# Thermal Energy dissapated from brakes.

1. Oct 25, 2008

### besenji

This is another one that I have given several attempts.

Calculate the thermal energy dissipated from brakes in a 1600 kg car that descends a 20 degree hill. The car begins braking when its speed is 95 km/h and slows to a speed of 30 km/h in a distance of 0.27 km measured along the road.

I tried using the conservation of energy equation.

1/2mv^2+mgh=1/2mv^2+mgh+Eloss

This doesn't seem to give me what I'm looking for.

Any help is greatly appreciated

2. Oct 25, 2008

### mgb_phys

You have the correct method.
Think of it terms of speed and kinetic energy change.
change in ke + change in pe = energy loss.

3. Oct 25, 2008

### borgwal

Just remember to use m/s instead of km/h, and make sure you get the right value for the change in height of the car.

In your equation, the v and h on the rhs should be different from those on the lhs. Don't use the same symbol for different quantities!

4. Oct 25, 2008

### besenji

Thank you soooo, much. The problem was just that I had the velocity in the wrong units.
I would have spent forever on that problem!

5. Oct 25, 2008

### borgwal

Excellent!

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