English Medium इयत्ता ९ - CBSE Question Bank Solutions for Science

Write the formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth.

How does the force of gravitation between two objects change when the distance between them is reduced to half?

What is the magnitude of the gravitational force between the earth and a 1 kg object on its surface? (Mass of the earth is 6 × 10²⁴ kg and radius of the earth is 6.4 × 10⁶ m).

The earth and the moon are attracted to each other by gravitational force. Does the earth attract the moon with a force that is greater or smaller or the same as the force with which the moon attracts the earth? Why?

If the moon attracts the earth, why does the earth not move towards the moon?

What happens to the force between two objects, if the mass of one object is doubled?

What happens to the force between two objects, if the distance between the objects is doubled and tripled?

What is the importance of the universal law of gravitation?

A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force (see the given figure). Let us take it that the force acts on the object through the displacement. What is the work done in this case?

When do we say that work is done?

Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Define 1 J of work.

A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?

Look at the activity below. Reason out whether or not work is done in the light of your understanding of the term ‘work’.

A sailboat is moving due to wind energy.

An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?

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.

In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east, as shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.

Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?

How does the sound produced by a vibrating object in a medium reach your ear?

Explain how sound is produced by your school bell.