# Why does work equals kinetic energy of friction?

1. Oct 23, 2011

### zzoo4

Why does work equals kinetic energy of friction??

1. The problem statement, all variables and given/known data
After hitting a long ﬂy ball that goes over the right ﬁelder’s
head and lands in the outﬁeld, the batter decides to keep going
past second base and try for third base. The 62.0-kg player begins sliding 3.40 m from the base with a speed of 4.35 m/s. If
the player comes to rest at third base, (a) how much work was
done on the player by friction?

2. Relevant equations
w=fd
ke=.5mv^2

3. The attempt at a solution

I already know how to do this but

KE= .5X6.2x4.35^2 = -587J

But Why do we use Kinetic energy equation to find the work of friction???

2. Oct 23, 2011

### aftershock

Re: Why does work equals kinetic energy of friction??

Energy can't be created or destroyed. Friction basically produces heat energy. In order to get this heat energy we need to take it from somewhere else, like kinetic energy.

Energy is the ability to do work so in your problem the work done by friction is equal to the loss of kinetic energy of the baseball player.

3. Oct 23, 2011

### vela

Staff Emeritus
Re: Why does work equals kinetic energy of friction??

How did you get a negative answer when multiplying a bunch of positive numbers?
The work-energy theorem says $\Delta KE=W$. What's ΔKE for the batter?

4. Oct 25, 2011

### zzoo4

Re: Why does work equals kinetic energy of friction??

Tyyyy