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Force dealing with vectors

  • Thread starter jimmyting
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  • #1
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[SOLVED] Force dealing with vectors

Homework Statement


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


Homework Equations


[tex]F=QV \times B[/tex]


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?
 
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Answers and Replies

  • #2
alphysicist
Homework Helper
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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?
 
  • #3
tiny-tim
Science Advisor
Homework Helper
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Since … the angle I calculated was 45, so it is 135 from the direction of the field.
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:
 
  • #4
13
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Haha, sadly Art isn't my forte, neither is music for that matter, but thank you guys for your help
 

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