# Homework Help: Work-Kinetic Energy

1. Oct 1, 2007

### chaotixmonjuish

With brakes fully applied, a 1590 kg car decelerates from a speed of 82.0 km/hr. What is the work done by the braking force in bringing the car to a stop?

I'm not really sure what to do here. I tried the whole (1590*82)/2. That didn't work.

2. Oct 1, 2007

### Mentz114

Work = energy, so the work done to stop the car is the same as its energy when the braking starts.

3. Oct 1, 2007

The work done by the brakes is the change in kinetic energy.

W=1/2*m(Vf^2-Vi^2)

That's all. Notice that final velocity is zero.

You didn't use the kinetic energy equation. First of all you need to change units, velocity should be in m/s not on km/hr and this velocity should be square.

Last edited: Oct 1, 2007
4. Oct 1, 2007

### chaotixmonjuish

I got 409655 Joules, I'm not sure if this is right.

The velocity I used was 22.77

5. Oct 1, 2007

### cristo

Staff Emeritus
That's correct (well, more or less; I used 22.78, but it depends upon your rounding).

6. Oct 1, 2007

### chaotixmonjuish

According to my lon capa it's wrong.

7. Oct 1, 2007

8. Oct 1, 2007

### cristo

Staff Emeritus
What's a "lon capa." Like I said, it depends upon your rounding and how many significant figures the answer needs to be to.

9. Oct 1, 2007

### chaotixmonjuish

It's some computer program that tolerates +/- 10%

I used 22.77, and I got 412.54 kJ