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## Homework Statement

A car of mass 2000kg moves along a horizontal road against a constant resistance of manitude (P)N. The total work done by the engine in increasing its speed from 4ms^-1 to 5.5ms^-1 while it moves a distance of 60m is 30000J. Find P.

## Homework Equations

ΔE

_{k}+W

_{P}=W

_{E}

## The Attempt at a Solution

Straightforward question. The correct solution is as follows:

ΔE

_{k}+W

_{P}=W

_{E}

1/2(2000)(5.5^2-4.0^2)+60P=30000

P=262.5J

However, another solution was proposed:

The work done by the engine is 30000J and the distance it moved was 60m so the average force it exerted was 30000/60=500N

500-P=ma

finding a using v^2=u^2+2as, and then multiplying it by 2000 (m) gives ma=237.5

P=262.5N

This alternative solution gives the same answer. However, a classmate pointed out that it is incorrect because he said it assumed a constant driving force, which is a wrong assumption. I think that dividing the total work done (30000J) by the total distance moved will give the AVERAGE value of this varying driving force and the value of a is the is the AVERAGE value of this varying acceleration. Is there a flaw in this method? If so, what is it?

Thanks in advance