Solving a Magnetic Force Problem: Finding q & r

[Seikon85]

Problem:

A positively charged particle of mass 7.21×10-8kg is traveling due east with a speed of 89.9m/s and enters a 0.290T uniform magnetic field. The particle moves through one-quarter of a circle in a time of 2.06×10-3s, at which time it leaves the field heading due south. All during the motion the particle moves perpendicular to the magnetic field. What is the magnitude of the magnetic force acting on the particle?

Work done so far:

F = ma = mv^2/r
F = B(q x v)sin90

Travels 1/4 of the circle so I figured the distance, X is (2pi*r)/4.

In order to find F(mag), need to find the two unknowns: q and r. So i figured I would try to find r first.

I used X = vt + 1/2at^2, substituted a with a = v^2/r, with X = (2pi*r)/4.

I ended up getting an unsolvable algebraic expression in terms of r as the unknown.

I know all I have to do, once r is determined is solve for q. Then I can calculate F from F = B(v x q)sin90.


Any suggestions??
Thanks!

 

[Shooting Star]

In order to find F(mag), need to find the two unknowns: q and r. So i figured I would try to find r first.

I used X = vt + 1/2at^2, substituted a with a = v^2/r, with X = (2pi*r)/4.

The particle travels with uniform speed along the arc when within the magnetic field. Equate the two F's you have written down.

There is no uniform accn here along the path, the eqns for which you are trying to use. Do you know in which direction the accn is?