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Hw can we relate conervation of momentum with newton's third law?

  1. Jun 8, 2012 #1
    Hw can we relate conervation of momentum with newton's third law??

    Hw can we relate conervation of momentum with newton's third law??.. Quantitavely and qualitatevely i want a clarification for the realation between these.. Assume an apple falling towards earth.. Thanks in advance
     
  2. jcsd
  3. Jun 8, 2012 #2
    Re: Hw can we relate conervation of momentum with newton's third law??

    Ok well it might help to imagine a situation that is "static". This means that the objects involved are not undergoing acceleration. A good example would be a rope pulling on a box that has yet to exceed its static friction.

    In that case you know that newtons first law holds so you have

    [itex] \sum F = 0 [/itex]

    now if you consider it a closed system then the only two forces acting are would be the force from the rope (f1), and the force opposing it from the static friction (f2).

    hence you have

    [itex] F1 + F2 = 0 [/itex]

    simple algera yields

    [itex] F1 = - F2 [/itex]
     
  4. Jun 8, 2012 #3
    Re: Hw can we relate conervation of momentum with newton's third law??

    I get this point but i need in terms of momentum!
     
  5. Jun 8, 2012 #4

    mfb

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    Re: Hw can we relate conervation of momentum with newton's third law??

    Forces are the time-derivative of momentum. If the time-derivative of momentum is always 0, momentum is constant (conserved).
     
  6. Jun 8, 2012 #5
    Re: Hw can we relate conervation of momentum with newton's third law??

    Also going back to my example. If [itex] F = ma [/itex] and we know that a = 0 then F must equal 0, and hence you can reach the conclusion using algebra above
     
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