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Can you please help me with Newtons 3rd Law

  1. Dec 2, 2009 #1
    how do you find the Tension force,acceleration,magnitude.
    Its those questions where you have pullys,boxes pushing each other...
    I am really confused for the 3rd law
  2. jcsd
  3. Dec 2, 2009 #2
    Do you have a specific example? I'd hate to just vapidly repeat the definition of the 3rd law, which you can easily find anywhere and everywhere....
  4. Dec 2, 2009 #3
    Two masses are places in a frictionless pulley system.Solve for the acceleration and tension forces.
    1st box is 4kg and the other is 3kg

    well i know,
  5. Dec 3, 2009 #4


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    You could start by saying that there is a net, unbalanced force of 10N (For GCSE level thought - or you can do 9.81m/s/s, if you like), pulling one up and one down.
    This is accelerating both the masses (7kg total), use your formula to find the acceleration.
    The tension will be ml times (g + this acceleration) for the light mass and also mh times (g- this acceleration) for the heavy mass. Those two values should be the same, natch - as a check.
    etc etc - you can put in the numbers
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