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Homework Help: Power\mass ratio

  1. Jul 30, 2004 #1
    how do i calculate power\mass ratio of something.
  2. jcsd
  3. Jul 30, 2004 #2


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    Science Advisor

    Divide power by mass?

    Other than that, your question is not very clear. Are you talking about a particular problem in which power is some function of mass?
  4. Jul 30, 2004 #3

    the problemo:
    The hind legs of a locust are extremely powerful!. it takes off with a speed of 3.0ms-1, the acceleration takes place in 25 milliseconds (25ms). The locusts mass is 2.5g.

    the Question states:
    What is the power/mass ratio of the locust (in J kg-1)?
  5. Jul 30, 2004 #4
    to solve this one you need to do a simple calculation.
    Power = Force x Velocity
    acceleration = v2 - v1 / t
    = (0.30m/s - 0m/s) / 0.0025s
    = 120m/s^2
    F = ma
    = (0.0025kg)(120m/s^2)
    = 0.3N
    P = FV
    = (0.3N)(0.30m/s)
    = 0.09J/s or W
    now the power is 0.09W and mass is 0.0025kg, the ratio is 36:1. Thats my calculation, if anybody finds anything wrong, please correct me, I hope Ive been a help.
  6. Jul 31, 2004 #5
    yes, thank you.
  7. Aug 1, 2004 #6

    Doc Al

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    Staff: Mentor

    The basic idea is correct. But the speed is 3 m/s, not 0.3; and the time is 0.025s, not 0.0025.
    Two problems: (1) as stated above, the speed is 3 m/s, (2) P = FV, where V = average speed, which is 3/2 m/s, not 3. So power = 0.45 J/s.
  8. Aug 4, 2004 #7
    he siad speed was 3m/s-1, I was assuming he meant to time 10^-1
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