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Homework Help: Newton's Second Law Help

  1. Oct 16, 2007 #1
    You are pulling your little sister on her sled acroos an icy(frictionless) surface. When you exert a constant horizontal force of 110 N, the sled has an acceleration of 2.5 m/s^2. If the sled has a mass of 7 kg, what is the mass of your little sister?

    Heres how I attempted it:
    A=2.5 m/s^2
    M1= 7kg
    M2= ?

    F=(M1+M2) A
    M2= 41.2 kg

    The answer is 37 kg, but I can't figure out how to arrive at this mass. Can anyone help me?
  2. jcsd
  3. Oct 16, 2007 #2

    Doc Al

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    Staff: Mentor

    Since you know F and A, you can solve for M1 + M2. Then just subtract out M1.
  4. Oct 16, 2007 #3
    I did, but it comes out to a wrong answer:

    F= (m1+m2)a



    The answer should be 37 kg, right?
  5. Oct 16, 2007 #4

    Doc Al

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    Staff: Mentor

    That last step doesn't follow. Do this:
    F = (m1 + m2)a
    F/a = m1 + m2

    What's that equal?
  6. Oct 16, 2007 #5
    hey it works, thanks you:)
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