### Question:

What is the value of ‘g’ at the centre of Earth ?

#### Answer:

g = 0 at the centre of Earth.

### Gravitation

#### Q 1.

How are gravitation and gravity related ?

#### Q 2.

Is mass a scalar quantity?

#### Q 3.

Fill in the blanks :

The acceleration which causes changes only in direction of the velocity of a body is called __________ and it is always directed ___________ the circle.

#### Q 4.

If the distance of Earth from the Sun were half the present value,how many days will make one year ?

#### Q 5.

Does Newton’s third law of motion apply to gravitational force ?

#### Q 6.

Explain what is meant by ‘free fall’and acceleration due to gravity.

#### Q 7.

A physics teacher explains to his students in class,the subject of motion under gravity.When a body is projected upwards from the ground with a certain velocity a,the velocity goes on decreasing at a constant rate due to gravitational pull of earth.At a certain height,this velocity becomes zero.The body cannot rise further.From this maximum height,the body begins to fall and its velocity goes on increasing at the same constant rate due to gravitational pull of earth.The body would strike the ground with exactly the same velocity (v = u),with which it was thrown initially. This is on the assumption that resistance due to air is zero, either way.

Read the above passage and answer the following questions:

(i) Why is velocity of body on striking the ground the same as initial velocity of projection of the body ?

(ii) Does the body take same time to fail to ground as the time it takes to reach the maximum height ?

(iii) What values of life do you learn from this brief study ?

#### Q 8.

Does the force of attraction between two objects depend on the properties of intervening medium?

#### Q 10.

How does value of 'g' vary on the earth? What are the factors responsible for this?

#### Q 12.

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

#### Q 13.

Then why Is the Earth not seen moving towards the stone ?

#### Q 14.

Distinguish between gravitation and gravity.

#### Q 15.

State Kepler’s Laws of planetary motion.

#### Q 16.

Whenever a body is thrown up with a certain velocity,the upward motion is opposed by gravitational pull of earth and also by the resistance of air.

Therefore, velocity of the body goes on decreasing. When this velocity becomes zero,the body cannot rise further.It has attained,what is called ‘maximum height’.From this height,the body begins to fall downwards under the action of gravity.

Read the above passage and answer the following questions:

(i) At the highest point,what are the velocity and acceleration of the body?

(ii) What is the maximum height attained by a body thrown upwards with a velocity of 19.6 m/s ? Take g =

$9.8\; m/s2$.

(iii) What values in life do you learn from this concept ?

#### Q 17.

Fill in the blanks :

Heliocentric model was proposed by ______.

#### Q 18.

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

#### Q 19.

State the universal law of gravitation.

#### Q 20.

A stone is thrown vertically upward with an initial velocity of 40 m/s.Taking g =

$10\; m/s2$,find the maximum height reached by the stone.What is the net displacement and the total distance covered by the stone ?

#### Q 21.

Who gave three laws of planetary motion ?

#### Q 22.

Where is ‘g' maximum, on the surface of Earth, above the surface or below the surface of Earth ?

#### Q 23.

Can you calculate mass of Earth from Newton’s law of gravitation ?

#### Q 24.

Compare gravitational force between light objects and heavy objects.

#### Q 25.

In Fig.,the line that joins a planet to the sun sweeps out areas

$A$_{1}$A$_{2},

$A$_{3}in time intervals 6 weeks,3 weeks and 2 weeks respectively.How are

$A$_{1} $A$_{2},

$A$_{3} realted ?

#### Q 26.

Fill in the blanks :

It is the ____________ of the earth, which holds our atmosphere in place.

#### Q 27.

Fill in the blanks :

The force of gravity _________ with altitude. It also varies on the surface of the earth, _________ from poles to the equator.

#### Q 29.

As you climb up a mountain, your ears “pop” because of the changes in atmospheric pressure. In which direction, outward or inward, does your eardrum move when you climb up?

#### Q 30.

Relative density of silver is 10.8. If the density of water is 10

^{3}kg/m

^{3}, find density of silver.

#### Q 31.

The volume of 50g of a substance is 20 cm

^{3}. If the density of water is 1g/cm

^{3}, will the substance float or sink?

#### Q 32.

A solid block of density 6000 kg/m

^{3} weighs 0.6 kg in air. It is completely immersed in water of density 1000 kg/m

^{3}. Calculate the apparent weight of the block in water and the buoyant force. (Take g = 10ms

^{-2})

#### Q 33.

What is the gravitational acceleration of a spaceship at a distance equal to two Earth’s radius from the centre of the Earth ?

#### Q 34.

The distance between two objects is doubled. What happens to gravitational force between them

#### Q 35.

A stone falling towards Earth also attracts the Earth with the same force.Is it true ?

#### Q 36.

What is the value of ‘g’ at the centre of Earth ?

#### Q 37.

A body is just dropped from a height What is its initial velocity ?

#### Q 38.

What is the relation between g and G ?

#### Q 39.

A body weighs 10 kg on the surface of earth.What would be its mass and weight at the centre of earth ?

#### Q 40.

Define g and G.Establish relation between them.

#### Q 41.

Fill in the blanks :

Geocentric model i.e. the Earth is the orbital centre of all celestial bodies was proposed by _____.

#### Q 42.

Fill in the blanks :

The value of Universal Gravitation constant (G) is ________ Nm

^{2}kgs

^{-2}.

#### Q 43.

Fill in the blanks :

The value of

**g** ________ (increases/decreases) as we go downwards from the earth's surface.

#### Q 44.

Why is G called the Universal Constant?

#### Q 45.

What is the gravitational force between the Earth and a body called? In which direction does it occur?

#### Q 46.

Calculate the force of gravitation between the earth and the Sun, given that the mass of the earth = 6 × 10

^{24} kg and of the Sun = 2 × 10

^{30} kg. The average distance between the two is 1.5 × 10

^{11} m.

#### Q 47.

What happens to the force between two objects, if(i) the mass of one object is doubled? (ii) the distance between the objects is doubled and tripled?(iii) the masses of both objects are doubled?

#### Q 48.

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

#### Q 49.

Fill in the blanks :

The SI unit of weight is _______.

#### Q 50.

Two bodies A and B having masses 2 kg and 4 kg respectively are separated by 2 m.Where should a body of mass 1 kg be placed so that the gravitational force on this body due to bodies A and B is zero ?