物理題目,請高人詳細解釋(急急)

1. Let q be the charges on the particle and m be its mass,
hence, electric force acting on the particle = q.E, where E is the electric field intensity
But E = (V/d),
hence, force acting on the particle = q(V/d)
When the particle travelled in a straight line, the electric force balanced the weight of the particle,
i.e. q(V/d) = mg, where g is the acceleration due to gravity
q/m = g.d/V

2. Let e be the electronic chagre and m be its mass.
Consider the vertical motion of the electron, use equation of motion:
s = ut + (1/2)at^2, with u = 0 m/s, a = eE/m = (e/m).(V/d), s = Q
i.e. Q = (1/2).(e/m).(V/d).t^2 ----------------- (1)

Consider the horizontal motion,
L = v.t, where v is the initial speed of the electron
i.e. t = L/v

substitute t = L/v into (1),
Q = (e/2m).(V/d).(L/v)^2
solve for v gives v = square-root[eVL^2/(2mdQ)]