When considering the motion of a body, velocity is defined as the rate of change of its position with respect to time and includes both speed and direction. A key aspect of velocity is that it is a vector quantity, meaning it has both magnitude and direction.
When a body changes direction, even if it maintains the same speed, its velocity changes because the direction component of velocity alters. For instance, if a car is moving in a straight line at 60 km/h and suddenly turns left, the direction of its motion changes. This change in direction means that the velocity cannot remain the same.
In physics, we often simplify our analysis by momentarily considering instances where the velocity is zero during a change in direction. This is because, at that exact moment of turning (like at the peak turning point), we can think of it as the object coming to a brief halt before moving in a new direction. In terms of the physics involved, the body has a momentary velocity of 0 m/s during that perfect transition.
To summarize, we consider the velocity of a moving body to be zero when it changes direction because it emphasizes that there is a change in vector direction, highlighting the distinct nature of velocity as both a speed and directional component.