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Energy Calculations please check work (thanks)

  1. Nov 5, 2006 #1
    Please someone check to see if on the right track:

    A woman lifts a barbell 2.0m in 5.0s, beginning and ending at rest. If she lifts the same distance in 10s, the work done by her is:

    I said it would be four times as much because I used 1/2mv^2. I placed the v I calculated from v=d/t into the kinetic equation. (.4)^2 (.2)^2
    .16 is four times as much as .04. Is this right.

    A 2kg block is thrown upward from a point 20m above the earth's surface. At what height above the earth's surface will the gravitational potential energy of the earth-block systme have increased by 500J.

    work:

    mgh= 2(9.81)(20m)= 392.4 --> potential
    500J= mgh
    500J = (2kg)(9.81) (h)
    h= 25m

    If someone could point me in the right direction of how to do this problem I would appreciate it. Thank you.
     
  2. jcsd
  3. Nov 5, 2006 #2

    rsk

    User Avatar

    I don't think so - I think the work will be the same - Work Done = Force x distance - taking longer means less power but the same work.

    This one I think is correct - except the actual height is 45, you need to add the 20m you started with.. When you're as close to the earth as this, the mgh formula is fine.
     
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