Question

38. A ball is thrown from a roof top at an angle of 45" above the horizontal. It hits the ground a few
seconds later. The ball has the smallest speed during its motion when
1) is at the highest point
2) it is at the point of projection
3) it hits the ground
4) both (2) and (3)​

Answer 1

In this problem total mechanical energy of the ball is conserved. As the ball is projected from point O, and covering the path OABC. At point A it has both kinetic and potential energy. But at point C it have only kinetic energy, (keeping the ground as reference where PE is zero.)

(a) At point B, it will gain the same speed u and after that speed increases and will be maximum just before reaching C.

(b) During upward journey from OtoA speed decreases and smallest speed attained by it is at the highest point, i.e., at point A.

(c) Acceleration is always constant throughout the journey and is vertically downward equal to g.