Is the GRAVITY, POSITIVE (+ve) or NEGATIVE (-ve)

Here is an example situation:

"A ball is thrown vertically upwards"

when the ball is moving UPWARDS

is the gravity positive or negative?
Meaning +9.81 or -9.81 ?

and in equation, will it be
this: v=u-gt
or this: v=u+gt ?

Your question is poorly formulated... I guess you want to know the sign of the gravitational acceleration, isn't it? Then the response should be one or the other propositions according to the choice you have made about the orientation of the vertical axis! The gravitational acceleration is always directed towards the center of the earth, then if the vertical axis direction is

(i) upwards:
acceleration is a = -g
celerity is v=-gt + u

(ii) downwards:
acceleration is a = +g
celerity is v= gt + u

You can choose option (i) or (ii) but you have to solve these equations with the axis system you have chosen at the beginning.

These equations of motion are always projections on a given axis system...

EDIT:
to whom had put a negative mark: you have things to learn...

To respond to your additional question (ball thrown horizontally) then you have to use projections on both vertical and horizontal axis. acceleration a=(0,az) then celerity V=(vox, az t + voz)
where Vo=(vox,voz) is the initial velocity ( voz= 0 if it is horizontally).

Report Abuse

gravity (on earth) is always negative because it causes matter with mass to stay on its surface all the time the downward pull of force...in space its much different in relation to distance gravity can be stable if not more or less because of the conflicting gravitational pull of spacial masses...i was in physics and is i remember right gravity is -9.813

when the ball is thrown upwards, the accleration due to gravity will be taken as negative as the gravity is opposing the motion (the ball is going upwards while gravity is pulling it down)....but as the ball comes down,gravity will be taken as positive as it is in the direction of motion. :)

while going upwards, eqn. would be v=u-gt

If gravitational force is against movement, gravity is negative.

negative

Ball is thrown vertically upward means against the gravity, so it will be -9.81
and in case of equation it will be v=u-gt