If you want a numerical root, you can try fsolve.  fsolve(5*lambda*tan(lambda)-1 = 0, lambda)

If you want a symbolic, you can try solve. solve(5*lambda*tan(lambda)-1 = 0, lambda)

Oh, I came across the same question. Your post helps me a lot. I use following code, it gave out the right effect as you expected. I just added an option scaleto(v).

with(plots):

ball := proc (a, b) fieldplot([a, b], x = -2 .. 2, y = -2 .. 2, fieldstrength = scaleto(20)) end proc;

proc(a, b) ... end;

animate(ball, [t, t], t = -10 .. 10, frames = 30);

Hope it helps.