# Homework Help: Positive and negative work

1. Nov 13, 2005

### Faira

Hello!
I need a help with one question:

Discuss weather any work is being done by each of the following agents, and if so, wether the work is positive of negative.
a) a chiken scratching the ground (what I think is that work is performed as
W=F∆x, and if the chicken moves her leg back and forth making ∆x equal to zero it means that there is no work done)
b) a person studying (the student does not move and does not have have any force it means that he does not make any work)
c) the leg muscles of a person in the act of sitting down (here I have not clue how to answer it)
Am I on the right way? And how to answer on the last question?

2. Nov 13, 2005

### Chi Meson

THink of work in this manner:

Work is done if a force is used to change the energy state of an object (either self or other). This could be a change in kinetic energy, gravitational potential energy, or elastic potential energy. (work can change thermal energy too, but save that for later). If the force causes the thing to have more energy (speeding up, for example), positive work has been done. IF the force causes the thing to have less energy, negative work has been done.

In a): think of the chicken scratching as a repeating process; over time (half an hour of scratching for example) has anything changed in energy? Is anything going faster or slower? Is anything higher or lower? No. This agrees with your asessment that no work is done. (Unless we are including tiny dust particles kicked up with each scratch, but I don't think so).

b) There is a force on the student, but you correctly note that the student does not move (the net force on the student is zero, but lots of forces are on the student). No change in energy, no work.

c)what happens to the energy of the person who sits down?

3. Nov 13, 2005

### dx

What the first one is asking you indirectly is whether friction is a conservative force. is it?

4. Nov 14, 2005

### Nm

Friction is a non-conservative force because it can convert mechanical energy such as kinetic energy into non-mechanical energy (heat).

5. Nov 14, 2005

### dx

A more precise statement would be

$$\oint F\cdot{ds} = 0$$

6. Nov 14, 2005

### Chi Meson

Faira:

has the term "conservative force" or "non conservative force" come up at all in your text or in class? This is kind of important to know in order to give you the right "level" of help.