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: A few vector problems

  1. Jan 13, 2005 #1
    ive been struggling with these for an hour and a half.
    they arent hard, i just cant seem to make them work.
    the first one is:
    A = (5,3,2)
    B = (-1,4,6)
    C = (8,2,0)

    im using this format (x,y,z)

    aA+bB+ C = a vector parallel to the y-axis.
    so, that means i need to find a vector with only z components right?
    well, the answers out of the back of the book are a = -12/7 and b = -4/7.
    these dont work out.

    heres the second one
    A = (a,1,4)
    B = (3,b,-6)
    C = (5,-2,c)

    Im supposed to find a,b, and c so that all three vectors are perpendicular to each other.
    that means i should use the dot product with A and B, A and C, and B and C and set the dot product equal to zero for all three. So I did this, and i got answers for a, b and c but they dont work out when i check them.
  2. jcsd
  3. Jan 13, 2005 #2
    if a vector is parallel to y axis, that's means the vector is pointing at y direction.... so what will the x and z component of this vector be? after you have the x and z component.. what you ganna to is write down the equation for x&z component... then you will have 2 equation, with 2 unkn..for the 2nd problem, your method is completely fine... check your algebra again
  4. Jan 13, 2005 #3
    okay, i tried this
    5a -b + 8 = 1
    3a + 4b +2 = 0
    this leaves a 1 in x, and a 0 in y

    i solved it this way, and got close to the right answer. am i on the right track?
  5. Jan 13, 2005 #4
    okay, this way should be right
    5a - b = -8
    2a + 6b = 0
    a = -1.5
    b = .5
    maybe the answers in the back of the book are wrong?
  6. Jan 14, 2005 #5
    Hi Formulajoe,

    Did you consider the questions of Vincenchan ?
    If a vector should be parallel to the y axis, what requirements can you put on the x, y, z coördinates. I'm sure you can work out the algebra to calculate the solutions, but I've got the impression you're a bit in the vague as to what you need to calculate...

  7. Jan 14, 2005 #6
    Hi formulajoe,

    you're right. Your solution is correct. Maybe the answers in your book are wrong.
  8. Jan 14, 2005 #7


    User Avatar
    Science Advisor
    Homework Helper

    If a = -12/7 and b = -4/7 then the z-component of aA+bB+C is:

    So it is not parallel to the y-axis.
    Check if you read the question correctly. If you did, then the books answer if wrong.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook