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# OCR Mechanics 2 - Why doesn't this method work

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1. I Was stuck on a question where I kept getting the wrong answer. I finally used a different method and got the right answer yet want to know where my first method went wrong.

The question says:
A car of mass 960kg moves along a horizontal road with its engine working at a constant rate of 20kW. Its speed at A on the road is 10m/s. Assuming that there is no resistance to motion, calculate the time taken for the car to travel from A until it's speed reaches 20m/s.

The method that worked was finding the change in KE (20^2*0.5*960 - 10^2*0.5*960) and dividing by 20,000W to get 7.2s.

But why doesn't this method work?:
p=fv 20000=F*10
F=2000
F=ma. 2000/960=2.083m/s^2
V=u + at. (20-10)/2.083 = 4.8s
2. (Original post by Bobrey)
...
works only when velocity is constant.
3. you can use the average velocity which would be halfway between 10 and 20 m/s ?
4. (Original post by Bobrey)
But why doesn't this method work?:
p=fv 20000=F*10
F=2000
F=ma. 2000/960=2.083m/s^2
V=u + at. (20-10)/2.083 = 4.8s
You can use p=fv to get the force for a given velocity But as the velocity changes, for a fixed power, so the force changes, and consequently the acceleration. So suvat isn't valid.

It's doable using p=fv, but you'll need to set up and solve a differential equation:

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