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Homework Help: Calculating acceleration of an object tied to another object

  1. Oct 2, 2011 #1
    1. The problem statement, all variables and given/known data
    Two objects are connected by a string of negligible mass. The 11 kg block is placed on a
    smooth table top 1.8 m above the floor, and
    the 7 kg block hangs over the edge of the table. The 7 kg block is then released from rest
    at a distance of 0.9 m above the floor at time
    t = 0.

    Determine the acceleration of the 7 kg block
    as it descends. The acceleration of gravity is
    9.8 m/s2
    Answer in units of m/s2

    2. Relevant equations

    I have no idea.
    Maybe- F=MA

    3. The attempt at a solution
    I have no idea where to start, how to do this. I googled and found nothing. I am just lost

    To see the diagram view the picture

    Attached Files:

  2. jcsd
  3. Oct 2, 2011 #2
    F=ma is a good start. What are the forces acting on the 7kg block?
  4. Oct 2, 2011 #3
    Gravity>>-9.8 m/s2

    That means that the force is F= 7*-9.8=-68.6

    So then that force acts on the 11kg which means for that weight, -68.6=11a would be the formula. a = -6.23636. Is that the acceleration?
  5. Oct 2, 2011 #4
    Is the force just acting on the 11kg block?
  6. Oct 2, 2011 #5
    I do not understand what you are asking. The force of gravity is pulling down the weight connected to the string so it pulls the 11kg weight at that force of gravity
  7. Oct 3, 2011 #6
    But the two masses are tied together, so they are accelerating at the same rate. If you cut string, then the 7kg mass would fall with acceleration g, and the 11kg mass wouldn't move at all.
    Another way to think about it is if you draw a diagram of the forces acting on the 7kg block: you have a downward force equal to its weight, but also an upward one which is from the tension in the string.

    You're really close to the answer but you've got to remember that the weight of the 7kg block isn't just acting on the 11kg block.
  8. Oct 3, 2011 #7
    So how would I include the other weight. Would I add the masses?
  9. Oct 3, 2011 #8
    You have correctly said that the force on the 7kg block due to gravity is equal to 68.6N. But this force is pulling on the two masses, causing them to accelerate at the same rate. Sooo, using a = F/m you should be able to get an answer.
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