When you are running and want to stop quickly, you must decelerate quickly. What is the origin of the force that causes you to stop? Estimate the maximum rate of deceleration of a person running at top speed to come to rest.

Answer 1

A runner decelerating ceases to push forward when the shoes touch the ground, resulting in a backwards force applied to the shoes by the ground.

For an estimate of the maximum deceleration rate, assume the shoe in contact with the ground is skidding, and get an estimate of the kinetic friction coefficient, or assume that motion can be stopped in about one second or 5 meters. These will result in different numbers for the maximum deceleration, but they should be reasonable estimates. There is no exact "right" answer.