Question

If a car generates 18hp (which is 13428Watts) when traveling at a steady speed of 88km/h (which is 24.44m/s), what must be the average force exerted on the car due to friction and air resistance??

I have worked this out correctly but I don't understand the logic final answer! Help please?

Answer 1

So I worked it out as follows...

\[P= \frac{ W }{ t } = \frac{ Fd }{ t } = Fv\]

So...

\[F = \frac{ P }{ v } = \frac{ 13428 }{ 24.44 }\]

Therefore, \[F = 549.43N\]

Answer 2

ok thats good!

Answer 3

This answer of 549.43N is correct, however, as I was calculating it I took this to be the TOTAL FORCE EXERTED BY THE CAR TO MOVE. I was then thinking that I'd have to minus some value from this to get the Force exerted by friction/air resistance but apparently not.

Can someone please explain how this final answer is the friction/air resistance and not the Force exerted by the car to move.

Answer 4

That's right..but note that at steady / constant speed there is no net force.. the 549.43 N goes into overcoming the frictional and drag forces. Hence no acceleration.

Answer 5

well imagine this way

if there was no friction or air resistance.. and you would put that much force.. would the speed remain same?

Answer 6

But if the force exerted by the car is this 550N (rounded) and the friction is also this value, then that's an equal and opposite force. Shouldn't the car be stationary?? I thought that the force exerted by the car had to be greater than that of friction in order for the car to move.

Answer 7

no no..

you see always remember... if there is a NET force.. there must be a NET acceleration
if not NET force.. no acceleration (doesn't mean the object would be stationary).. the object will simply keep moving at constant speed.. remember inertia!? newton's first law?

Answer 8

Oh right! Got it. The whole 'net force' thing was confusing me. Thank you!

Answer 9

glad i could help :)