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Homework Help: Find Power

  1. Mar 21, 2010 #1
    A motorcycle (mass of cycle plus rider = 2.5 102 kg) is traveling at a steady speed of 15 m/s. The force of air resistance acting on the cycle and rider is 1.5 102 N. Find the power necessary to sustain this speed under the following conditions.
    b) the road is sloped upward at 27° with respect to the horizontal
    m = 250
    v = 15
    t = 90
    distance = 1350
    angle = 27 degree
    The force of air resistance acting on the cycle and rider = 150
    P = ?
    ( I FOUND THE TIME AND DISTANCE ,IM POSITIVE THE VALUES ARE CORRECT SINCE I SOLVED ANOTHER PART OF THIS QUESTION WITH THE SAME VALUES)

    **I used the Formula of Power = Work/time
    Power = 150*1350* cos 27 / 90 which comes out to be 2004.76
    marked wrong . any suggestions???
     
  2. jcsd
  3. Mar 21, 2010 #2

    Filip Larsen

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    It may help to draw a diagram of the relevant forces acting on the vehicle both when it drives level (2 forces) and when it drives up the incline (3 forces). Think about how these forces relate to power for the given speed. Note, that you do not need to know distance or time when calculating this power.
     
  4. Mar 21, 2010 #3
    which are the m, g, Cofcos 27 ... correct?
     
  5. Mar 21, 2010 #4
    thats -Cof cos27, m, g. but since we r calculating Power why dont i need time or distance?
     
  6. Mar 21, 2010 #5
    this is similar to wat i made
     

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  7. Mar 21, 2010 #6

    Filip Larsen

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    Think about (or read about it in your textbook) how force and velocity are related to power. When the vehicle is travelling horizontal there is only two forces that can affect the velocity: the engine force and the drag force. Given that the vehicle is not accelerating, these two must be equal.

    The vehicle then drives up a 27 degree incline which means that gravity now enters the equation so to speak. There is now 3 forces at play that can affect velocity and again these must sum to zero for the velocity to be constant. And when you know the force the engine has to deliver and know the speed it does it with you also know the power (that is, once you are done thinking like I mentioned first in this post).

    By the way, I have some trouble understand what you write when you shorten your words or sentences too much. Please write things in as full english as you can to help others understand what you write without too many misunderstandings. The more clearly you write the more likely you are to get assistance.
     
  8. Mar 21, 2010 #7
    I apologize for the misunderstandings.
    So when the vehicle drives up a 27 degree incline, the 3 forces at play are the GRAVITY, THE FORCE * cos27 , THE COEFFICIENT which will be negative
    *** And power is directly proportional to Force and Velocity
    ***In other words, I find the Force by drawing a free body diagram in terms of x-axis and since i know the Velocity, i Multiply it by that therefore Power= FV
     
  9. Mar 21, 2010 #8

    Filip Larsen

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    That sounds correct.
     
  10. Mar 21, 2010 #9
    thank you sooooooooooooooooooo much!!! now it feels that the problem was so easy but i fried my brain over it! I appreciate your help and patience :)
     
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