Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

My physics teacher doesn't teach Physics.

  1. Sep 10, 2006 #1
    It's true. First day of class he branded himself as a "learning assistor" and he wasn't kididng. He literally doesn't teach and requires us to read the book and learn it on our own. I have a test tomorrow, and I don't have a textbook (problems with ebay) but I do have review sheets so I have a few questions:

    Magnitude: What is the formula for calculating it?

    Given that a force extends from the origin to (120 N, -60 N) find the magnitude of the vector.

    Ok well to be honest, I have no idea how to do this problem :D I think you draw a graph and then draw the vector between 120, -60?

    Here's another one:

    Vector A has a magnitude of 3.0 units and makes an angle of -90.0 with the positive x-axis, vector B has a magnitude of 4.0 units and makes an angle of -120 with the positive x-axis. What is the direction of the vector sum of A+B referenced to the positive x-axis?

    I think the wording confuses me here. I know to draw a graph, and vector A is on the y-axis going downward (negative) and is 3 uints long. Vector B, I'm not sure which way to draw it... does it go in quadrant III or IV? Anyway, because i know its 120 degrees, that means the intersection o fA and B is 30 degrees.. but I'm not sure what hte problem means by what is the direction of the vector sum.....

    Any help would be much appreciated.
  2. jcsd
  3. Sep 10, 2006 #2

    Chi Meson

    User Avatar
    Science Advisor
    Homework Helper

  4. Sep 11, 2006 #3
    Just to add on, you will need to know your Trigonometry quite well to do these type of question. Namely Pythagorus theorem, sine and cosine rule.
  5. Sep 11, 2006 #4
    BTW vector B is in 3rd quadrant
  6. Sep 11, 2006 #5
    I have a test (today!) and I have no idea when my book will becoming so I really need to learn this. I thought about this problem more and here's what I've come up with:

    so I draw a vector from origin (0,0) to (120, -60), an dfrom there I draw a line down to complete the right triangle. So now I have hte opposite and adjacent being 120 and 60 units in length, so from here I can use sin and cosine to solve it? Or pythagorem theorem to solve it?
  7. Sep 11, 2006 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Pythagorean theorem if you're solving for the magnitude of the vector.
  8. Sep 11, 2006 #7


    User Avatar
    Homework Helper

    Create a Free Home User Trial account on


    to gain access to the Gizmo's (otherwise there is a five minute limit on their use, which is too short to learn anything). Go to the Physics Gizmo's in the Science category. Click on the Motion and Force link. You will see a Vectors Gizmo there. Launch it.

    Drag the origin of the vectors to the origin of the coordinate systems. You can also change the direction and magnitude of the vectors by dragging their heads (the cursor changes to a hand). These Gizmos are very effective learning tools.
    Last edited by a moderator: Apr 22, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook