Constant power vs constant acceleration for car engine problem

[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 don't 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 transferred 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 don't 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]

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)

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]

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?

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]

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. .

24.167^2*940/2 = 274500

ehild

 

[ruck101]

Problem

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)