Which statement describes the effect of a forward-propelling ground reaction force on momentum?

Which statement describes the effect of a forward-propelling ground reaction force on momentum?

• A. Decelerates the center of mass forward
• B. Has no effect on momentum
• C. Only affects vertical motion
• D. Accelerates the center of mass forward

Answer: (D)

Momentum changes when an external force acts on a body; in gait, the ground exerts a reaction force on the body. If this reaction has a forward component, it provides forward impulse to the center of mass, speeding it up in the forward direction. That is why a forward-propelling ground reaction force accelerates the center of mass forward and increases forward momentum. The other statements either imply no horizontal impulse or only vertical influence, which ignores the forward component of the reaction force.

Momentum changes when an external force acts on a body; in gait, the ground exerts a reaction force on the body. If this reaction has a forward component, it provides forward impulse to the center of mass, speeding it up in the forward direction. That is why a forward-propelling ground reaction force accelerates the center of mass forward and increases forward momentum. The other statements either imply no horizontal impulse or only vertical influence, which ignores the forward component of the reaction force.