# NCERT Class-9 Science Solutions: Chapter 10-Gravitation Part 4

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**Question 21:**

Calculate the average density of the earth in terms of g, G and R.

**Answer:**

**Question 22:**

The earth is acted upon by gravitation of Sun, even though it does not fall into the Sun. Why?

**Answer:**

The gravitational force is responsible for providing the necessary centripetal force.

The earth is orbiting round the sun in a stable orbit (nearly circular) such that the gravitational attraction of the sun just provides the centripetal force to the earth for its orbital motion so, net force on the earth is zero and consequently, the earth does not fall into the sun

**Long Answer Questions**

**Question 23:**

How does the weight of an object vary with respect to mass and radius of the earth? In a hypothetical case, if the diameter of the earth becomes half of its present value and its mass becomes four times of its present value, then how would the weight of any object on the surface of the earth be affected?

**Answer:**

Weight of an object is directly proportional to the mass of the earth and inversely proportional to the square of the radius of the earth. i.e.

Weight of a body

Original weight

When hypothetically becomes and becomes

The weight will become

**Question 24:**

How does the force of attraction between the two bodies depend upon their masses and distance between them? A student thought that two bricks tied together would fall faster than a single one under the action of gravity. Do you agree with his hypothesis or not? Comment.

**Answer:**

This hypothesis is not correct. The two bricks, like a single body, fall with the same speed to reach the ground at the same time in case of free-fall. This is because acceleration due to gravity is independent of the mass of the falling body

**Question 25:**

Two objects of masses having the same size are dropped simultaneously from heights respectively. Find out the ratio of time they would take in reaching the ground. Will this ratio remain the same if?

one of the objects is hollow and the other one is solid and

Both of them are hollow, size remaining the same in each case. Give reason.

**Answer:**

Ratio will not change in either case because acceleration remains the same.

In case of free-fall acceleration does not depend upon mass and size.