Chapter-9 Gravitation

NCERT Class 9 Science 

Question 1

State the universal law of gravitation.

Answer:

The universal law of gravitation states that:

Every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres. The force acts along the line joining the centres of the two objects.

Mathematically,

$$F=G\frac{Mm}{d^2}$$

where

• $F$ = gravitational force

• $M,m$ = masses of the two objects

• $d$ = distance between their centres

• $G$ = universal gravitational constant

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Question 2

Write the formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth.

Answer:

The formula is:

$$F=G\frac{Mm}{R^2}$$

where

• $F$ = gravitational force between earth and object

• $G$ = universal gravitational constant

• $M$ = mass of the earth

• $m$ = mass of the object

• $R$ = radius of the earth

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Page 104 

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Questions 

Question 1

What do you mean by free fall?

Answer:

Free fall is the motion of an object when it falls towards the earth under the influence of gravitational force alone, without any other force acting on it.

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Question 2

What do you mean by acceleration due to gravity?

Answer:

The acceleration produced in an object due to the gravitational force of the earth is called acceleration due to gravity.
It is denoted by g and its value near the earth’s surface is 9.8 m s⁻².

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Page 106 – 

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Question 1

What are the differences between the mass of an object and its weight?

Answer:

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Question 2

Why is the weight of an object on the moon $\frac{1}{6}$th its weight on the earth?

Answer:

The mass of the moon is much smaller than the mass of the earth.
Due to this, the gravitational force of the moon is weaker, and the value of acceleration due to gravity on the moon is about one-sixth of that on the earth.
Therefore, the weight of an object on the moon becomes one-sixth of its weight on the earth.

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Questions (Page 109)

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Question 1

Why is it difficult to hold a school bag having a strap made of a thin and strong string?

Answer:

A thin strap has a small area of contact with the shoulder.
Since pressure = thrust / area, a smaller area produces more pressure.
Therefore, the bag hurts the shoulder and becomes difficult to hold.

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Question 2

What do you mean by buoyancy?

Answer:

Buoyancy is the upward force exerted by a fluid (liquid or gas) on an object when it is immersed in it.
This upward force is also called buoyant force.

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Question 3

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

Answer:

An object floats or sinks depending on its density compared to water.

• If the density of the object is less than the density of water, the buoyant force is greater than its weight → object floats

• If the density of the object is greater than the density of water, the buoyant force is less than its weight → object sinks

Questions (Page 110)

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Question 1

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

Answer:

Your mass is more than 42 kg.

Reason:
A weighing machine actually measures weight, not mass.
Due to the buoyant force of air, the apparent weight is slightly less than the true weight.
Hence, the actual mass is slightly more than 42 kg.

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Question 2

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 the other. Can you say which one is heavier and why?

Answer:

The bag of cotton is actually heavier.

Reason:

• The cotton bag has a larger volume, so it displaces more air.

• Greater air displacement causes a greater buoyant force, reducing the apparent weight more.

• Therefore, although both show the same mass on the weighing machine, the true weight of the cotton bag is more than that of the iron bar.