Question

In-depth analyses A. We are going to think a lot about the knees of a 70-kg athlete. 1. First, Estimate the amount of upward force that each knee must produce when this athlete is standing in place. Explain your reasoning 2. Estimate the amount of upward force that a knee must produce when this athlete supports his entire weight on one leg. Describe the types of situations when an athlete might be doing this 3. There are many situations in which an athlete's knees would need to produce even more upward force. Describe some of these. 4. Estimate the maximum static force of friction in this athlete's knee when he is supporting his entire weight on one knee. Show the equations and/or reasoning that you used to make this estimate. 5. Compare this force of friction to the maximum static force of friction that occurs when you try to slide a 70-kg wooden box across a wooden floor. What mass of wooden box would give approximately the same force of static friction as the knee in this situation?

Answer 1

1. The weight of athlete W = 70×9.81 = 686.7N

This weight is divided equally on both knees i.e. 686.7÷2=343.4N

Hence upward force produced by each knee=343N

2. When the athlete supports his entire weight on one leg, then the upward force produced by that knee =686.7N

3. When athlete take up, accelerate or jump his knees produce more upward force.

4. The static force of friction should be able to balance the total weight of athlete on one knee, and the maximum normal force on one knee is 686.7N hence the maximum static force of friction should be always less than 686.7 N