Class 10th chapter 1 gravitation short note.
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Answer:
Gravitation : Chapter Notes

Universal Law of Gravitation
The Universal law of Gravitation states that any two bodies having mass attract each other with force directly proportional to the product of their mass and inversely proportional to the square of distance between them. The force acts along the line joining the centres of the objects.

Here, G is Universal gravitational constant = 6.673 × 10-11 Nm2 kg-2. The value of G was found out by Henry Cavendish.
Note:
If mass of object is large, force will be more. If mass of object is small, force will be less.
If distance between two object is more, force exerted will be less and vice versa.
Importance of Gravitational Force
It binds us to Earth.
Moon revolves around Earth due to gravitational force. Planets revolve around Sun due to gravitational force.
Tides in seas are caused due to gravitational force of moon on earth.
Free Fall
The earth attracts objects towards it due to gravitational force. When an object moves such that only gravitational force of earth acts on it, it is said to do “Free Fall”.
Attraction of Earth on Objects close to its Surface
When an object is near the surface of earth, the distance between object and centre of the earth will be equal to the radius of earth (i.e. RE) because the distance of object is negligible in comparison of the radius of earth.
Therefore,
F=GMEmR2E
⇒F=m(GMER2E)
⇒F=mg .............. (i)
Where, g=(GMER2E)
g = acceleration due to gravity. Value of g = 9.8 m / s2.
Velocity and Acceleration
When an object moves with constant velocity, such motion is called uniform motion.
When an object moves with increasing velocity, motion is called accelerated motion.
When an object moves with decreasing velocity, motion is called retarded motion.
Direction of velocity is the direction in which body is moving.
Falling Bodies
Case 1: When an object is released from top, it falls down due to gravitational force of earth. As force acts downwards, acceleration ‘a’ would be in downward direction.
As object is moving downwards and acceleration is also in downward direction, velocity increases. So when an object is released from top, it does accelerated motion (increasing velocity).

Calculation of acceleration ‘a’:
According to Newton’s 2nd Law
F=ma
⇒a=Fm
From equation (i)
⇒a=mgm
⇒a=g=9.8m/s2
Thus acceleration acted on a falling object is equal to ‘g’ (acceleration due to gravity).
Case 2: If we throw a ball upward in the air, a gravitational force acts downwards on it. As force acts downwards, acceleration is in downwards direction. It means velocity will decrease with time and at one point, it becomes zero.
Due to the gravitational force, the ball takes U-turn and reaches to ground with same speed with which it was thrown up. As object is moving downwards and acceleration is also in downward direction, velocity increases.

Note: We have studied three equations of motion.
v=u+at
s=ut+12at2
v2=u2+2as
These equations can be used to calculate the value of velocity, distance, etc by replacing ‘a’ by ‘g’.
The value of ‘g’ is taken as positive in the case of object moving towards earth and taken as negative in the case of object is thrown in opposite direction of earth.
Air Resistance
Theoretically, all objects of different shape, size and mass should take same time to fall through same distance. But practically it is observed that two objects of different mass hit the ground at different times due to different air resistance on them.
Factors affecting Air resistance:
what is meant of gravity