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Mechanics (chain rule)

  1. Sep 28, 2008 #1
    1. The problem statement, all variables and given/known data


    6. A particle of mass m moves along a frictionless, horizontal plane with a speed given by

    v(x) = α / x. Where x is the distance of the object from the origin and α is a constant.

    Working with F = ma, we want to get the acceleration. You have v = v(x). You want a = dv/dt. Find (dv/dx)(dx/dt). Find the force F(x) to which the particle is subjected to.




    3. The attempt at a solution

    I guess my problem is I dont understand why I need to use chain rule since v = dx/dt. I thought I could take the derivative in respect to t on both sides, and get dv/dt = - α / x^2, then multiply both sides by m to get the force equation.

    the answer is given, -m α^2/ x^3. So can someone explain what I'm missing here....
     
  2. jcsd
  3. Sep 29, 2008 #2

    Kurdt

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    You need to use the chain rule because x is some function of t. What you have done above is find dv/dx. Now you have correctly identified dx/dt as v and you know v = a/x, so what is (dv/dx)*(dx/dt)?
     
  4. Sep 29, 2008 #3
    wow awesome thanks that clears up a lot. I almost gave up on anyone answering me. I read that there was a way to close a thread or say that the problem is solved but I'm not seeing it on here so I guess'll have to leave it as is.
     
  5. Sep 29, 2008 #4

    Kurdt

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    The forum software was upgraded recently and I think only mentors can mark it solved at the minute. Just leave it as it is for now. :smile:
     
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