Let a=qE/m

 

Then at any time t

x=0.5at^2

y=v_ot

 

When x=x_o

t=sqrt(2x_o/a)

y=v₀ sqrt(2x_o/a)

 

v_x=a sqrt(2x_o/a)

v_y = v_o

 

a_x=a

a_y = 0.

 

From these three you now velocity and acc of particle at x=x_o

Find magnitude of velocity (v) and component of acc perpendicular to velocity (a_per)

 

Radius of curvature r = v²/a_per