Hughcifer
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#1
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#1
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
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BTAnonymous
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#2
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#2
if you take the velocity to be positive then you must take the frictional force to be negative.
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Hughcifer
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#3
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#3
(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?
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BTAnonymous
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#4
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#4
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.
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BTAnonymous
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#5
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#5
(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?
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Hughcifer
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#6
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#6
(Original post by BTAnonymous)
what should the answer be?
I don't know, that's why i'm asking Student Room.
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BTAnonymous
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#7
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#7
(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.
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Hughcifer
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#8
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#8
(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.
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BTAnonymous
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#9
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#9
(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.
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Hughcifer
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#10
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#10
(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.
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BTAnonymous
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#11
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#11
(Original post by Hughcifer)
No sorry, Isaac Physics says it is incorrect.
last answer, 58500W
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Hughcifer
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#12
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#12
(Original post by BTAnonymous)
last answer, 58500W
Sorry, but they say that's wrong too.
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BTAnonymous
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#13
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#13
(Original post by Hughcifer)
Sorry, but they say that's wrong too.
lol, no idea then.
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Joinedup
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#14
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#14
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)?
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Hughcifer
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#15
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#15
(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!
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Joinedup
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#16
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#16
(Original post by Hughcifer)
6800N, thanks!
Watts! :facepalm:
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