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Power problem

  1. Oct 17, 2005 #1
    A motorcycle (mass of cycle plus rider = 245 kg) is traveling at a steady speed of 29.1 m/s. The force of air resistance acting on the cycle and rider is 161 N. Find the power necessary to sustain this speed if (a) the road is level and (b) the road is sloped upward at 33.7° with respect to the horizontal.

    i got that the initial KE is 1.04E5 and i know i need the final KE so i get get the Work and then divide it by the change in time to get the power... but im not sure how to go about that... help!!! please! thanks
  2. jcsd
  3. Oct 17, 2005 #2


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    If the speed is constant ("steady"), how does the initial kinetic energy compare to the final kinetic energy? That, I'm afraid, is a dead end.

    Power is the rate of using energy. Is the motorcycle using any energy to maintain a constant speed? If so, where is the energy going? Why does it have to use any?

    Think about the definition of work - that should get you in the vicinity.
  4. Oct 17, 2005 #3
    the motorcycle must be using energy in order to be sustaining the speed since there is air resistance. work is the force through a distance but we only know the force of the air resistance and we don't know a distance though
  5. Oct 17, 2005 #4


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    That is true but power is work done per unit time. What is the distance covered per unit time? :)
  6. Oct 17, 2005 #5
    29.1 m/s but i'm not sure how to use that
  7. Oct 17, 2005 #6


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    If the force is constant then the power should be force times distance per unit time! :)
  8. Oct 17, 2005 #7
    how do i find the force... we only know the force of air resistance... don't i have to find the force that propels the bike forward? i'm not sure how to find that
  9. Oct 17, 2005 #8


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    Since the speed is constant, the force that propels the bike forward exactly balances the opposing force of the wind, which you are given.
  10. Oct 17, 2005 #9
    when then that would make the net force 0 and multiplying 0 by the velocity would make the power 0??
  11. Oct 18, 2005 #10

    Doc Al

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    They want the power supplied by the motorcyle, not by the net force.
  12. Oct 14, 2008 #11
    Hi, I am working on a very similar problem right now (just dif values) and have managed to find the answer to part a but am a little confused abt part b... i know that you have to consider the additional force motorcycle's weight down the slope with respect to the angle... which would be (in this case) 245kg * 9.8 m/s^2 * cos33.7 * s(which i think would be 29.1 m given the velocity?)... a is this right? and then would i add this value to my answer for part a or does the above already consider that? or use this value in the equation p=Fv as the value of force.. confused. help appreciated; thanks!
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