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: Cross product proof

  1. Dec 16, 2007 #1
    1. The problem statement, all variables and given/known data
    We know that T(t) = r'(t)/||r'(t)||, or equivalently, r'(t) = ||r'(t)||·T(t). Differentiate this equation to find r''(t), then show that

    r'(t) x r''(t) = ||r'(t)||^2 · (T(t) x T'(t)).

    2. Relevant equations
    r''(t) = ||r'(t)||'·T(t) + T'(t)·||r'(t)||

    3. The attempt at a solution
    I was able to get as far the differentiation above but from there I got stuck. I attempted to cross both sides, which i'm pretty sure needs to be done, but I'm confused how to get the right side of the equation to simplify to the desired form. Any help would be greatly appreciated...Thanks a lot
  2. jcsd
  3. Dec 17, 2007 #2


    User Avatar
    Homework Helper

    Actually I don't see any puzzle here. Are you sure this is what the question is asking for? All you have to do is to differentiate r'(t)=|r'(t)|T(t) to get r''(t)=|r'(t)|T'(t). Then simply do the cross product for r'(t) and r''(t) from the above 2 expressions. You should get the answer as shown.
  4. Dec 17, 2007 #3
    Use the fact that the cross product of a vector with itself is zero.

    Last edited by a moderator: Apr 23, 2017
  5. Dec 17, 2007 #4


    User Avatar
    Science Advisor

    No. r''(t) is not |r'(t)|T'(t). Since |r'(t)| is a function of t itself you have to use the product rule.
  6. Dec 17, 2007 #5


    User Avatar
    Homework Helper

    Oh, yeah. I kept thinking |r'(t)| could be treated as a constant.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook