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Question on Newton second's law

  1. Oct 11, 2008 #1

    This is my first post on the forum, I hope that I will get an answer for my question as soon as possible. :rolleyes:


    Thanks in advance
  2. jcsd
  3. Oct 11, 2008 #2


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    Dearly Missed

    Well, what have you done so far? :smile:
  4. Oct 11, 2008 #3

    Did you understand my question ?
    because it is translated and my english is not good enough .

    My try:
    Total Mass = 48 kg ==> 480 Newtons

    a = 4.79 m/s^2

    F = 32(10-4.79) = 166 newtons

    I know it is wrong but after I read the sticky topic I saw that I should put my try

    help me please
  5. Oct 11, 2008 #4


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    There is another force acting on the boy+seat which you are not accounting for. Look at the picture carefully to see what that might be.

    Also, be careful about direction (upward or downward) and +/- signs. I suggest using + for the upward direction, and - for downward direction.
  6. Oct 12, 2008 #5
    I couldn't find the third foce :cry:
  7. Oct 12, 2008 #6


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    In addition to the downward force of weight (gravity):

    One end of the rope is attached to the scale, pulling upward.

    The other end of the rope is attached to the boy+board.
  8. Oct 12, 2008 #7
  9. Oct 12, 2008 #8

    Doc Al

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    In your FBD for the board: You have the force "b" acting upward, but the boy exerts a downward force on the board.

    In your FBD for the boy: Again, the direction of force "b" is backward. The board pushes up on the boy. Also, you neglected the force on the boy due to the rope he's holding.
  10. Oct 12, 2008 #9
    Thank PF members especially Doc Al

    I solved it and I checked the answer from the final answer that is provided under the question and it is correct
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