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Homework Help: A question on work, power and energy

  1. May 1, 2014 #1
    1. The problem statement, all variables and given/known data
    The two blocks in an Atwood machine have masses 2 kg and 3 kg. Find the work done by gravity during the fourth second after the system is released from rest.

    2. Relevant equations
    Work done by gravity= mgh

    3. The attempt at a solution
    Both the masses will have the same acceleration 'a'. Let T be the tension in the string.
    m1= 2 kg, m2= 3 kg, g= 10 m/s^2

    therefore, T- 20= 2a

    m2g- T= m2a
    therefore, 30 - T= 3a
    on solving both the equations, a= 2 m/s^2

    distance travelled by the blocks would be 1/2(at^2). So, the heavier block would travel 16 m downwards and the lighter block 16 m upwards.
    Net mass= 3-2= 1 kg
    hence, the work done should be
    W= mgh= 1 x 10 x 16 = 160J

    but the answer is 67J.
    where am I going wrong?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. May 1, 2014 #2
    You have used distance traveled in 4 seconds .Whereas you need distance traveled during the 4th second i.e between 3rd and 4th sec.
  4. May 1, 2014 #3
    Got it! Thanks! :D
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