Q7 of 28 Page 159

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.

Nobody is wrong. Potential energy of an object depend upon the consideration of zero potential energy.

If we consider ground as zero potential energy lifting a 1kg object to a height of 3m taking g=10 brings the object to 20J potential energy , whereas considering the surface at a vertical height of 1 m above ground as zero potential lifting a 1kg object to 2m makes the potential to 30J.


More from this chapter

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5

State the principle of conservation of 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.

 8

In which of the following cases is the work done positive or zero or negative?

a) Work done by the porter on a suitcase in lifting it from the platform on to his head.


b) Work done by the force of gravity on suitcase as the suitcase falls from porter’s head.


c) Work done by the porter standing on platform with suitcase on his head.


d) Work done by force of gravity on a ball thrown up vertically up into the sky.


e) Work done by force applied by hands of a man swimming in a pond.

 9

What is potential energy? Derive an equation for gravitational potential energy of a body of mass ‘m’ at a height ‘h’.