# Need Help Understanding KE, PE and Mechanical Energy lost due to friction

1. Oct 19, 2009

### sarahjt1

1. The problem statement, all variables and given/known data

A 70 kg base runner begins his slide into second base when he is moving at a speed of 4.0m/s. The coefficient of friction between his clothes and the earth is 0.70. He slides so that his speed is zero just as he reaches the base.
a) How much mechanical energy is lost due to friction acting on the runner?

b) How far does he slide?

So, I figured out b no problem but a) I have no idea where to even begin. I was using the conservation of energy equation KE + PE = KEi + PEi. But that obviously is not correct. How does mechanical energy EVER get calculated into these equations? I don't understand where it goes and what represents it....

2. Relevant equations

For part b) I used the KEf-W=KEi because PEi=PEf due to the fact that there is no y-direction.... right?

3. The attempt at a solution

2. Oct 20, 2009

### tiny-tim

Welcome to PF!

Hi sarahjt1! Welcome to PF!

(Yes, you can ignore PE completely … in fact, I don't think the examiner will even expect you to mention it)
This isn't physics, it's just arithmetic

(yeah, sometimes the problems are that easy! )

you know the mechanical energy before, and the mechanical energy after, so the mechanical energy lost is … ?

3. Oct 20, 2009

### sarahjt1

I got -560J as my answer but I am not sure if that is right. If it is, I don't fully understand the rationale I used to get there. I guess it is more that I don't fully understand the concepts involved.

This is where my trouble lies... just in understanding the equations that are relevant to the questions being asked... :(

4. Oct 20, 2009

### Jebus_Chris

Well for this question the runner as an initial kinetic energy of 560J. Friction does work on the runner which brings his velocity to zero and thus, his kinetic energy. So you would have...
$$KEi -Wf = KEf$$
And you'll see that he losses 560j of energy.

5. Oct 20, 2009

### tiny-tim

Hi sarahjt1!
It is, except that it's plus 560J (the question asks for the energy lost, which of course is positive ).
Are you talking just about part a)?

You're just calculating the KE before and the KE after (using the standard 1/2 mv2 definition), and subtracting …

what is worrying you about that?