Final answer:
Newton's second law is the one that explains how the momentum of a system can change when an externally applied force is applied. It states that the net external force equals the change in momentum of a system divided by the time over which it changes.
Step-by-step explanation:
The momentum of a body can change when an externally applied force is present. The Newton's law that explains how the momentum of a system can change with an externally applied force is Newton's second law. This law, in terms of momentum, states that the net external force equals the change in momentum of a system divided by the time over which it changes. The formula for Newton's second law in terms of momentum is Fnet = Δp / Δt, where Fnet is the net external force, Δp is the change in momentum, and Δt is the change in time.
Therefore, when a force is applied to a body, the momentum changes in the direction of the applied force. To alter the momentum, either the speed or the direction of the body must change. This is essential when trying to start a body moving, speed it up, slow it down, stop it, or change its direction.