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Homework Help: Power & Work newb question

  1. Jul 7, 2011 #1
    1. The problem statement, all variables and given/known data
    A skydiver is falling at a terminal velocity of 45 metres per second. If her weight is 700N, at what rate is gravity doing work on her?



    2. Relevant equations
    P = W / t
    P = F * (d/t) = f*v
    v = d/t
    w = F * d



    3. The attempt at a solution

    Pretty confused at this question, this is all i got:

    v = 45m/s

    p = 45 * 700 = 31500
    31500 = 700 * (d/t)

    thats about it, i'm not even sure if i'm on the right track.

    PS, sorry for asking such a newbie question hope someone can help, thanks...
     
  2. jcsd
  3. Jul 7, 2011 #2

    rock.freak667

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    Well gravity would be cause the work done to be entirely kinetic energy. So you just need to apply that formula.
     
  4. Jul 7, 2011 #3
    i don't see how i could apply that as i dont have the mass to find out the kinetic energy gained.
     
  5. Jul 7, 2011 #4

    rock.freak667

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    You have her weight which is related to mass and acceleration due to gravity.
     
  6. Jul 7, 2011 #5
    uhm i'm not quite sure what you mean, i dont see how the 700N can relate to the kinetic energy.

    ek = 0.5 * m * v^2

    why would m = 700N? and if it does that means the gain in kinetic energy would be 708750J, where would i go from there?

    sorry but i'd need more htan a sentance or two explanation as i'm trying teach myself these things, thanks anyway.
     
  7. Jul 7, 2011 #6

    rock.freak667

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    700 Newtons is her weight, weight is related to mass 'm' by W = mg where g is acceleration due to gravity (on Earth this is 9.81 m/s2)


    You will need to calculate m from the 700 N weight.

    Her kinetic energy will be the work done by gravity since she is at terminal velocity.
     
  8. Jul 7, 2011 #7
    weight (force) = mass times gravity

    mass = weight (force) divided by gravity

    Her weight is 700N and gravity is 9.81m/s^2
     
  9. Jul 7, 2011 #8
    Okay so m = 71.4kg ( 700/9.81 )

    using this the gain in kinetic energy is 72292.5J

    i've taken a look at the answer and it's 31 500W, i thought i'd be figuring out the work done which is calculated in joules, now my head is going everywhere. :\
     
  10. Jul 7, 2011 #9

    rock.freak667

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    Sorry sorry, I initially read the question as what is the work done by gravity.

    Your initial way is correct :redface:
     
  11. Jul 7, 2011 #10
    No worries, i learnt how weight is related to mass due to gravity so thanks for that.

    Anyone have any ideas how to go on for this question then?
     
  12. Jul 8, 2011 #11

    rock.freak667

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    The same way you did it, using power = force*velocity and in this case the force = 700 N.
     
  13. Jul 8, 2011 #12
    Lol, i had the answer from the start, sure feel like a idiot now for not understanding the question. Thanks anyway :)
     
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