# NCERT Class 9 Science Solutions: Chapter 11 – Work and Energy Part 6

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Question 9:

Q. A certain household has consumed 250 unit of energy during a month. How much energy is this joules?

**Answer:**

1 unit of energy is equal to 1 kilowatt hour

1 unit

1

Therefore, units of energy

Question 10:

Q. An object of mass is raised to a height of above the ground. What is its potential energy? if object is allowed to fall, find its kinetic energy when it is half-way down.

**Answer:**

Gravitational potential energy is given by the expression,

Where,

Vertical displacement

Mass of the object

Acceleration due to gravity

At half-way down, the potential energy of the object will be

At this point, the object has an equal amount of potential and kinetic energy. This is due to the law of conservation of energy. Hence, half-way down, the kinetic energy of the object will be

Question 11:

Q. What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.

**Answer:**

Work is done whenever the given two conditions are satisfied:

(i) A force acts on the body.

(ii) There is a displacement of the body by the application of force in or opposite to the direction of force.

If the direction of force is perpendicular to displacement, then the work done is zero. when a satellite moves around the Earth, then the direction of force of gravity on the satellite is perpendicular to its displacement. Hence, the work done on the satellite by the Earth is zero.

Question 12:

Q. Can there be displacement of an object in the absence of any force acting on it? think. Discuss this question with your friends and teacher.

**Answer:**

Yes. For a uniformly moving object

Suppose an object is moving with constant velocity. The net force acting on it is zero. But, there is a displacement along the object. Hence, there can be a displacement without a force.

Question 13:

Q. A person holds a bundle of hay over his head for minutes and gets tired. Has he done some work or not? Justify your answer.

**Answer:**

Work is done whenever the given two conditions are satisfied:

(i) A force acts on the body.

(ii) There is a displacement of the body by the application of force in or opposite to the direction of force.

When a person holds a bundle of hay over his head, then there is no displacement in the bundle of hay. Although, force of gravity is acting on the bundle, the person is not applying any force on it. Hence, in the absence of force, work done by the person on the bundle is zero.