Gravitation Class IX Chapter 10

Question - Answer

Q.No. 1. State the universal law of gravitation.

Answer:

The statement of universal law of gravitation is as follows.

“Every object in the universe attracts every other object with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them”.

Q.No. 2. Write the formula to find the magnitude of the gravitational force between the earth and object on the surface of the earth.

Answers:

The formula which is used to find the magnitude of the gravitational force between the earth and object on the surface of the earth is given below.

F = GMm/d²

 

Here,

F denotes gravitational force.

M and m denote mass of the earth and object respectively.

d denotes distance between the earth and the object.

G is the universal gravitational constant.

The value of G is 6.673x10^-11 Nm²/kg².

Q.No. 3. What do you mean by free fall?

Answer:

                   When an object falls towards the earth. There is no change in the direction of the motion of the object. But due to the earth’s attraction, there will be a change in the magnitude of the velocity.Therefore, whenever an object falls towards the earth, an acceleration is involved due to the earth’s gravitational force.

                   This acceleration is called acceleration due to the gravitational force of the earth or acceleration due to gravity. It is denoted by g. The SI unit of g is ms^-2.

                   According to universal law of gravitation, we know that
F = GMm/d² ................. (i)
F = mg ................. (ii)

Now, from (i) and (ii)

mg =

Therefore,

g = GM/d²

Q.No. 4. What do you mean by acceleration due to gravity?

Answer:

                   Whenever an object falls towards to the earth, acceleration is involved due to gravitational force. This is called acceleration due to gravity. This is equal to

g = GM/d²

Here,

G is the gravitational constant and equal to 6.673x10^-11 Nm²/kg².

M is the mass of the earth.

Q.No. 5. What are the difference between the mass of an object and its weight?

Answer:

                   The mass of an object is the measure of inertia. The mass of an object is constant. It does not change from place to place. Therefore, the mass of an object remains same whether on the surface of the earth, moon or outer space.

                   The weight of an object is variable. It changes from place to place because it depends on gravitational force. Therefore, weight of an object is not same whether on the surface of the earth, moon and outer space.

Q.No. 6. Why is the weight of an object on the moon 1/6^th its weight of the earth.

Answer:

                   Let the acceleration due to gravity on the surface of the earth is g and mass is m.

The weight of the object on the surface of the earth = w_e = mg

The weight of the object on the surface of the moon = w_m =mg/6

We know that acceleration due to gravity on the surface of the moon is 1/6^th of earth.

Therefore,

w_m =we/6

Therefore, the weight of an object on the moon is 1/6^th its weight of the earth.