Physics problem on stress?
The legs of a weight lifter must ultimately support the weights he has lifted. A human tibia (shinbone) has a circular cross section of approximately 3.6 cm outer diameter and 2.80 cm inner diameter. (The hollow portion contains marrow.)
If a 90.0 kg lifter stands on both legs, what is the heaviest weight he can lift without breaking his legs, assuming that the breaking stress of the bone is 230MPa?
I used the equation stress = Force/Area and got 575000N, but that doesn't seem to be right.
Comments
Definitely not right.
Area of one bone = 4.02 sq.cm.
2 legs =8.04 sq.cm.
Breaking force = stress x Area
Stress = 230x10^6 N/sq. meter
Let's convert to N/sq.cm.
Stress = 230x10^6 /10^4 sq. cm/sq. meter = 23000 N/sq.cm.
Breaking force = 23000 x 8.04 = 184,920 N
Subtract the lifter's weight, but that has to be converted to N.
90 kg x 9.81 = 883 N
184,920 - 884 = 184,036 N
That still doesn't look right. It's almost 19,000 Kg. I'm pretty sure nobody's legs would hold that.
I am thinking 230 MPa is way too high for breaking stress of bone,
Wikipedia says 130, which still sounds pretty high to me.