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# M2 Power.. argh! watch

1. Okay, seems like a simple enough question but can't do it..

A car is travelling along a straight road against resisitances to motion which are constant and total 2000N. When the engine of the car is working at a rate of H kW, the max speed of the car is 30m/s

a) Find a value of H:

assuming 30m/s constant speed, driving force = 2000N

so Power = Force x Velocity
H = 2000 x 30 = 60000W = 60kW

The car driver wishes to overtake another vehicle so she increases the rate of working of the engine by 20% and this results in an initial acceleration of 0.32m/s^2.. Assuming the resistances to motion remain constant..

b) Find the mass of the car:

forces no longer balanced due to acceleration, so let Driving force of engine = D;

F = m.a
D - 2000 = m.(0.32)

And then i'm totally lost.. you don't know what the initial power of the car was before the 20% increase.. and you can't work out the driving force!?
2. (Original post by Sahir)
And then i'm totally lost.. you don't know what the initial power of the car was before the 20% increase.. and you can't work out the driving force!?
Yes you do - you worked it out in part a!
3. (Original post by Helenia)
Yes you do - you worked it out in part a!
No that's the power output of the car when it's at it's MAXIMUM velocity! You can't have a 20% increase can you!?
4. (Original post by Sahir)
No that's the power output of the car when it's at it's MAXIMUM velocity! You can't have a 20% increase can you!?
Oh okay wait, or is it a badly worded question?! WELL DONE edexcel.

So if 60kW power.. + 20% increase = 72kW

Then Driving force = Power / Velocity

D = 72000 / v

what value of v should be used then?! ARGH!
5. (Original post by Sahir)
Oh okay wait, or is it a badly worded question?! WELL DONE edexcel.
Its not badly worded at all.

It clearly says:

WHEN THE ENGINE OF THE CAR is working at a rate of H kW, the max speed of the car is 30m/s

so obviously if the power goes up so does the maximum speed
6. (Original post by Lagz)
Its not badly worded at all.

It clearly says:

WHEN THE ENGINE OF THE CAR is working at a rate of H kW, the max speed of the car is 30m/s

so obviously if the power goes up so does the maximum speed
That could also imply that when the engine of the car is working at a rate of H kW, the is at its maximum speed, of 30m/s

Yeah nevermind.. whatever. Still can't do the question

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