Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Deriviate proof d/dt[r (v a)]= r(v a)

  1. Sep 25, 2010 #1
    Hey guys, I really do not even know how to get this question started.

    1. The problem statement, all variables and given/known data

    \frac{d}{dt}[r (v x a)] = r (v x a)

    the last a is supposed to have a period on top

    as such it is \frac{d}{dt}[r (v x a)] = r (v x \frac{d}{dt}a)

    d, v, and a are position, velocity, and acceleration

    the last a, after the = sign is d/dt of a, as mentioned above

    3. The attempt at a solution

    I do not even know how to start...I tried doing different degrees of deriviatives of both left and right sides but, it seems like I need to get ride of one unit of /s (time).
  2. jcsd
  3. Sep 25, 2010 #2
    that was an epic fail on trying to use brackets...

    derivative of [r (v times a)] = r ( v times a*)

    a* is the derivative of a.

    question asks to prove that both sides are equal.
  4. Sep 25, 2010 #3
    Simply apply the chain rule and remember that the cross product of a vector with itself is 0, and that [tex]\vec a \cdot (\vec a \times \vec b)=0[/tex] (Convince yourself this is true, because a x b is perpendicular to a, and the dot product of a vector with another vector to which it is orthogonal, is 0)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook