9781422274828

Objects have a natural tendency to not change their state of motion. This is called inertia, and Newton’s Third Law is also known as the law of inertia. In this discussion, the unbalanced force Newton refers to is gravity. If the runner had no gravity and no friction from contact with the track, he or she could run forever. Going back to the cannonball model, if you shot a cannonball straight out of a cannon, the cannonball would keep going in horizontal motion if there were no gravity. If you dropped the cannonball straight down off a cliff (free fall with no horizontal motion and only vertical motion), it would drop straight to the ground. Horizontal motion is the distance the runner travels. Dividing that distance by the runner’s time gives the velocity, or speed.

v = d t —

Sprinters use powerful horizontal force to overcome gravity and propel themselves forward.

Gravity is acting on the runner to slow him or her down.

Think of gravity as an invisible force pulling the runner’s shoes back to the track. Only the runner’s tremendous horizontal force and motion can overcome it.

CHAPTER 1 : RUNNING

11