# Homework Help: W=dke=dpe potential energy

1. Apr 14, 2014

### yolo123

A conservative force of 5 N causes a body to move 2 m in the direction of the force. The change of the body’s potential energy associated with the force is:
(a) zero,
(b) +10 J,
(c) -10 J, (d) infinite.

Let's say the force is pushing the body for 2m, there must be a change in KE. But, for change of KE, there must be change in PE.
I don't understand this. Can you explain to me how W=DKE=DPE?

2. Apr 14, 2014

### PhanthomJay

you might want to consider both the work energy theorem and the conservation of energy theorem to show the relationship between work done by a conservative force and the PE change.

3. Apr 14, 2014

### yolo123

I think I found the answer: -10J. Say it is gravity, object goes down two meters, -10J of DPE.

4. Apr 14, 2014

### PhanthomJay

Yes that is correct.
You should satisfy yourself (with no other non conservative forces doing work ) that the KE change is ?

5. Apr 15, 2014

### nasu

It is not. Look up the definition of potential energy. There is a minus sign there.

6. Apr 15, 2014

### yolo123

10j!

7. Apr 15, 2014

### nasu

Is the work positive or negative?

8. Apr 15, 2014

### yolo123

Nasu, I was answering PhantomJay's question! The change in KE= +10J

9. Apr 15, 2014

### PhanthomJay

Correct. So you originally stated that that Wc=dKE=dPE.
Do you wish to correct that statement, for the special case as in your example when only conservative forces are doing work?