Question about gravitational force

[Ellio]

Homework Statement

none

Relevant Equations

g= 9.81 / or -9.81 m/s²

Hello, I hope you are all very well !

Let's say a man, standing on point "0 m" throws a ball 10 m in the air. The gravitational force goes in the opposite (down (always)) of the action (up): I supposed it's -9.81 m/s².
The same action but this time the man stands on the point "10 m" and the stone goes up to 0 m. The gravitational force is still going on the opposite of the stone but this time is going in the same direction (down) as the direction of the action (down): here it would be 9.81 ?

 

[PeroK]

By definition:

$g = 9.81 m/s^2$

When you set a problem up, you get to choose whether up or down is taken to be the positive direction. Then $a = \pm g$, as appropriate.

 

[Cutter Ketch]

To elaborate on what PeroK said, you are free to choose whether up is positive or down is positive. However, it isn’t just the sign of the gravitational acceleration you are choosing. It is the direction of the coordinate system. For example, if you choose up as positive, then gravitational acceleration which points down will be negative, but also position is more positive as you go up and positive velocity refers to traveling upward.

So, you are welcome to choose up or down as positive, but you must keep the same convention for the whole problem.

 

[Ellio]

To elaborate on what PeroK said, you are free to choose whether up is positive or down is positive. However, it isn’t just the sign of the gravitational acceleration you are choosing. It is the direction of the coordinate system. For example, if you choose up as positive, then gravitational acceleration which points down will be negative, but also position is more positive as you go up and positive velocity refers to traveling upward.

So, you are welcome to choose up or down as positive, but you must keep the same convention for the whole problem.

Ok thank you very much for your answers !