Inertia is the property of an object due to which it opposes the change in its present state.
(A) Inertia of rest
It is the inability of a body to change by itself, its state of rest. This means a body at rest remains at rest and cannot start moving on its own
For example :
(1) Suppose we are standing in a stationary bus and the driver starts the bus suddenly. We get thrown backwards with a jerk. Let us understand why?
Our feet are in touch with the floor of the bus. There is some friction between the feet and the floor If start of the bus is not that sudden, the frictional force would be enough to accelerate our feet along with the bus. But our body is not strictly a rigid body. It is deformable, allowing some relative displacement between different parts. Therefore, while our feet go with the bus, the rest of the body remains where it is, due to inertia. Therefore, relative to the bus, we are thrown backwards. At this instant, the muscular forces on the rest of the body by our feet come into play and move the body along with the bus.
(2) When a horse starts suddenly, the rider tends to fall backwards on account of inertia of rest of upper part of the body as explained above.
(3) The dust particles in a blanket fall off when it is beaten with a stick. This is because the beating sets the blanket in motion whereas the dust particles tend to remain at rest and hence separate.
(4) When we place a coin on smooth piece of cardboard covering a glass and strike the cardboard piece suddenly with a finger, the cardboard slips away and the coin falls into the glass, This happens on account of inertia of rest of the coin.
(5) When we shake a branch of a mango tree, the mangoes fall down. This is because the branch comes in motion and the mangoes tend to remain at rest. Hence they get detached.
(B) Inertia of motion
It is the inability of a body to change by itself, its state of uniform motion i.e. a body in uniform motion can neither accelerate nor retard on its own and come to rest.
For example :
(1) Suppose we are standing in a moving bus, and the driver stops the bus suddenly. We are thrown forward with a jerk. Let us understand why.
As the bus is suddenly stopped, our feet stop due to friction which does not allow relative motion between the feet and the floor of the bus. But the rest of our body continues to move forward due to inertia. That is why we are thrown forward. The restoring muscular forces exerted on our body by our feet come into play and bring the body to rest.
(2) When a horse at full gallop stops suddenly, the rider falls forward on account of inertia of motion as explained above.
(3) A person jumping out of a speeding train may fall forward. This is because his feet come to rest on touching the ground and the remaining body continues to move due to inertia of motion.
(4) An athelete runs a certain distance before taking a long jump.
This is because velocity acquired by running is added to the velocity of the athelete at the time of jump. Hence he can jump over a longer.
(c) Inertia of direction
It is the inability of a body to change by itself, its direction of motion i.e. a body continues to move along the same straight line unless compelled by some external force to change it.
For example :
(1) An umbrella protects us from rain. It is based on the property of inertia of direction. The rain drops vertically downwards cannot change their direction of motion and wet us, with the umbrella on.
(2) When a stone tied to one end of a string is whirled and the string breaks suddenly, the stone flies off the tangent to the circle. This is because the pull in the string was forcing the circle. As soon as the string breaks, the pull vanishes. The stone in a bid to move along the straight line flies off tangentially
(3) When a car rounds a curve suddenly, the person sitting inside is thrown outwards. This is because the person tries to maintain his direction of motion due to directional inertia while the car turns,
(4) The rotating wheels of any vehicle throw out mud, if any, tangentially, due to directional inertia.
The mud guards over the wheels stop this mud, protecting the clothes etc. of the driver of the bike or motor bike.
(5) When a knife is sharpened by pressing it against a grinding stone, the sparks fly off along the tangent to the grinding stone, on account of directional inertia.