The figure shows the bone structure at the hip joint. The bones are normally not in direct contact, but instead are covered with cartilage to reduce friction. The space between them is filled with waterlike synovial fluid, which further decreases friction. Due to this fluid, the coefficient of kinetic friction between the bones can range from 0.0050 to 0.020. (The wide range of values is because motion such as running causes more fluid to squirt between the bones, thereby reducing friction when they strike each other.) Typically, approximately 65% of a person's weight is above the hip - we'll call this the upper weight.

Part A When a person is walking, each hip now supports up to 2.5 times his upper weight, depending on how fast he is walking. (Recall that when you walk, your weight shifts from one leg to the other and your body comes down fairly hard on each leg.) For a 61 kg person, what is the maximum kinetic friction force at the hip joint if ?k has its minimum value of 0.0050?

Part B As a person gets older, the aging process, as well as osteoarthritis, can alter the composition of the synovial fluid. In the worst case, this fluid could disappear, leaving bone-on-bone contact with a coefficient of kinetic friction of 0.3. What would be the greatest friction force for the walking person in part A? The increased friction causes pain and, in addition, wears down the joint even more.
 

Hip joint- ANA