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Homework Help: Force dealing with vectors

  1. May 9, 2008 #1
    [SOLVED] Force dealing with vectors

    1. The problem statement, all variables and given/known data
    A uniform magnetic field of magnitude 1.47 T acts in the positive z direction.
    Find the magnitude of the force exerted by the field on a proton if the proton's velocity is <3Mm/s i + 3Mm/s j>. Answer in pN

    2. Relevant equations
    [tex]F=QV \times B[/tex]

    3. The attempt at a solution

    Being a cross product, I thought that it would just take the sin of the velocity vector. Since it is going at the same speed in x and y, the angle I calculated was 45, so it is 135 from the direction of the field. So after doing all that calculation, I get .7056. Where am I doing this wrong?
    Last edited: May 9, 2008
  2. jcsd
  3. May 9, 2008 #2


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    The magnetic field is in the z direction; the velocity is in the x-y plane. So the angle you need is the angle between the z direction and the xy plane. What do you get?
  4. May 9, 2008 #3


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    Hi jimmyting! :smile:

    Usually I'd recommend "draw a diagram"

    but I guess that it's your diagram that's misled you.

    You've got three arrows for, say v y and z, and you've correctly labelled the angles vy and yz as 45º and 90º. :smile:

    And then you've just added them, because your diagram doesn't look three-dimensional.

    hmm … take art classes … or see http://en.wikipedia.org/wiki/Perspective_(graphical) :smile:
  5. May 9, 2008 #4
    Haha, sadly Art isn't my forte, neither is music for that matter, but thank you guys for your help
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