How Is Force Calculated on a Proton in a Magnetic Field?

[jimmyting]

[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

$$F=QV \times B$$

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?

 

[alphysicist]

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?

 

[tiny-tim]

Since … the angle I calculated was 45, so it is 135 from the direction of the field.

Hi jimmyting!

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.

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)

 

[jimmyting]

Haha, sadly Art isn't my forte, neither is music for that matter, but thank you guys for your help