Rate of potential energy loss equals weight times velocity

Steps:

• At constant speed, net force is zero, so drag force equals weight: mg.
• Power from drag equals drag force times velocity: mgv.
• This power matches the rate of losing gravitational potential energy: R = mgv.
• Solve for mass: m = R/(gv).

Why B is correct:

• It directly follows from R = mgv, the formula for power as force times velocity at terminal speed.

Why the others are wrong:

• A: Inverts the formula, yielding gv/R, which dimensions to force, not mass.
• C: v² lacks connection to energy rate or gravity, ignoring R and g.
• D: Omits g and uses 2R/v, resembling kinetic energy formulas but incorrect here.

Final answer: B