# Knee injuries in athletes

This passage will test your knowledge on Newton's laws and equilibrium.

### Problem

Chronic knee injuries to athletes can be caused by repeated shock to the knees during running and jumping. The most common of these are strains and tears of the anterior cruciate ligament (ACL) and the medial collateral ligament (MCL). It is estimated that 200, comma, 000 ACL tears occur annually. This typically happens when an athlete attempts to decelerate, stop, turn, or pivot more quickly than the knee can handle. See Figure 1.
Figure 1. A depiction of a torn ACL often seen in agility sports.
A sports podiatry research team is trying to figure out ways to help prevent ACL injuries. To investigate the forces experienced by the knees, the research team had an athlete of mass 100, space, k, g jump as high as possible off the ground. The velocity of the athlete’s knees were monitored during the entire jumping process.
During a trial, the athlete bends his knees to prepare for the jump, then pushes off the ground, moves up through the air, then falls back down and lands on the ground again. The graph below represents the vertical velocity of the athlete’s knees during a trial.
Kim, J. (2009). ANTERIOR CRUCIATE LIGAMENT INJURY (ACL). Sports Medicine. University of California, San Francisco.
According to the graph, at what time did the athlete’s feet lose contact with the ground?
Please choose from one of the following options.
Get 5 answers correct in a row