# Power Question: help please!

1. Oct 2, 2005

### wr1015

What is the average power needed to accelerate a 630 kg car from 0 to 65 mi/h in 6.0 seconds? Assume that all forms of frictional losses can be ignored.

i've calculated change in kinetic energy to be 20475 J using: 1/2mv(final) - 1/2mv(initial). Since, change in kinetic energy is equal to work, i applied it to P = W/t. The answer I am getting is wrong and i've also tried converting mi/h to m/s and the answer is still not right, what am i doing wrong??

Last edited: Oct 2, 2005
2. Oct 2, 2005

### Fermat

it looks like you used 65 mph in your calculation!

You should try to be consistent with your units.
If you're going to have the work done expressed in Joules, then use SI units all the way. Convert the speed to metric first of all.

Oh yes, the formula you used is incorrect, I'm afraid.
The KE is given by (1/2)mv² - not (1/2)mv.

3. Oct 2, 2005

### Grogs

To get joules, you're going to have to convert miles/hour to m/s. It's best to do this before you try and calculate Work. What's the final speed of the car in m/s?