# Kinetic energy: car stopping, finding speed of car

1. Nov 25, 2007

### enso

nothing!

Last edited: Nov 25, 2007
2. Nov 25, 2007

### Fizzicist

W = -Fd

W = $$\Delta$$KE

W = (1/2)mv2^2 - (1/2)mv1^2

-Fd = -(1/2)mv1^2

but F = $$\mu$$s(mg)

-$$\mu$$s(mg)d = -(1/2)mv1^2

$$\mu$$sgd = (1/2)v1^2

v1 = sqrt(2$$\mu$$gd)

The flaw in your method was that you were using newton's second law, F = ma. While it's true in this case that the only force affecting the car's velocity is static friction, and so this is equal to ma, you don't know what a is, so it's necessary to formulate it differently.

Last edited: Nov 25, 2007
3. Nov 25, 2007

### hage567

Fizzicist:

It is against forum rules to provide complete solutions to homework questions. But anyway, you've make a mistake in your answer.

The rules can be found at: https://www.physicsforums.com/showthread.php?t=5374