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To find tenstion in the string

  1. Dec 12, 2014 #1
    1. The problem statement, all variables and given/known data
    Two blocks of equal mass m are tied to each other through a light string. One of the blocks is pulled along the line joining them with a constant force F. Find the tension in the string joining the blocks.

    2. Relevant equations

    I tried doing this by this method.
    (I don't know how to draw free body diagram here. It is just two bodies connected by string)
    I got

    Hence, F=3ma/2
    I got T=F/3
    But the answer given at the back is F/2

    Can someone help me with this?
    Where am I going wrong?
  2. jcsd
  3. Dec 12, 2014 #2


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    Staff Emeritus
    Science Advisor

    You really need to draw a free-body diagram. You have the "front" block that has a force [itex]F[/itex] pulling it in one direction and a force [itex]T[/itex] pulling it in the opposite direction. You have the "rear" block that only has a force [itex]T[/itex] pulling it. The accelerations are the same. So just apply [itex]F_{total} = m a[/itex] for each block separately.
  4. Dec 12, 2014 #3
    I did draw a free body diagram. I don't know how to draw it here. Is there some way where I can upload it?
    I will do it again.
  5. Dec 12, 2014 #4


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    Staff Emeritus
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    No, that's okay. Just write down "F = ma" for each block. Remember that the front block has two forces acting on it--whatever is pulling the blocks (force "F") and the tension ("T", acting in the opposite direction) The second block only has "T" acting on it. So write down your two F=ma equations and post them.
  6. Dec 13, 2014 #5
    I got it.
    Thank you.
    I made the mistake of including F even for the second block

    System: Second Block(behind)
    System: First Block(Front)
    Hence, T=F/2

    Thanks, again
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