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Homework Help: Can someone tell me how they did cross product?

  1. Nov 7, 2015 #1
    1. The problem statement, all variables and given/known data

    Angular momentum is the cross product of r and mv. But why is there mvR outside of the paranthesis? And where did the v go in the second paranthesis - shouldn't the second paranthesis be (-v*sin(ωt), v* cos(ωt)). Does anyone have any idea how they did the cross product, because I'm totally lost.

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 7, 2015 #2
    Go through how vector cross products are done :

    Also, m,v, R - all being scalar ( just numbers, see they are without arrows) are multiplied dierctly and so kept outside while the main (vector) components of cross product is kept within the third bracket.

    Second paranthesis, v/R is replaced by angular velocity "ω".

    See the link for the process.
  4. Nov 7, 2015 #3


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    The cross product is bilinear. That means that for all vectors ##\vec x,\vec y,\vec z## and all numbers ##a,b##, we have
    $$(a\vec x+b\vec y)\times \vec z =a(\vec x\times\vec z) +b(\vec y\times\vec z)$$ and
    $$\vec x\times (a\vec y+b\vec z)=a(\vec x\times y)+b(\vec x\times\vec z).$$
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