Forces in Equilibrium on Charged Oil Drop

Steps:

• Charged oil drop in electric field between parallel plates experiences electrostatic force $\vec{F_e} = q\vec{E}$.
• Drop has mass, so gravitational force $\vec{F_g} = mg$ acts downward.
• Stationary drop implies net force is zero, requiring balance of electric and gravitational forces.
• Both forces must act to achieve equilibrium.

Why A is correct:

• Newton's first law requires balanced forces for stationary motion; electric force opposes gravity exactly.

Why the others are wrong:

• B: Gravity acts on all massive objects, cannot be ignored.
• C: Charged drop in electric field experiences electrostatic force.
• D: Both forces are present and necessary for equilibrium.

Final answer: A