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Confused on MEI M2 Work Energy Power question... watch

    • Thread Starter

    Ok heres there Question:

    A cyclist of mass 84kg is travelling, initially, at a steady speed of 5m/s for 100m against a constant resistance of 49N
    (i) find the work done against resistance.
    Work done = Fd = 49*100 = 4900Nm
    She now travels at a steady speed of 5m/s up a constant slope at an angle arcsin(  \frac {1}{21} ) to the horizontal, there is still a constant resistance of 49N
    (ii) show power is 441W
    resolved forces to get  (84g* \frac {1}{21}+49)*5 = 441
    Travelling at 5m/s she now comes to another horizontal section and maintains her power, after three seconds her speed is 6m/s
    (iii) explain why the driving force is not constant
    fv=p, if p is constant and v changes, so must f
    (iv) calculate the total work done against the resistance to motion during the three seconds
    heres where I need help
    this is what i did...
     P = fv = \frac {w}{t}
    therefore 441*3 = 1323, but the answer in the back is 861... What am I doing wrong?
    • Study Helper

    Study Helper
    (Original post by Tim_Lehner)
    You've worked out the total work done in the 3 seconds which is the sum of the work done against resistance PLUS the work done in accelerating from 5 to 6 ms-1. So if you subtract the latter (change in K.E.) you'll end up with the correct answer.
    • Thread Starter

    cool, thanks
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Updated: February 9, 2010

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