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    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!?
    Please help! Cheers.
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    (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!?
    Please help! Cheers.
    Yes you do - you worked it out in part a!
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    (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!?
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    (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!
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    (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
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    (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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