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: Vectors equations

  1. Oct 30, 2005 #1
    we have r = a + ut, r = b + vt, where a,u,b,t are all vectors and t is the variable for time. We need to show that the collision time is
    a.(b X u)/v.(b X u) where . is the dot product and X is the cross product.
    this is quite straightforward. we then need to find the time if a,b,u,v are coplanar. This means that v.(b X u) = 0, which is not allowed in our expression for the time as the denominator would go to zero. I can derive a different expression for the time using the scalar product,
    but I have a feeling this may be equivalent. I can't see why this eq'n would only apply when the vectors were coplanar.
    The question does not specify that a does not equal b and that u does not equal v. It might be that the two lines are skew and the collision never takes place (i.e time is infinite).

  2. jcsd
  3. Oct 30, 2005 #2
    so what is your question?
  4. Oct 30, 2005 #3
    i just wanted to know what the time was if the vectors were coplanar. sorry if I rambled a bit in the question.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook