# Work done by gravity problem

## Homework Statement

An object of mass m is moved straight up by a distance h. The work that gravity does on the object is:
(a) -mh
(b) +mh
(c) -mgh
(d) +mgh
(e) None of the Above

Work = ∫F dl
PE = mgh

## The Attempt at a Solution

I have a feeling the answer is (c) -mgh because work is done to lift the object. The object is gaining potential energy so it's gaining +mgh and due to Newton's 3rd law, the work done by gravity is -mgh. Is that right?

SammyS
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## Homework Statement

An object of mass m is moved straight up by a distance h. The work that gravity does on the object is:
(a) -mh
(b) +mh
(c) -mgh
(d) +mgh
(e) None of the Above

Work = ∫F dl
PE = mgh

## The Attempt at a Solution

I have a feeling the answer is (c) -mgh because work is done to lift the object. The object is gaining potential energy so it's gaining +mgh and due to Newton's 3rd law, the work done by gravity is -mgh. Is that right?

Yes, it's answer (c) . But you can get it directly from the definition of work.

What is the direction of the displacement?

What is the direction of the gravitational force?

Ah, right, hadn't thought of that. The direction of the displacement is opposite the direction of the force. Thank you.