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Homework Help: Find the acceleration in the string

  1. Dec 29, 2005 #1
    http://img514.imageshack.us/img514/2535/test4uv.th.jpg [Broken]

    The 1st block has a mass of 10kg and the 2nd block has a mass of 20kg..

    I need to find the acceleration in the string when a 50 N force is applied to the 2nd block...

    I was thinking to find the acceleration I would do 50 N = (30kg)a
    a = 1.6 but according to another problem that I looked at the acceleration would be 5.0 so now I'm confused for the correct way to find it.
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Dec 29, 2005 #2


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    I don't know what you mean by "acceleration in the string" and I think you've left something out. Also, we have no way of comparing the present problem with the other one you mention and, therefore, have no basis for judging whether the solution of the other problem has any relevance to this one.

    Please elaborate.
  4. Dec 29, 2005 #3
    it looks like he means that he wants to know the acceleration of the system after the 50 N force has been applied to the string?

    it's kind of unclear.

    what's the second problem like?
  5. Dec 30, 2005 #4
    Shouldn't there be the assumption that both objects have the same acceleration a ?

    Newton's second law for the composite system then yields
    [tex] 50 = (10 + 20) a[/tex]
    [tex] a = \frac{5}{3} \frac{m}{s^2}[/tex]

    Newton's second law for the left block yields
    [tex] 10a = T[/tex]
    [tex] \frac{50}{3} N = T[/tex]

    Newton's second law for the right block yields
    [tex] 20a = 50-T[/tex]

    Which again must give the correct same T
    [tex] T = \frac{150}{3} - \frac{100}{3} = \frac {50}{3} N[/tex]

  6. Dec 30, 2005 #5
    He has done that. May be he is asking about finding some thing else. He needs to state the problem properly so we can stop guessing.
  7. Dec 30, 2005 #6
    Yeah, sorry I was talking about the acceleration for the system.
    That second problem I was talking about was really weird and I think I might of screwed up on that.

    I can understand what marlon did,
    thanks all
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