When a person runs to the right, as shown above, which of...

When a person runs to the right, as shown above, which of the
following could be the direction of the force from the ground on
his/her foot?
(A) see image
(B) see image
(C) see image
(D) None of these are possible.

Key Concepts

Newton's Third Law

Newton's Third Law states that for every action there is an equal and opposite reaction. When a runner pushes against the ground with a backward-directed force, the ground responds with an equal and opposite force acting on the foot. This reaction force is what ultimately contributes to the forward motion of the runner.

Ground Reaction Force

When a person’s foot is in contact with the ground during running, the ground applies a reaction force that typically has both vertical and horizontal components. The vertical component supports the runner’s weight while the horizontal component, usually provided by friction, facilitates acceleration or deceleration. This concept is essential in understanding how forces are transmitted between the runner and the ground.

Friction

Friction is a resistive force that arises when two surfaces interact. In the context of running, static friction prevents the foot from sliding relative to the ground. This frictional force not only stops unwanted slipping but also provides the necessary force for propulsion by acting in the direction opposite to the relative motion between the foot and the ground.

Force Vector Decomposition

Understanding the overall force exerted by the ground involves decomposing it into its horizontal and vertical components. The vertical component counteracts the gravitational force while the horizontal component (arising from friction) provides the necessary push or traction. This method of breaking down forces helps in analyzing the actual direction and magnitude of forces during dynamic activities such as running.

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