If the net external force on an object is zero, what is the change in its momentum over a given time interval?

If the net external force on an object is zero, what is the change in its momentum over a given time interval?

• A. Δp/Δt
• B. 0
• C. Δp
• D. F

Answer: (B)

Momentum changes only when an external impulse acts. The impulse-momentum relation says Δp = F_net Δt, and the rate of change of momentum is F_net. If the net external force is zero, the impulse is zero, so the momentum remains constant and the change in momentum over that interval is zero. The force is zero as given, and the rate of change would be zero as well, but the quantity asked for is the actual change in momentum, which is zero.

Momentum changes only when an external impulse acts. The impulse-momentum relation says Δp = F_net Δt, and the rate of change of momentum is F_net. If the net external force is zero, the impulse is zero, so the momentum remains constant and the change in momentum over that interval is zero. The force is zero as given, and the rate of change would be zero as well, but the quantity asked for is the actual change in momentum, which is zero.