# Question #38493

Mar 2, 2017

See below.

#### Explanation:

First, gravity always acts radially inward toward the centre of the earth.

These sign issues are a different issue and arise when you choose your co-ordinate system.

• In the first case, if the ball is thrown upwards, and you choose up as your positive z-axis, in which case you must use $- g$ as your "acceleration".

• In the second case, if the ball is dropped, and you choose down as your positive z-axis, then you must use $g$ as your "acceleration".

In projectile motion, it is most common to choose up as the positive z-axis, then clearly you must use $- g$ as your "acceleration".

But you could also choose down as the positive z-axis, provided you were sure to keep all the signs in order. Then clearly you would use $g$ as your "acceleration".