# Word done by (non)conservative force

1. Nov 5, 2007

### jack1234

In the following question, the answer is a
http://tinyurl.com/2musg7

My understanding is,
amount of work done of conservative force
=amount of decrease of potential energy
=amount of increase in kinetic energy

decreases of word done of nonconservative force
=decreases of mechanical energy of the entire system

Am I correct?

Last edited: Nov 5, 2007
2. Nov 5, 2007

### learningphysics

Yes, you are right. a is the answer.

change in kinetic energy is the net work done by all forces... so that's -30J + 50J = 20J

change in mechanical energy is work done by all non-conservative forces = -30J

3. Nov 5, 2007

### jack1234

But is that always the case that work done of the non-conservative force(-30J) will contribute to the kinetic energy, but not potential energy? I can' find this information in the reference book that I have.

4. Nov 5, 2007

### learningphysics

Why do you say it only contributes to the kinetic energy and not potential energy? work done by non-cons. forces = change kinetic energy + change in potential energy.

The basic physics is:

Work done by all forces = change in kinetic energy

Work done by non-conservative forces + work done by conservative forces = change in kinetic energy

The above is the physics involved... the stuff below is just bookkeeping (ie math/algebra)

Work done by non-conservative forces = -work done by conservative forces + change in kinetic energy

now we introduce the idea of change in potential energy as -work done by cons. forces

Work done by non-conservative forces = change in potential energy + change in kinetic energy

I advise you not to think of it in terms of contributions to potential energy vs. contributions to kinetic energy... let the math take care of it...