Work Done by Gravity - Positive or Negative

[axer]

Homework Statement

Homework Equations

The Attempt at a Solution

I first said it's B, since force throws up = positive.
gravity down= also positive, please help asap thanks

 

[Doc Al]

What's the definition of the work done by a force? What determines whether that work is positive or negative?

(Is this thread mistitled?)

 

[axer]

What's the definition of the work done by a force? What determines whether that work is positive or negative?

(Is this thread mistitled?)

OOps yeah it is, ill change it. D: if the displacement in the same direction as force then there is work, so going down is 100% positive.

But going up is tricky.EDIT: i can't edit it.

 

[Doc Al]

if the displacement in the same direction as force then there is work

Careful. Work is done even if they are in opposite directions. But the sign is different.

so going down is 100% positive.

RIght, because force and displacement are in the same direction.

But going up is tricky.

It shouldn't be. :)

EDIT: i can't edit it.

Done.

 

[axer]

Careful. Work is done even if they are in opposite directions. But the sign is different.RIght, because force and displacement are in the same direction.It shouldn't be. :)Done.

so the work going up is negative, since gravity pulls down and displacement is going up.?

 

[Doc Al]

so the work going up is negative, since gravity pulls down and displacement is going up.?

Exactly.

Work = Force x Displacement. No matter what sign convention you use, when the force and displacement are in the same direction, work is positive; when opposite, work is negative. (Try it and see.)

 

[axer]

Exactly.

Work = Force x Displacement. No matter what sign convention you use, when the force and displacement are in the same direction, work is positive; when opposite, work is negative. (Try it and see.)

thanks, you're a life saver.