# Homework Help: A 1200kg car skids with uk = 0.45. If it skids 45.2m, find the thermal energy and the

1. Oct 25, 2009

### bigmac

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

A 1200kg car skids with uk = 0.45. If it skids 45.2m, find the thermal energy and the initial speed.

m = 1200 kg
Vi = ?
Vf = 0 m/s (i think..because it wants to stop)
uk = 0.45 (coefficient of friction)
delta d = 45.2m

2. Relevant equations

Well for Frictional force we use Ff = uk x Fn (0.45x11760) = 5292N

3. The attempt at a solution

So I found the Fn which is 1200 x 9.8 = 11760 N

Then I found Ff which is 0.45 x 11760 = 5292 N

Then I found the work done (I don't know why thermal energy is the work can someone explain this please)

W = F x d = 5292 x 45.2 = 239198.4 J

But my textbook's answer key has a different answer for the thermal energy, it says it should be 5.3 kJ and 72km/h for the speed. How do we find the speed?

2. Oct 26, 2009

### tiny-tim

Hi bigmac!

work done = loss in energy (the work-energy equation), and nearly all the energy lost is thermal.

So they're different things, but they're equal in value.

And once you have the thermal energy, you can equate it to ∆(KE)
Are you sure it doesn't say 5.3 kN ?

Using your figure for W, I do get 72km/h for the speed.