Force as Rate of Change of Momentum

Steps:

• Recall Newton's second law: force equals the rate of change of momentum for a body free to move.
• Evaluate option A: it describes F = ma, valid only for constant mass.
• Evaluate option B: it matches the general definition F = dp/dt.
• Eliminate C and D: they relate to work and torque, not force directly.

Why B is correct:

• It aligns with Newton's second law in its most general form, F = Δp/Δt, where momentum p = mv.

Why the others are wrong:

• A: Limits to constant mass; ignores variable mass cases like rockets.
• C: Defines average force via work-energy theorem (W = Fd), not the fundamental definition.
• D: Describes torque (τ = Fd), not force itself.

Final answer: B