First Law of MotionA body at rest continues to be at rest unless an external force acts on it. Likewise a body in motion continues to remain in motion unless an external force acts on it. Newton's first law of motion states that,
"each and every object remains in the state of rest or of uniform motion in a straight line unless it is compelled by an external force to change that state".
We shall have a first-hand experience of inertia of our body when we travel in a bus.
What happens when a moving bus suddenly stops?
We are thrown forward and our head may hit the front seat. The reason is that our body is in motion with the bus and tends to stay in motion even after the bus has stopped.
On the other hand, when the bus suddenly starts, we are thrown backwards, because, in this case, our body is at rest and tends to stay at rest even after the bus has started.
When a bus takes a sharp turn we are thrown to one side. This is because the motion of our body continues along a straight line. When an unbalanced force is applied by the engine to change the direction of motion of the bus, the inertia of our body makes us slip to one side of the seat.