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: Two Vector Problem

  1. Sep 20, 2015 #1
    1. The problem statement, all variables and given/known data

    The diagram below shows two vectors, A and B, and their angles relative to the coordinate axes as indicated.


    DATA: α=45.7°; β=55.6°; |A|=3.50 cm. The vector AB is parallel to the −x axis. Calculate the y-component of vector B.

    2. Relevant equations


    3. The attempt at a solution

    A-B is parallel to x so A_{y} must equal B_{y} ? Therefore, 3.5sin45.7 = A_{y} = B_{y} = 2.50 cm ; which is wrong. I'm a bit confused here. It tells me "Since the sum vector (AB) has no y-component, vector A must have the same y-component as vector B. As shown, 'east' is for +x, and 'north' for +y, thus the answer can be negative. " Did I mess up the calculation?
  2. jcsd
  3. Sep 20, 2015 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Why do you say it's wrong? They haven't given you a magnitude for B so you just need to choose a magnitude for B that, given its angle, makes its vertical component equal to 2.50.
  4. Sep 20, 2015 #3
    It's part of an online assignment, to which when I enter 2.50 cm it tells me it's incorrect.
  5. Sep 20, 2015 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Perhaps they really meant to ask you the magnitude of B, because if they didn't want that they wouldn't have needed to give you the angle ##\beta##. Try calculating and entering that and see if it accepts it.

    If that doesn't work, the next thing I'd try is the x component of B.
  6. Sep 20, 2015 #5
    Sorry, I should of specified. I supposed they give you β because there's other parts to the question:

    Calculate the x-component of the vector AB.

    Calculate the magnitude of the vector A+B.

    I do think it's being explicit though, in wanting the y-component rather than the vector. I could try but I have four more tries left.
  7. Sep 20, 2015 #6


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Perhaps they want a minus sign on the 2.5. The word 'component' can refer either to an unsigned vector or to a signed scalar. If they mean the latter, the answer would be -2.50 since the vertical component is downwards and the positive y direction is up..
  8. Sep 20, 2015 #7


    I still don't understand though. So is it that if the vector is 'moving downwards' in a Cartesian plane, the y-component, regardless of the quadrant it resides in, will be negative? Vice versa for 'moving upwards'?
  9. Sep 20, 2015 #8


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Yes that's right, although it would be more accurate to say pointing downwards rather than moving.

    Vectors don't have a location. A vector is just a direction and a magnitude. In fact it's a little odd that the two vectors are drawn the way they are, rather than head-to-tail as is the usual convention. I guess that in this case they didn't want to draw them head to tail because they want you to calc things about both A - B and A + B and those two give different head to tail diagrams.
  10. Sep 20, 2015 #9
    The y component of A is -2.5. The y component of A - B is yA - yB=0. Therefore, yB=yA= -2.5
  11. Sep 22, 2015 #10
    Having trouble with "Calculate the magnitude of the vector A+B. " My friends and I are stuck and what seems like a easy problem.
  12. Sep 22, 2015 #11
    What are the x- and y components of the vectors A and B?
  13. Sep 22, 2015 #12
    A_{y} = 2.5
    B_{y} = -2.5
    A_{x} = 2.44
    B_{x} = 3.65 (-3.65? it shouldn't matter since Pythagoras)

    We did:

    [tex] \sqrt{(2.44+3.65)^{2} + (2.5+2.5)^{2} } = 7.88 cm [/tex]

    Which it tells me is wrong.
  14. Sep 22, 2015 #13
    Would A+B be parallel to the y-axis?
  15. Sep 22, 2015 #14
    No. In the figure, is the x component of A positive or negative? In the figure, is the x component of B positive or negative?

  16. Sep 22, 2015 #15

    I got it now, thanks. Answer was 5.14 cm
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted