I need help with part D, I got the answer -8.72 but it was incorrect. To keep th

Question

I need help with part D, I got the answer -8.72 but it was incorrect.

To keep the calculations fairly simple, but still reasonable, we shall model a human leg that is 92.0 cm long (measured from the hip joint) by assuming that the upper leg and the lower leg (which includes the foot) have equal lengths and that each of them is uniform. For a 70.0 kgperson, the mass of the upper leg would be 8.60 kg , while that of the lower leg (including the foot) would be 5.30 kg . Take that x-axis is directed horizontally and the y-axis is directed vertically downward.

Part A

Find the x-coordinate of the center of mass of this leg, relative to the hip joint, if it is stretched out horizontally.

40.5

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Correct

Part B

Find the y-coordinate of the center of mass of this leg, relative to the hip joint, if it is stretched out horizontally.

0

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Correct

Part C

Find the x-coordinate of the center of mass of this leg, relative to the hip joint, if it is bent at the knee to form a right angle with the upper leg remaining horizontal.

31.8

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Correct

Part D

Find the y-coordinate of the center of mass of this leg, relative to the hip joint, if it is bent at the knee to form a right angle with the upper leg remaining horizontal.

40.5

Explanation / Answer

It is given that Y-axis is vertically downward. So the Center of masses are at co-ordinates C1(23,0) for upper thigh and C2(46,23) for lower leg. SO center of mass vertically would be at y= {(8.6x0) + (5.3x23)} / 13.9 = + 8.77 cm.

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