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Potential energy

  1. Nov 5, 2009 #1
    1. The problem statement, all variables and given/known data

    i wasnt really sure where to put this question. its kind of a maths question i suppose

    the potential energy funtion u along the x-axis is given as

    U=-U0x2

    the question is really why do i not use the product rule on this equation as it is two variable multiplied together

    i am told that the answer is u02x
    but im a bit confused as to why you dont seem to do anythign with the first term apart from multiplied by -1 but still remains there. Is it to be treated as a constant? if so why does it not = 0 when differentiated.

    any help would be appreciated =]


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 5, 2009 #2

    Doc Al

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    Staff: Mentor

    I presume the question is to find the force given that potential function. In any case, U0 is a constant. Go ahead and use the product rule if you like; the derivative of a constant is zero.

    If y = ax2, what's dy/dx? (a is a constant)
     
  4. Nov 5, 2009 #3
    sorry ye the question to find the force. and y=ax2 will differentiate to 2x but i dont understand in the question the answer has the constant still in it after its be differentiated

    :/
     
  5. Nov 5, 2009 #4
    aaahh wait i see if i differentiate it using the product rule i do get that answer..
    i see why now

    dy/dx= da/dx.x2+2x.a but because da/dx = 0 it removes the whole of the first term.

    makes more sense now
    cheers
     
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