# Homework Help: Newton's 2nd law and Lorentz Force

1. May 13, 2009

### Dvsdvs

ok, the lorentz force equation is F=q(E+v x B) and Newton's 2nd law is F=ma. I need to combine the two to show that ma.v=qE.V
I don't really know what to do first here...
i can see the simple substitution so that ma=qE+q(v x B) but beyond that...do u change the a to dv/dt. Any type of help to get me started will work.

Last edited: May 13, 2009
2. May 13, 2009

### Staff: Mentor

Good. Now just multiply each side by v (scalar multiplication, aka dot product).

3. May 13, 2009

### Dvsdvs

nice. thank you very much. How do i show that v.(qE+(v x B)=qE.v in other words why is it that v.(V x b)=0. i know this is probably an elementary question but im not too good at working with vectors.

4. May 13, 2009

### diazona

The cross product of two vectors is always perpendicular to those vectors. So $$\vec{v}\times\vec{B}$$ is perpendicular to v (and to B). What does that tell you about the dot product $$\vec{v}\cdot(\vec{v}\times\vec{B})$$ (and why)?

5. May 13, 2009

### Dvsdvs

oh wooow i feel so stupid really. yeah so dot product = 0 b/c they are perpendicular...Also the overall question for the exercise was to show that if speed is constant. then show that E does now work along the path of the particle.

For this it means that //v(t)// is constant which is to say that
v(t).a=0 by a proposition I proved in a previous prob. Does this mean that
ma.v=0=qE.v?? and if it does this shows that E is perpendicular to v. Being perpendicular to direction of motion, is it sound to say that it does no useful work along path of particle? This is the last part of this whole thing thank you very much for help so far!!!

6. May 13, 2009

### diazona

Well, if you've shown that v.a = 0, and that v.a is proportional to E.v, then that would mean that E.v = 0 ... that's just simple math ;-)

7. May 13, 2009

Yep.
Yep.
Yep again.