1. The problem asks: After a collision, the change in an athlete's motion is due to the application of a ___ (fill in the blank). 2. This is a physics problem related to motion and forces. 3. The key concept is Newton's Second Law, which states that a change in motion (acceleration) is caused by a net force applied to an object. 4. The formula is: $$ F = m \times a $$ where $F$ is the force applied, $m$ is the mass, and $a$ is the acceleration (change in motion). 5. Therefore, the change in an athlete's motion after a collision is due to the application of a **force**. 6. In plain language: When two objects collide, the athlete's motion changes because a force acts on them during the collision, causing acceleration or deceleration. Final answer: **force**