NCERT Gist: Gravitation | Additional Study Material for UPSC PDF Download

- The change in direction involves change in velocity or acceleration.
- The force that causes this acceleration and keeps the body moving along the circular path is acting towards the centre.
- This force is called the centripetal (meaning centre-seeking) force.
- The motion of the moon around the earth is due to the centripetal force.
- The centripetal force is provided by the force of attraction of the earth.
- If there were no such force, the moon would pursue a uniform straight line motion.
- Newton concluded that not only does the earth attract an apple and the moon, but all objects in the universe attract each other. This force of attraction between objects is called the gravitational force.
- The universal law of gravitation successfully explained several phenomena which were believed to be unconnected:
(i) the force that binds us to the earth;
(ii) the motion of the moon around the earth;
(iii) the motion of planets around the Sun; and
(iv) the tides due to the moon and the Sun.
- Whenever objects fall towards the earth under this force alone, we say that the objects are in free fall.
- This acceleration is due to the earth’s gravitational force.
- Acceleration is called the acceleration due to the gravitational force of the earth (or acceleration due to gravity).
- Mass remains the same whether the object is on the earth, the moon or even in outer space.
- The force acting on an object perpendicular to the surface is called thrust.
- All liquids and gases are fluids.
- The upward force exerted by the water on the bottle is known as upthrust or buoyant force.
- The force due to the gravitational attraction of the earth acts on the bottle in the downward direction.
- So the bottle is pulled downwards.
- But the water exerts an upward force on the bottle.
- Thus, the bottle is pushed upwards. Objects of density less than that of a liquid float on the liquid.
- The objects of density greater than that of a liquid sink in the liquid.
- When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.