NCERT Class 9 Science Solutions: Chapter 11 – Work and Energy Part 7

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Question 14:

Q. An electric heater is rated 1500 w. How much energy does it used in 10 hours?


Energy consumed by an electric heater can be obtained with the help of the expression,


Power rating of the heater,

Time for which the heater has operated,

Work done Energy consumed by the heater

Therefore, energy consumed

Hence, the energy consumed by the heater in is kWh.

Question 15:

Q. Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why do the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?


The law of conservation of energy states that energy can be neither created nor destroyed. It can only be converted from one form to another.

Consider the case of an oscillating pendulum.

Q 15 Image Showing Oscillating Pendulum.

Q 15 Image Showing Oscillating Pendulum

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When a pendulum moves from its mean position to either of its extreme positions it rises through a height above the mean level P. At this point, the kinetic energy of the bob changes completely into potential energy. The kinetic energy becomes zero, and the bob possesses only potential energy. As it moves towards point, its potential energy decreases progressively. Accordingly, the kinetic energy increase. As the bob reaches point P, its potential energy becomes zero and the bob possesses only kinetic energy. This process is repeated as long as the pendulum oscillates.

The bob does not oscillate forever. It comes to rest because air resistance resists its motion. The pendulum loses its kinetic energy to overcome this friction and stops after some time.

The law of conservation of energy is not violated because the energy lost by the pendulum to overcome friction is gained by its surroundings. Hence, the total energy of the pendulum and the surrounding system remain conserved.

Question 16:

Q. An object of mass, m is moving with a constant velocity, v. How much work should be dine on the object in order to bring the object to rest?


Kinetic energy of an object of mass, m moving with a velocity, v is given by the expression.

To bring the object to rest, amount of work is required to be done on the object.