Knee injuries in athletes
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- Knee injuries in athletes
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Knee injuries in athletes
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[ID] => 556517
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[post_date] => 2025-01-09 07:25:01
[post_date_gmt] => 2025-01-09 12:25:01
[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
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 200,000 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 kg 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.
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[question] => According to the graph, at what time did the athlete’s feet lose contact with the ground?
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[answer] => 1
[description] => Reason for the Correct Answer:
Think about when during the jumping process a person has a maximum velocity.
After a jumper loses contact with the ground the only significant force is gravity, which causes an acceleration of approximately -10 m/s²
The maximum velocity of a person will be attained right before the person leaves the ground.
Since the person’s knees have a maximum velocity at time t=1.0 seconds, that is the moment when the person is leaving the ground. Also, because the acceleration after 1.0 seconds is -10 m/s2 we know that the person is in the air during that time accelerating only due to gravity.
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[each_answer] => A. At 1.0 seconds
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[1] => Array
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[each_answer] => B. Between 0.51 seconds and 0.65 seconds
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[each_answer] => C. At 0.5 seconds
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[each_answer] => D. At 1.5 seconds
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Between time t=0 and t=0.4 seconds, what was the upwards force exerted on the feet of the athlete by the ground?
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[answer] => 1
[description] => Reason for the Correct Answer:
Before time t=0.5 seconds the jumper has not yet started the jumping process and is simply standing up at rest. So the acceleration is zero.
Use ΣF= 0
The only other force is gravity. The mass of the athlete was 100 kg.
Let’s call F(up) the upward force. F(up)-mg=0, so F(up)=(100kg)(9.8m/s²) = 980N
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[each_answer] => A. 980 N
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[each_answer] => B. 950 N
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[each_answer] => C. 1050 N
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[each_answer] => D. 0 N
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => At time t = 1.25 seconds, what’s the best description of the motion of the athlete’s knees?
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[answer] => 3
[description] => Reason for the Correct Answer:
Moving up means positive velocity, moving down means negative velocity.
Since at 1.25 seconds the velocity is positive, the knees are moving up.
The horizontal axis on this graph represents the line where velocity is zero.
Since the graph is approaching zero velocity the knees are slowing down, and since the velocity is positive the knees are moving up.
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[0] => Array
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[each_answer] => A. The knees are moving downwards at a constant velocity.
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[each_answer] => B. The knees are moving upwards and speeding up.
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[each_answer] => C. The knees are moving upwards and slowing down.
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[each_answer] => D. The knees are moving downwards and slowing down.
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[quiz_unique_key] => 2261298308
[question] => While the athlete is in the air during his jump, how does the magnitude of the gravitational force exerted on the athlete due to the Earth compare to the magnitude of the gravitational force exerted on the Earth due to the athlete?
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[answer] => 1
[description] => Reason for the Correct Answer:
Think about Newton’s third law.
Newton’s third law says that the force exerted on object A from object B, will be equal and opposite to the force on object B from object A.
Because of Newton’s third law, the gravitational forces will have the same magnitude.
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[each_answer] => A. Both gravitational forces have the same magnitude.
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[each_answer] => B. The gravitational force on the Earth is larger because the mass of the Earth is larger.
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[each_answer] => C. The gravitational force on the athlete is larger since the mass of Earth is larger.
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[each_answer] => D. The gravitational force on the athlete is smaller since the mass of the athlete is smaller.
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[question] => According to the graph, during which time interval did the athlete’s knees experience the largest upwards force?
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[answer] => 4
[description] => Reason for the Correct Answer:
Think about Newton’s second law ΣF=ma.
Assume that F(up) is the upward force. Then, F(up)-mg = ma
Because F(up) = mg + ma, the athlete’s knees will have the largest upward force F(up) when the acceleration has the largest positive value
The acceleration has the largest positive value between 2.0
to 2.1 seconds so that is where the athlete’s knees will experience the largest upwards force.
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[each_answer] => A. From 1.0 to 1.5 seconds
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[each_answer] => B. From 0.65 to 1.0 seconds
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[each_answer] => C. From 0.5 to 0.6 seconds
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[each_answer] => D. From 2.0 to 2.1 seconds
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Kim, J. (2009). ANTERIOR CRUCIATE LIGAMENT INJURY (ACL). Sports Medicine. University of California, San Francisco.