Homework Help: Power and Work

1. Jun 19, 2011

wolves5

A car with a mass of 1200 kg accelerates from rest to 20 m/s in 6.0 seconds. What average power must the car produce to cause this acceleration?

a. 5 kW
b. 15 kW
c. 30 kW
d. 40 kW

For this problem, I did (1200)(9.81) and got the force. Then I used d= (6 seconds) x ((0 +20)/2) to find the distance. I got 60 meters as my distance. I then did force times distance to get the Work. Then I divided the Work by 6 seconds because thats how you get power. I got an answer that was none of the options above. What am I doing wrong?

2. Jun 19, 2011

Staff: Mentor

The weight of the car is not the force doing the acceleration (unless of course it's fallen off a cliff!).

The work done while accelerating the car goes into giving it kinetic energy, expressed as velocity. If you know the velocity at the end of 6.0 seconds you should be able to calculate the work done; what's the kinetic energy for mass 1200kg moving with velocity 20 m/s?

3. Jun 19, 2011

wolves5

The kinetic energy is 240000 J.

4. Jun 19, 2011

wolves5

Do you divide 240,000 by 6 seconds? I got 40,000 Watts when I did that which is 40 kW. So is it D?

5. Jun 19, 2011

Staff: Mentor

You've already stated that average power is given by work done divided by the time over which it is done. So you should have confidence in your result.

6. Jun 19, 2011

wolves5

So the kinetic energy or the 240,000 J was the Work?