

Identify the wrong statement among the following. Rewrite them by making necessary corrections.

a) Work and energy have different units.


b) When an aero plane takes off, the work done by its weight is positive.


c) The potential energy of spring increases when it is extended and decreases when it is compressed.


d) If the work done by external forces on a system is negative then the energy of the system decreases.


e) When a body is falling freely from a height, its kinetic energy remains constant.


f) The unit of power is watt.

 4

What is mechanical energy?

 6

When you lift a box from the floor and put it on an almirah the potential energy of the box increases but there is no change in its kinetic energy. Is it violation of conservation of energy? Explain.

 7

One person says that potential energy of a particular book kept in an almirah is 20 J and other says it is 30J . Is one of them necessarily wrong? Give reasons.