Atennis ball is shot vertically upward in an evacuated chamber inside a tower with an initial speed of 20.0 m/s at time t=0s. what is the magnitude of the acceleration of the ball when it is at its highest point?

at the highest point of the path the acceleration of ball is same as acceleration due to gravity

Explanation:

At the highest point of the path of the ball the speed of the ball becomes zero as the acceleration due to gravity will decelerate the motion of ball due to which the speed of ball will keep on decreasing and finally it comes to rest

So here we will say that at the highest point of the path the speed of the ball comes to zero

now by the force diagram we can say that net force on the ball due to gravity is given by

now the acceleration of ball is given as

so at the highest point of the path the acceleration of ball is same as acceleration due to gravity

scientific laws are the result of testing theories and proving them to be facts.