# NCERT Class-9 Science Solutions: Chapter 10-Gravitation Part 3

Question 15:

An apple falls from a tree because of gravitational attraction between the earth and apple. If is the magnitude of force exerted by the earth on the apple and is the magnitude of force exerted by apple on earth, then

Question 16:

What is the source of centripetal force that a planet requires to revolve around the Sun? On what factors does that force depend?

Gravitational force. This force depends on the product of the masses of the planet and sun and the distance between them.

The source of centripetal force is the huge gravitational forces between the planet and the sun. It depends upon masses of the planet and the sun and also the distance between them.

Question 17:

On the earth, a stone is thrown from a height in a direction parallel to the earth’s surface while another stone is simultaneously dropped from the same height. Which stone would reach the ground first and why?

Both stones will take the same time to reach the ground because the two stones fall from the same height.

Question 18:

Suppose gravity of earth suddenly becomes zero, then in which direction will the moon begin to move if no other celestial body affects it?

The moon will begin to move in a straight line in the direction in which it was moving at that instant because the circular motion of moon is due to centripetal force provided by the gravitational force of earth.

Question 19:

Identical packets are dropped from two aero planes, one above the equator and the other above the North Pole, both at height h. Assuming all conditions are identical, will those packets take same time to reach the surface of earth. Justify your answer.