Understanding Circular Motion: The Importance of F=(mv^2)/r in Mass Spectrometry

[v_pino]

I managed to do part a) of the question but got stuck on b). Why use F=(mv^2)/r for circular motion? And even if I use this equation, how do I find r, the radius?

Thank you
PV

 

[Kurret]

When the ions leaves the electric field, they are only affected by the magnetic field and will therefor move in a circle. Altough they will only finnish half of the circle before they hit the plate. So, if you have a semicircle, where can you find the radius of the circle?

 

[v_pino]

thanks :)