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Conservation of Energy in Mechanics for Point Mass

  1. Mar 4, 2015 #1
    Reading "Atmospheric Thermodynamics" I'm stumped almost as soon as I've started. I've probably bitten off more than I can chew and this also might even be more of a math question than a physics one but where I'm stuck is where they "simplify" from:

    mv . dv/dt = -mgv . ez (where ez is a unit vector on the z-axis and the dots signify scalar multiplication by v)

    to

    d/dt 1/2mv . v = d/dt 1/2 mv2 = -mg dz/dt


    What I do get is that on the right we have v . ez = dz/dt, since velocity is the derivative of displacement (z) with respect to time.

    What I don't get is the operation on the left by which the d/dt is just magically pulled out, leaving the v behind to work its own kind of magic on the other v. The text simply says "This equation can be simplified:" so I guess the authors presume a reader of more skill than I currently have in terms of playing with derivatives. What is it about a derivative that allows that d/dt to be stripped of its v and yanked out in front like that? And where the heck does that 1/2 appear from?
     
  2. jcsd
  3. Mar 4, 2015 #2
    It is just the derivative
    [tex] \dfrac{dv^2}{dt} = 2 v \dfrac{dv}{dt} [/tex]
    Which comes from the rule
    [tex] \dfrac{d f(t)^\alpha}{dt} = \alpha f(t)^{\alpha-1} \dfrac{df(t)}{dt} [/tex]
     
  4. Mar 4, 2015 #3
    Thanks Matteo, that's somewhat familiar but I'm so rusty on this stuff it's not funny.
     
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