1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Is this all correct? (more bl dy vectors!)

  1. Mar 6, 2008 #1
    Is this all correct? (more bl**dy vectors!)

    I hope these are right, but I have the nagging suspicion that they're not? Where have I gone wrong (if I have indeed gone wrong)?
    1. The problem statement, all variables and given/known data
    line P has equation (x+3)/2 = y = z-1
    (a) Show that point Q (2,1,2) does not lie on line P.
    (b) Write down the parametric equations for the line.
    (c) Use dot product properties to find the co-ords of point R which is on P, such that QR is perpendicular to P.

    3. The attempt at a solution
    (a) sub (2,1,2) into the equation:
    (2+3)/2 = 1 = 2-1
    5/2 = 1 = 1
    they don't equal, therefore point Q does not lie on line P.
    Is it that simple, or do I need to use [v(P) = (-3,0,1) + t(2,1,1)] and the parametric equations such that x=2t-3=2, y=t=1 & z=t+1=2 and attempt a solution. Here it is inconsistent.

    (b) parametric equations:

    (c) dot product:
    let u = (2,1,2) and v=(-3,0,1)
    u.v = (2 1 2).(-3 0 1) |i 2 -3 |
    = det |j 1 0 | = i - 8j + 3k
    |k 2 1 |
    point R = (1, -8, 3).
    again, I feel I've skipped a step here, but it's 11.30pm and I'm too tired to think any further!

    many thanks in advance! I'll be checking again in the am.
  2. jcsd
  3. Mar 6, 2008 #2


    User Avatar
    Science Advisor

    Yes, it really is that simple!

  4. Mar 6, 2008 #3
    thanks alot for that.
    I have this problem of always thinking the solution has to be more complex than it appears to be!
    I really must go out and buy another Occam's razor. Mine's obviously too blunt and ain't working as well as it should.
  5. Mar 6, 2008 #4
    very minor quibble here: shouldn't that 4/3 be 5/3 ? :wink:
  6. Mar 7, 2008 #5


    User Avatar
    Science Advisor

    Yes, of course it should.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook