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Homework Help: 2 boxes with energy attempt

  1. Jan 4, 2007 #1
    1. The problem statement, all variables and given/known data
    Two boxes are connected over a pulley and hel AT REST. Box A has a mass of 15kg and box B has a mass of 12kg. If the bottom of box A is originally 85cm above the floor, with what speed will it contact the floor when the system is released? Use conservation of energy and assume that friction is negligible. (Hint: Think abou the total energy of the system before and after the block A is released)


    2. Relevant equations
    ET= mgh + 1/2mv squared
    Ek=1/2mv squared
    Ep=mgh


    3. The attempt at a solution
    Before released: Eta= (15)(9.8)(.85)
    Eta= 124.95J
    After released: Etb= mgh
    Etb= 12(9.8)(.85)

    up to this point, the test says I have it write (as seen from the teachers marking)

    then this is where i somehow went wrong: 124.95-99.96= Change in Energy
    *at this point, the marker said, "B HAS Ek TOO!"

    then i did: 24.99= Change in Energy
    24.99= 1/2mv squared
    24.99= 1/2(15)v squared
    3.332m/s=v
    3.3m/s=v

    This is unfortunately not the correct answer. If anyone can assist me in where i went wrong that would be great!
     
  2. jcsd
  3. Jan 4, 2007 #2
    I think block B is still moving when A hits the floor. That's probably what your teacher was talking about (there's no floor to stop B).

    So

    [tex] U_A = K_A+K_B+U_B[/tex]

    Dorothy
     
  4. Jan 4, 2007 #3
    what is that?

    what does U stand for Dorothy?
     
  5. Jan 4, 2007 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Not only is B moving, but what is the relationship between the speeds of A and B? (They are connected by a rope!)

    A useful way to view conservation of energy is in terms of changes:
    [tex]\Delta{E} = \Delta{KE} + \Delta{PE} = 0[/tex]

    Remember: You want the change in energy of the system--both boxes.

    (Dorothy is using U to stand for potential energy.)
     
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