# Average rate of energy transfer & rate of energy dissipation.

1. Nov 11, 2007

### ihopeican

1. The problem statement, all variables and given/known data
1.) A 70kg boy runs as fast as he can up a flight of stairs which has vertical height of 10metres. He achieves this in 8 seconds. Calculate the average rate of energy transfer into gravitational potential energy that must have occured.

2.) A700kg car travelling at 20ms-1 breaks to rest in 5 seconds. Calculate the rate of energy dissipation the car must achieve.

Hello, i am not certain how to calculate these.

2. Nov 11, 2007

### catkin

3. Nov 11, 2007

### ihopeican

Hello,

i thought maybe mgh and mgh/t?

70x9.8x10?

700x9.8x(20)/5

Last edited: Nov 11, 2007
4. Nov 11, 2007

### Vidatu

E=mgh only works for vertical changes. You will use it in the first one, but not in the second one.

If I understand the question, you almost have the first one. However, it asks for the rate, which implies a time. So, you have to put the answer in the form J/s.

The second one requires a different formula, one which uses kinetic energy. See if you can find this formula; theres a handy little sticky with some of the more common formulas at the top of this page.

5. Nov 11, 2007

Hello,

1/2mv^2/t

?