Physics Questions About Vector

  1. Vector A is 3.00 units in length and points along the positive x-axis. Vector B is 4.00 units in length and points along the negative y-axis. Use graphical methods to find the magnitude and direction of the vectors.
    Find
    A) A+B
    B) A-B

    I don't get it.
     
  2. jcsd
  3. arildno

    arildno 12,015
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    Welcome to PF!
    Post in some more detail why you don't get it.
     
  4. Well I'm taking ap physics right now and I dont get it my teacher just puts two problems on the board every day and then goes on solving them. He doesnt teach, so far I dont get any thing in physics.

    Thats a question from my textbook. Is A) 3+-4? That seems wrong. Since R = Vector of (A) + Vector of (B)

    but isnt the vector of A = 3?
    since 3cos90 = 3

    How would you solve this problem?
     
  5. Pyrrhus

    Pyrrhus 2,273
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    Use the paralelogram addition of vectors.
     
  6. I have no clue what to do? I read the chapter three times and I just dont get it. I know what hte parallelogram addition of vectors is.

    Its when you make a diagonal line between the two constants.

    Can someone solve it with an explanation for me? Since even if you say all these rules I still wont get it. I have this text book

    http://www.brookscole.com/cgi-wadsw...uct_isbn_issn=003023798X&discipline_number=13
     
  7. arildno

    arildno 12,015
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    All right, let's take it easy!
    1. Suppose you draw an arrow on a piece of paper.
    That arrow has two features:
    a) It has some length
    b) It has a direction
    You get that?
     
  8. yep, the coordinates of X would be (0,0) to (0,3)
    the coordinates of Y would be (0,0) to (0,-4)
     
  9. arildno

    arildno 12,015
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    Good!
    So the arrowhead of X lies at (0,3) and the arrowhead of Y at (0,-4)
    Do you agree with this?
     
  10. yes i got that much
     
  11. A useful property of vectors is that they remain the same vector if you move them as long as you don't change their magnitude or direction. So, you can move vector B out along the positive x axis until its "foot" is at the "head" of vector A, and it's still the same vector. Now, what would the vector be that you could draw from the origin to the tip of vector B in it's new position?
     
  12. (3,0) to (3,-4) so it would look like

    --->
    ....|
    ....|
    ....|
    ....|
    ....\/
     
  13. arildno

    arildno 12,015
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    Allright!
    At this position, both X and Y has the "foot" at the origin.
    You are asked to ADD Y to X.
    That means:
    "Slide" the foot of Y along X (without rotating Y!), such that when the foot of Y coincides with the arrowhead of X, you've got a line segment parallell to Y attached to the arrowhead of X (that line segment (the "translated Y") is of equal length as Y).
    Draw this.

    We say that the SUM of X and Y is the vector which has its foot in the origin, and its arrowhead coincident with the arrowhead of "translated Y"

    Get that?
     
  14. Looks like I was a day late and a dollar short here! Sorry for jumping in the middle of this!
     
  15. like that right? and then you draw a line? and use the pythagorean theorem

    --->
    \...|
    .\..|
    ..\.|
    ...\|
    ....\/
     
  16. arildno

    arildno 12,015
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    Precisely!
    This is the graphical way in summing two vectors together; how can you do this arithmetically if you're given the vector's components?
     
  17. a squared + b squared = c squared?
    3*3 + (-4)*(-4) = 25

    square root of 25 = 5?
     
  18. arildno

    arildno 12,015
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    The Sum of (3,0) and (0,-4) is given by:
    (3,0)+(0,-4)=(3+0,0-4)=(3,-4)
    (It seems you did this somewhere..)
    The magnitude (length) is, as you said, 5.
    What is the vector's direction (given, for example, as the angle the vector makes with the x-axis)
     
  19. 45 degrees, going southeast, Oh when they ask what is A + B they are asking for the point at which it ends
     
  20. arildno

    arildno 12,015
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    45 degrees are incorrect; think again.
     
  21. negative 45?
     
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