to be at rest, the net external force on it must be zero, according to the first law. This implies that the normal force R must be equal and opposite to the weight W ” . Fig. .
(a) a book at rest on the table, and (b) a car moving with uniform velocity. The net force is zero in each case. Consider the motion of a car starting from rest, picking up speed and then moving on a smooth straight road with uniform speed (Fig. ( .
(b)). When the car is stationary, there is no net force acting on it. During pick-up, it accelerates. This must happen due to a net external force.
Note, it has to be an external force. The acceleration of the car cannot be accounted for by any internal force. This might sound surprising, but it is true. The only conceivable external force along the road is the force of friction.
It is the frictional force that accelerates the car as a whole. (You will learn about friction in section . ). When the car moves with constant velocity, there is no net external force.
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