NCERT Class 9 Science Solutions: Chapter 11 – Work and Energy Part 5

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Q 3 Image Showing Battery Light Bulb.

Q 3 Image Showing Battery Light Bulb

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Question 3:

Q. A battery lights a bulb. Describe the energy changes involved in the process.


When a bulb is connected to a battery, then the chemical energy of the battery is transferred into electric energy. When the bulb received this electrical energy, then it converts it into light and heat energy. Hence, the transformation of energy in the given situation can be shown as:

Question 4:

Q. Certain force acting on a mass changes its velocity from to .calculate the work done by the force.


Kinetic energy is given by the expression,


Kinetic energy of the object moving with a velocity,

Mass of the object

(i) Kinetic energy when the object was moving with a velocity

(ii) Kinetic energy when the object was moving with a velocity

Work done by force is equal to the change in kinetic energy.

Therefore, work done by force

The negative sign indicates that the force is acting in the direction opposite to the motion of the object.

Question 5:

Q. A mass of is at a point on a table. It is moved to a point B. if the line joining and is horizontal, what is the work done on the object by the gravitational force? Explain your answer.


Work done by gravity depends only on the vertical displacement of the body. It does not depend upon the path of the body. Therefore, work done by gravity is given by the expression,


Vertical displacement,

Hence, the work done by gravity on the body is zero.

Question 6:

Q. The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?


No. the process does not violate the law of conservation of energy. This is because when the body falls from a height, then its potential energy changes into kinetic energy progressively. A decrease in the potential energy is equal to an increase in the kinetic energy of the body. During the process, total mechanical energy of the body remains conserved. Therefore, the law of conservation of energy is not violated.

Question 7:

Q. What are the various energy transformation that occur when you are riding a bicycle?


While riding a bicycle, the muscular energy of the rider gets transferred into heat energy and kinetic energy of the bicycle. Heat energy heats the rider’s body. Kinetic energy provides a velocity to the bicycle. The transformation can be shows as:

During the transformation, the total energy remains conserved.