A ball having a mass of 100 g and a charge of 4.9 × 10^ -5 C is...

The forces acting on the ball are the gravitational force acting downwards and the electric force acting horizontally.

Gravitational force is $F_g = mg = 0.1 \times 9.8 = 0.98 \text{ N}$.

Electric force is $F_e = qE = 4.9 \times 10^{-5} \times 2.0 \times 10^4 = 0.98 \text{ N}$.

Both forces are constant in magnitude and direction, so their resultant force is also a constant vector.

Since the ball is released from rest, its initial velocity is zero. Under the action of a constant resultant force with zero initial velocity, the acceleration is constant and the particle moves in a straight line along the direction of the resultant force.