## Chapter 9

NCERT
Class 9
Science
Solutions

### Question:

What is the importance of universal law of gravitation?

The importance of the universal law of gravitation has been given earlier.

Newton's third law of motion and gravitation.

According to Newton's third law of motion, for every action there is an equal and opposite reaction. That is, action and reaction forces act on different objects or we can say that a force of the same magnitude an the first object but in opposite directions to each other.

For example, a piece of stone falling freely from a height, falls towards the center of the earth. That is, it is accelerated this acceleration is produced due to the force of gravitation on the piece of stone. A piece of stone also attracts the earth towards itself with the same gravitational force. Here's the question, why can't we see the earth being pulled towards the stone?

To answer this question, we assume that the mass of a piece of stone is 1 kg, the distance of the stone from the center of the earth = the radius of the earth = 6400km.

Hence, the magnitude of the gravitational force

F = GMm/R2

F = (6.67 x 10-11 N -m2/kg) (6.0 x 1024 kg) (1kg) / (6.4 x 106 m)2

F = 9.8N

By, the Newton's second law of motion acceleration of stone,

a = F/m

a = 9.8N/1kg

a = 9.8 m/s2

Since the stone also attracts the earth towards itself with a force of 9.8N by Newton's third law of motion.

acceleration of the earth.

a1 = F/m

a1 = 9.8 N / 6.0 x 1024 kg

a1 = 1.6 × 10–24 m/s2

The acceleration of the earth is very small compared to the acceleration of the stone, so we can not see the earth pulling towards the stone.