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

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Question 2:

Q. What do you mean by buoyancy?

The upward force exerted by a liquid on an object immersed in it is known as buoyancy. when you try to immerse an object in water, then you can feel an upward force exerted on the object, which increases as you push the object deeper water.

Question 3:

Q. Why does an object float or sink when placed on the surface of water?

If the density of an object is more than the density of the liquid, then it sinks in the liquid. This is because the buoyant force acting on the object is less than the density of the liquid, then it floats on the surface of the liquid. This is because the buoyant force acting on the object is greater than the force of gravity.

Question 1:

Q. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?

When you weigh your body, an upward force acts on it. This upward force is the buoyant force. As a result, the body gets pushed slightly upward, causing the weighing machine to show a reading less than the actual value.

Question 2:

Q. You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality one is heavier than other can you say which one is heavier and why?

The iron bar is heavier than the bag of cotton. This is because the surface area of the cotton bag is larger than the iron bar. Hence, more buoyant force acts on the bag than that on an iron bar. This makes the cotton bag lighter than its actual value. for this reason, the iron bar and the bag of cotton show the same mass on the weighing machine, but actually the mass of the iron bar is more that of the cotton bag.

Question 1:

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

According to the universal law of gravitation, gravitational force acting between two objects is inversely proportional to the square of the distance between them, i.e.,

If distance becomes then the gravitational force will be proportional to

Hence, if the distance is reduced to half, then the gravitational force becomes four times larger than the previous value.

Question 2:

Q. Gravitational force acts on all objects in proportion to their masses. why then, a heavy object does not fall faster than a light object?

All objects fall on ground with constant acceleration, called acceleration due to gravity (in the absence of air resistance). It is constant and does not depend upon the mass of an object. Hence, heavy objects do not fall faster than light objects.

Question 3:

Q. What is the magnitude of the gravitation force between the earth and a 1 kg object on its surface? (Mass of the earth is kg and radius of the earth is m).

According to the universal law of gravitation, gravitation force exerted on an object of mass is given by:

Where,

Mass of Earth,

Mass object,

Universal gravitational constant,

Since the object is on the surface of the Earth, radius of the earth

Gravitation force,