# Homework Help: Force dealing with vectors

1. May 9, 2008

### jimmyting

[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
$$F=QV \times B$$

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. May 9, 2008

### 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?

3. May 9, 2008

### tiny-tim

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)

4. May 9, 2008

### jimmyting

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