Constant Power

TraceBusta

A car manufacturer claims that their car can accelerate from rest to 87 km/hr in 6.3 s. The car's mass is 940 kg. Assuming that this performance is achieved at constant power, determine the power developed by the car's engine.
Ok, I solved the problem by first solving for constant acceleration. a=[v(f)-v(0)]/t
Then using that I found the Force, using F=ma. Then plugging into P=F(dot)v

The answer that i got, 87143.056W is wrong. I think it could be because the problem says it is constant power, not constant acceleration.

How can I go about solving this problem? I dont really understand the concept of constant power instead of constant acceleration

Sirus

Remember what power is. The unit for power, watts, is equal to joules per second. Power is energy transfered in a given amount of time. Can you solve the problem using energy concepts?

TraceBusta

ok, i know that Kinetic energy is = .5mv^2

so the kinetic energy at v=87 km/hr (24.167 m/s) is .5(940 kg)(24.167 m/s)^2= 11358.49 J. I dont know where to go from here.
I divided it by 6.3 sec to get 1802.93 J/s, but that is wrong. Atleast the units are right, heh.

Tide

I think you have the right idea and your numbers look right. Was there any discussion of thermodynamic efficiency in your class or your textbook?

TraceBusta

in the last lecture the prof. talked about it for a few minutes at the end. my book has some examples, but nothing related to this problem, (or so i think)

Tide

That may be a big clue! Just to get an idea is "the answer" approximately 3 times as large as the one you got?

TraceBusta

well i won't know the right answer until the computer says "OK"after i enter it. until then it just says "NO"

I'm curious, why would it be 3 times larger?

Tide

It's only a guess but thermodynamic efficiency is generally about 30%

If I might ask, how do you know the answer you got is wrong?

TraceBusta

it says "no" when i put in a wrong answer

ehild

24.167^2*940/2 = 274500

ehild

ruck101

How would you find this same type of problem, but in addition, there's a constant air resistance force? Thanks.

Sirus

Subtract work done by frictional forces from the total energy. (W=FdcosΘ)