# NCERT Class 9 Science Solutions: Chapter 10 – Gravitation Part 8

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Question 16:

Q. Calculate the force of gravitation between the earth and the sun, given that the mass of the earth and of the sun kg. The average distance between the two is m.

According to the universal law of gravitation, the force of attraction between the Earth and the Sun is given by:

Where,

Mass of the sun kg

Mass of the Earth kg

Average distance between the earth and the sunm

Universal gravitational constant

Question 17:

Q. A stone is allowed to fall from the top of a tower m high and at the same time another stone is projected vertically upward from the ground with a velocity of . Calculate when and where the two stones will meet.

Let the two stones meet after a time

For the stone meet after time

Initial velocity,

Let the displacement of the stone in time from the top of the tower be .

Acceleration due to gravity,

From the equation of motion,

For the stone thrown upwards:

Initial velocity,

Let the displacement of the stone from the ground in time be.

Acceleration due to gravity,

Equation of motion,

The combined displacement of both the stones at the meeting point is equal to the height of the tower 100m.

The combined displacement of both the stones at the meeting point is equal to the height of the tower m.

In, the falling stone has covered a distance given by equation (1) as

Therefore, the stones will meet after at a height from the ground.