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Homework Help: Work power and energy question

  1. Jan 14, 2008 #1
    1. The problem statement, all variables and given/known data

    A Student, starting from rest, does 2750 J of work to propel himself on a scooter across level ground. The combined mass of the scooter and the student is 68.0 kg. Assuming no friction, find...

    a) How fast is he travelling?
    b) What is his kinetic energy?
    c) If he then coasts up a hill, to what height does he rise before stopping?

    2. Relevant equations

    Ek=1/2mv^2 ???

    3. The attempt at a solution

    Not even sure where to start on this given only 3 completely different variables. We can figure out c) if we are given some sort of start on a) and possibly b).

    Thanks in advance
  2. jcsd
  3. Jan 14, 2008 #2


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    All the work he is doing should be converted into the kinetic energy. So you can find v from the equation
  4. Jan 14, 2008 #3
    Also, remember the work-kinetic energy theorem. Work is change in kinetic energy. For C, you can use conservation of energy with Kinetic all being transferred to gravitational potential. But, first you need to do a and b.
  5. Jan 14, 2008 #4
    Just note (we're trying to do this equation as you reply) that the "relevant equation" I gave is likely not hte only one that we haev to use. Others such as Eg=mgh, Mt=Mtprime, and so on, are likely to be involved.
  6. Jan 14, 2008 #5
    Heh ... thanks. I think the mentors on this site probably know what equations are relevant. :wink:

    Just think about the energy relations in this problem. The work that the student does is initially (i.e. before he reaches the hill) entirely converted to KE, so you can just equate the two. After he's rolled up the hill to a stop, all the KE (which is equal to the work he did at the start) is now converted to gravitational PE.

    You have the equations giving these quantities in terms of things you know. Can you identify the relevant equations now?
  7. Jan 14, 2008 #6
    Just to be clear: you don't have to do (a) and (b) first, since you know the amount of energy that is being converted to gravitational PE. You might as well answer them in order, though.
  8. Jan 14, 2008 #7
    Soooo, 2750J=Ek .... Ek = 1/2mv^2 .... Am I on a roll here?
  9. Jan 14, 2008 #8
    ORRR, wait a tick, another equation I have is W=delta E so... E=Ek

    W=Ekmax - Ekmin(zero)

    Sub it all in and we get approx. 9.00m/s for v .... Does that sound correcT?
  10. Jan 14, 2008 #9

    That's right.
    Now, parts b and c follow logically. All of the student's kinetic energy becomes gravitational potential.
  11. Jan 14, 2008 #10


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    Correct, now for part c you need gravitational potential energy E = m g h
  12. Jan 14, 2008 #11
    So, just to confirm then (because it sounds far too easy) Ek (the asnwer for b)) will be 2750 J ???
  13. Jan 14, 2008 #12
    sadly, yes
  14. Jan 14, 2008 #13
    OK! Thanks you guys. c) Isn't to hard just because we had done something similar already.
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