Imagine you throw a tennis ball vertically into the air. At the exact top of its trajectory it is at rest. What is the magnitude of its acceleration at this point? a. 9.8 m/s2.b. -9.8 m/s2.c. 0.d. other(specify).

a) 9.8 m/s²

Explanation:

At any point during its trajectory, the only influence on the ball is due to gravity, so the ball  is always accelerated by it.This is true even if the ball momentarily stops, like when it reaches to its maximum height before starting to fall down.Close to the surface of  the Earth, the magnitude of this acceleration is 9.8 m/s², as the magnitude is always a positive quantity, so the right answer is a).

Idont understand the question

The answer is orbit around nucleus