Question

How can we say that the speed becomes zero when we are finding the difference between speed and velocity??

Answer 1

If speed is zero that means the distance covered by the particle is zero and the particle is in rest so that consequently means there is no motion and in that case velocity will also be zero.

In some cases the particle comes back to the starting point then in that case the velocity becomes zero as displacement becomes zero but speed is there as there will be path covered (distance).

So zero speed does not mean anything but zero velocity still has a meaning that particle comes back to starting position.