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Math Help - Energy

  1. #1
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    Energy

    A man on a motorcycle starts from rest from a point A at the bottom of one hill and travels to a point B at the bottom of another hill. The total distance of the journey is 120 km. The resistance to motion is constant at 300 N throughout the journey. Point A is 1500m higher than point B.

    The total mass of the man and his motorcycle is 240kg. When the motorcyclist reaches the point B, his speed is 90 km/h. Calculate the work done by the motorcycle during the journey.


    OK basically my tutor has left me with this equation.



    Total energy at (t=t) = Total energy at t=0

    mgh + \frac{1}{2}mv^2 = mgh + \frac{1}{2}mv^2

    ok when t=0

    v=0

    m=240000

    h= I dont know what h is!!!! is this 1500??

    When t=t

    v = 90 km/h (does this need changing to mph?)

    m = 240000

    h = 0?????


    I also know the equation work = force*distance
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  2. #2
    Junior Member rubix's Avatar
    Joined
    Apr 2009
    Posts
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    your tutor left you with Energy Equation:

    mgh_1 + \frac{1}{2}mv_1^2 = mgh_2 + \frac{1}{2}mv_2^2


    hope you notice that in this problem:

    1) the mass is constant

    i.e. mass (m) = 240kg

    { there is no need to change this to grams (it will only complicate if you do so), so leave it in kg...see #4 }

    2) you can also assume that gravity (g) is constant

    i.e. g = 9.8 m/s^2

    3) Point A is 1500m higher than point B.

    i.e. Distance A - Distance B = 1500 m

    For your convenience assume B is ground level. So, h2 would be 0 and h1 would be 1500 m as you guessed

    4)) The resistance to motion is constant at 300 N

    since, 1 N = 1 kgm/s^2

    let us keep everything consistent to that unit.

    i.e. keep mass in kg, distance in m, and time in seconds.

    keeping that in mind, you should change velocity to m/s^2


    edit: i think you should visit http://www.physicshelpforum.com/physics-help/ for further help. Afterall, it is a physics problem.
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