# Homework Help: Mgh = 1/2mv^2

1. Sep 29, 2006

### ZGMF - X20A

I just wanna know in what conditions does mgh = 1/2mv^2 when a guy goes down a slope and in what conditions does mgh =/= 1/2mv^2. Thanks in advance.

2. Sep 29, 2006

### semc

according to COE, (initial)PE + KE = PE + KE(final) so i think that its better to use this instead of your equation.For example,if the object were to be released with some initial velocity,the KE(initial) would not be 0 so total initial energy is mgh+1/2mv^2 instead of mgh.Btw,it is more precise to write change of PE = -change of KE as this shows that energy is conserved.

3. Sep 29, 2006

### HallsofIvy

And energy is conserved as long as the are no "non-conservative" forces- i.e. as long as there is no friction.

4. Sep 30, 2006

### ZGMF - X20A

So when there is no friction P.E = K.E?

5. Sep 30, 2006

### Tomsk

Yes, decrease in PE= increase in KE and vice versa. If there's friction, decrease in PE= increase in KE + energy 'lost' to surroundings, so increase in KE<decrease in PE.