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Energetic Question Varification & help

  1. Mar 29, 2005 #1
    hey, we were asked to run up some stairs and find the power and then convert to horsepower.
    Heres what i did. Converted my mass into newtons, and got 644.84N.
    The distance ran was 9.7m in a time of 3.7s.
    using the formula W=F*DeltaD*cos(theta) I got a total energy of 6254.95J
    Using P=W/t I got a power of 1690.53watts, and converted this into horsepower and got 2.265hp. Does this sound reasonable?
    We also had to get the height of the stares...why??? We were not asked to find GPE but im guessing it plays some roll in the question.

    Also, what would the ratio of GPE to KE look like going up the stairs? Because you are expending energy going up the stairs by increasing your GPE and you also had to use some energy to get your KE. My thought is that they would not be the same because it takes more energy to go up then down.
    Last edited: Mar 29, 2005
  2. jcsd
  3. Mar 29, 2005 #2


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    If you know the geometry (the inclination angle & the distance travelled while climbing the stairs),then u can easily find the height,using the "sin" function in a right triangle...

    Yes,the result of 2.265 horsepower sounds reasonable.

  4. Mar 29, 2005 #3
    Sorry i musnt have been clear, i meant that i DO know the hieght it was 4.7m. I was wondering why we would need to know something like this in a question asked to find power. Im guessing it has something to do with potential energy
    any ideas??? and what about the ratios? (mentioned above)
  5. Mar 30, 2005 #4


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    I dunno,u haven't posted the text of the problem exactly,u know...

  6. Mar 30, 2005 #5
    KE = PE. PE = F * h

    You need h to know how much work you did, its not enough to know you went against a force.
  7. Mar 30, 2005 #6


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    Well,he computed the work and apparently forgot to add to this work done against gravity the KE of his movement...He's given the distance & the time,so that would give him an average velocity with which he can compute the KE...

  8. Mar 30, 2005 #7
    Well he could have found the energy either using GPE or KE, they would both give the same result, but KE I believe would require some trig. GPE is much simpler :D

    PE = MGH = MG (your weight) x H (height of climb)
    PE = 644.84N x 4.7m = 3031J

    Yay for inconsistencies. You either measured something incorrectly or reported the wrong figures. Either that or you were running up a 30 degree incline.
    Last edited: Mar 30, 2005
  9. Mar 30, 2005 #8
    wouldn't you use you average velocity and then find KE, and add this energy to PE??
    and then divide that number by time to get watts?
  10. Mar 30, 2005 #9
    w= Fdcos(theta) is for a constant force or average force, i mean unless you're doing physics with calc. How did you find the force if you were running up the steps, was someone pushing you with a constant force up the stairs. I think you were probably supposed to use the graviational potential energy to find out how much power you used to get yourself to a higher gravitational potential. However, i don't really know what you were supposed to do so this is just a guess.
  11. Mar 31, 2005 #10
    read above, i already did that, im wondering about if u find GPE do u add KE to that to get total Energy?
  12. Mar 31, 2005 #11
    Theres two different energies here. The energy provided by your body and the energy provided by GPE. You gain GPE and gain kinetic energy at the expense of the energy put out by your body. I would omit this part.

    What I'm not getting is if you ascended 4.7meters, the work done should be mgh. 644.84*9.8*4.7 is 3030Nm. In 3.7s this is 819W, or about 1.1hp.
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