# Center of Mass question (help)?

1. Mar 25, 2009

### driftk

I tried this problem but only managed to somewhat understand Part A. But that answer is wrong as well, so can someone guide me through steps of finding the answer: Thank you.

To keep the calculations fairly simple, but still reasonable, we shall model a human leg that is 0.92 m 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 Kg person, the mass of the upper leg would be 8.60 Kg, while that of the lower leg (including the foot) would be 5.25 Kg.

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

- i got 0.35 as my answer but it was wrong. My method was this: (5.25*0.92)/ total mass of 13.85. What am I doing wrong?

B. 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.

Last edited: Mar 25, 2009
2. Mar 25, 2009

### LowlyPion

The Center of Mass is the ∑ m*x/ ∑ m

What they want you to do is calculate the CoM using both the two leg parts, upper and lower. Since they say that each piece is uniform, then you can assume that the CoM's of each piece are located half way on each part.