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Power Question

Please could I get some help with the following question:

A 1300kg car travels at a steady speed, covering 75m in 5.0 seconds. Frictional forces are constant and are 450N in total. Work out the power output of the engine, assuming 100% efficiency.

Here is my attempt:

Ke = 1/2mv^2 1/2*1300*(75/5)^2 = 146250J
w=fd 450*75 = 33750J
146250 + 33250 = 180,000J
p = e/t 180,000/5 = 36000W
(edited 6 years ago)
if you take the velocity to be positive then you must take the frictional force to be negative.
Reply 2
Original post by BTAnonymous
if you take the velocity to be positive then you must take the frictional force to be negative.


Thanks, but I have tried this and:

146250-33750 = 112500 J
112500/5 = 23000 W

Comes up as incorrect, am I still doing it wrong?
the frictional force = -450 and the distance covered by that constant force is 75m. work out the work done by the frictional force and then subtract it from the kinetic energy of the car. then divide this new energy by the change in time.
Original post by Hughcifer
Thanks, but I have tried this and:

146250-33750 = 112500 J
112500/5 = 23000 W

Comes up as incorrect, am I still doing it wrong?


what should the answer be?
Reply 5
Original post by BTAnonymous
what should the answer be?


I don't know, that's why i'm asking Student Room.
Original post by Hughcifer
I don't know, that's why i'm asking Student Room.

so how do you know 23000W is incorrect? I got 22500W but I'm assuming you rounded up.
Reply 7
Original post by BTAnonymous
so how do you know 23000W is incorrect? I got 22500W but I'm assuming you rounded up.


This question is from Isaac Physics, they tell you whether your answer is right or not, but don't tell you the answer or how to get it.
Original post by Hughcifer
This question is from Isaac Physics, they tell you whether your answer is right or not, but don't tell you the answer or how to get it.

oh right. have you tried 65,250? if it's correct, I'll work through it.
Reply 9
Original post by BTAnonymous
oh right. have you tried 65,250? if it's correct, I'll work through it.


No sorry, Isaac Physics says it is incorrect.
Original post by Hughcifer
No sorry, Isaac Physics says it is incorrect.


last answer, 58500W
Reply 11
Original post by BTAnonymous
last answer, 58500W


Sorry, but they say that's wrong too.
Original post by Hughcifer
Sorry, but they say that's wrong too.


lol, no idea then.
Power = Work done / time

Work done = Force * distance

---
in one second the car travels 75/5 meters against a constant force of 450N
so what is the work done per second (i.e. power)?
Reply 14
Original post by Joinedup
Power = Work done / time

Work done = Force * distance

---
in one second the car travels 75/5 meters against a constant force of 450N
so what is the work done per second (i.e. power)?


6800N, thanks!
Original post by Hughcifer
6800N, thanks!


Watts! :facepalm:

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