NCERT Class 9 Science Solutions: Chapter 11 – Work and Energy Part 9

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Q 20 Image Showing Unit Of Electricity.

Q 20 Image Showing Unit of Electricity

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Question 20:

Q. Find the energy in consumed in 10 hours by four devices of power each.


Energy consumed by an electric device can be obtained with the help of the expression for power,


Power rating of the device,

Time for which the device runs,

Work doneEnergy consumed by the device

Therefore, energy consumed Power Time

Hence, the energy consumed by four equal rating devices in will be

Question 21:

Q. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?


When an object falls freely towards the ground, its potential energy decreases and kinetic energy increases. As the object touches the ground, all its potential energy gets converted into kinetic energy. As the object hits the hard ground, all its kinetic energy gets converted into heat energy and sound energy. It can also deform the ground depending upon the nature of the ground and the amount of kinetic energy possessed by the object.