Terminal velocity balances forces

Steps:

• Terminal velocity occurs when gravitational force (weight) equals drag force, making net (resultant) force zero.
• Both spheres reach terminal velocity, so each has zero net force.
• Resultant force is the vector sum of all forces; zero for both means they are equal.
• Denser sphere has greater weight, so greater drag force at terminal velocity, but net force remains zero.

Why B is correct:

• By Newton's first law, at terminal velocity, resultant force is zero for any object in steady motion, so equal for both spheres.

Why the others are wrong:

• A: Drag force equals weight for each; denser sphere has larger weight, so larger drag.
• C: Terminal velocity $v_t = \sqrt{\frac{2mg}{\rho A C_d}}$ differs due to unequal masses from densities.

Final answer: B