Work done by gravity problem

  • #1

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

Homework Equations


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?
 

Answers and Replies

  • #2
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

Homework Equations


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?
 
  • #3
Ah, right, hadn't thought of that. The direction of the displacement is opposite the direction of the force. Thank you.
 

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