Every body continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise.
If the net external force on a body is zero, its acceleration is zero. Acceleration can be non zero only if there is a net external force on the body.
if the net external force is zero, a body at rest continues to remain at rest and a body in motion continues to move with a uniform velocity. This property of the body is called inertia. Inertia means ‘resistance to change’. A body does not change its state of rest or uniform motion, unless an external force compels it to change that state.
The property of inertia contained in the First law is evident in many situations. Suppose we are standing in a stationary bus and the driver starts the bus suddenly. We get thrown backward with a jerk. Why ?
Our feet are in touch with the floor. If there were no friction, we would remain where we were, while the floor of the bus would simply slip forward under our feet and the back of the bus would hit us.
However, fortunately, there is some friction between the feet and the floor. If the start is not too sudden, i.e. if the acceleration is moderate, 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, i.e. it allows some relative displacement between different parts.
What this means is that while our feet go with the bus, the rest of the body remains where it is
due to inertia. Relative to the bus, therefore, we are thrown backward.
As soon as that happens, however, the muscular forces on the rest of the body (by the feet) come into play to move the body along with the bus.
A similar thing happens when the bus suddenly stops. Our feet stop due to the friction which does not allow relative motion between the feet and the floor of the bus. But the rest of the body continues to move forward due to inertia. We are thrown forward. The restoring muscular forces again come into play and bring the body to rest.
Types of Inertia with Examples
Inertia of Rest
An object stays where it is placed, and it will stay there until you or something else moves it. The property of a body to oppose any change in its state of rest is known as inertia of rest.
- When a tree is vigorously shaken, some of the leaves fall from the tree: When the branch of a tree is vigorously shaken, the branch comes in motion as the force is applied on the branch. But the leaves want to remain at rest due to inertia of rest and fall down.
- The carpet is beaten with a stick to remove the dust particles: When carpet is beaten with stick, the carpet comes in motion but the dust particles remain at rest due to inertia of rest.
Inertia of Motion
An object will continue at the same speed until a force acts on it. The property of a body to oppose any change in its state of uniform motion is known as inertia of motion.
- Luggage is usually tied with a rope on the roof of a bus. When the bus stops suddenly, the luggage kept on the roof may fall from the roof of the bus due to inertia of motion therefore, it is advised to tie any luggage kept on the roof of a bus with a rope.
Inertia of Direction
An object will stay moving in the same direction unless a force acts on it. The property of a body to oppose any change in its direction of motion is known as inertia of direction.
- If you jump from a car or bus that is moving, your body is still moving in the direction of the vehicle. When your feet hit the ground, the grounds act on your feet and they stop moving. You will fall because the upper part of your body didn’t stop, and you will fall in the direction you were moving.
- When a bus driver is negotiating a curve on the road, passengers fall towards the centre of the curved road. Whenever a driver is negotiating a curve, the passengers experience a force acting away from the centre of the curve; it happens due to the tendency of the passengers to continue moving in a straight line.