## Note on Gravity

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It is the force due to which a relatively small object is attracted towards the center of the larger objects like planets or satellites.

#### Acceleration due to gravity

It is the acceleration produced on a freely falling object towards the center at any planet or satellite.

The value of acceleration due to gravity differs from planet to planet and satellite to satellite.

E.g.:

Acceleration due to gravity of the earth = 9.8m/s2

Acceleration due to gravity of the moon = 1.6 m/s2

Acceleration due to gravity of the Jupiter = 25 m/s2

#### Prove that: g = $$\frac{GM}{r^2}$$

Let,

M = mass of the planet

R = radius of the related body

m = mass of falling body on the surface of planet or object.

F = Gravitational force of attraction between the two bodies

g = acceleration due to gravity.

From Newton's universal law of gravitation,

F= ------------- (i)

From Newton's second law of motion,

F = mg -------------- (ii)

From Eqn (i) and (ii) we get

mg =

g =

g =   --------------- (iii)

Further g ∝   {when G and M are held constant}

The acceleration due to gravity on earth is 9.8 m/s means that the velocity of every falling body above the earth surface increases 9.8m/s in each second.

It is demonstrated by the following diagram.

#### Differences between acceleration due to gravity (g) and gravitational constant (G)

 Acceleration due gravity(g) Gravitational Constant(G) The acceleration produced on a freely falling body under the effect of gravity is called acceleration due to gravity. The gravitational constant is defined as the force of attraction between two bodies each of unit masses kept at a unit distance. It is a variable quantity. Its value changes from place to place. It is a constant quantity.

#### Differences between acceleration due to gravity and gravity

 Acceleration due to gravity Gravity The acceleration produced, on a freely falling body under the effect of gravity is called acceleration due to gravity Gravity is the force by which body is attracted towards the center of the earth or any other heavenly body Its SI unit is Its SI unit is N It is the effect of gravity. It is the cause of acceleration due to gravity.

#### Difference between gravity and gravitation

 Gravity Gravitation Gravity is the force by which body is attracted towards the center of the earth or any other heavenly body It is the force of attraction between two bodies Its SI unit is N Its SI unit is

• Force due to which a relatively small object is attracted towards the center of the larger objects like planets or satellites is known as gravity.
• The acceleration produced on a freely falling body under the effect of gravity is called acceleration due to gravity.
.

### Very Short Questions

The force of gravity on the surface of the moon is 6 times less than that of the earth. Therefore, one can jump higher on the surface of the moon than on the earth.

The two differences between gravity and gravitation are:

 Gravity Gravitation The force of attraction with which a planet pulls an object towards its center is called gravity. The force of attraction produced between any two objects is known as gravitation. It depends upon the mass of the planet or satellite. It depends upon the product of the two masses and distance between them.

The two differences between mass and weight are:

 Mass Weight The quantity of matter contained in a body is called its mass. The force applied by the earth to pull any object towards its centre is called the weight of that object. It does not vary from place to place. It varies from place to place.

The earth attracts every object towards its center due to gravity. Therefore, the stone thrown up the sky return back to the earth.

The gravity of moon is not sufficient to hold atmosphere around its surface like that of the earth and hence moon has no atmosphere.

The three effects of gravity are:

1. We can stay on earth's surface due to gravity.
2. The reason behind the stability of large buildings is also due to gravity.
3. If there is no gravity, the river will not flow, the atmosphere will not cover the earth's surface.

When two objects each having unit mass is separated by unit distance, then gravitational constant 'G' becomes equal to gravitational force 'F'.

We know, gravitation is an interaction. Hence, force on the apple by the earth must be equal and opposite of the force on the earth by the apple.

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• ### The force that pulls falling objects toward Earth is called ______.

free fall.

acceleration.

gravity.

friction

• ### The force that one surface exerts on another when the two rub against each other is called ______.

freefall

acceleration.

inertia.

friction.

• ### In science, a push or a pull is called ______.

freefall

acceleration.

force.

inertia.

• ### According to Newton's third law of motion, when a hammer strikes and exerts force on a nail, the nail ______.

disappears into the wood.

moves at a constant speed.

creates a friction with the hammer.

exerts an equal force back on the hammer.

• ### Objects falling through air experience a type of friction called ______.

terminal velocity.

air resistance

inertia.

air friction

• ### If you are launching a matchstick rocket, the action force is the rocket pushing the gases down. What is the reaction force?

The table pushing the rocket down.
The gases pushing the rocket up.
none of the answers are right
The gases pushing the rocket down.
• ### An object is dropped from a height of 1 km above the Earth. If air resistance is ignored, the acceleration of the object is dependent on the ______.

mass of the object

weight of the object

mass of the earth

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