NCERT Class-9 Science Solutions: Chapter 11-Work and Energy Part 5 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 23:

A girl having mass of sits on a trolley of mass . The trolley is given an initial velocity of by applying a force. The trolley comes to rest after traversing a distance of .

  1. How much work is done on the trolley?
  2. How much work is done by the girl?


Work done on the trolley

Work done by the girl,

Question 24:

Four men lift a g box to a height of and hold it without raising or lowering it.

  1. How much work is done by the men in lifting the box?
  2. How much work do they do in just holding it?
  3. Why do they get tired while holding it?


  1. Zero; as the box does not move at all, while holding it.
  2. In order to hold the box, men are applying a force which is opposite and equal to the gravitational force acting on the box. While applying the force, muscular effort is involved. So they feel tired.

Question 25:

What is power? How do you differentiate kilowatt from kilowatt hour? The Jog Falls in Karnataka state are nearly high. tonnes of water falls from it in a minute. Calculate the equivalent power if all this energy can be utilized?


Power is the rate of doing work. Kilowatt is the unit of power and kilowatt hour is the unit of energy.

Question 26:

How is the power related to the speed at which a body can be lifted? How many kilograms will a man working at the power of , be able to lift at constant speed of vertically? (



We have to find mass m,