Question #38493

1 Answer
Mar 2, 2017

See below.


This question is misleading.

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".