Physics: How much is this person's femur compressed?

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SUMMARY

The discussion centers on the calculation of the force supported by a femur, emphasizing the distinction between mass and weight. The correct calculation involves using the formula for area, specifically A = π(0.02m)², which results in an area of approximately 0.001256 m². Participants highlight the importance of understanding fundamental concepts such as Hooke's law, stress, strain, and Young's modulus to accurately analyze the problem. Misinterpretations regarding units and calculations are addressed, reinforcing the need for precision in scientific computations.

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Homework Statement
A 89.0 kg person stands on one leg and 90% of the weight is supported by the upper leg connecting the knee and hip joint-the femur. Assuming the femur is 0.650 m long and has a radius of 2.00 cm, by how much is the bone compressed?
Relevant Equations
weight supported by femur=0.9(85.0kg)= 76.5kg
L= 0.650m
A=(pi)(0.02m)^2=0.1256m
Thats how far I got with my equation. Help me please.
 
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brittanyr said:
weight supported by femur=0.9(85.0kg)= 76.5kg
That's a mass, not a weight. Weight is a force.
brittanyr said:
A=(pi)(0.02m)^2=0.1256m
What is 0.022? What is (m)2?
 
In your course work, have they been talking to you about Hooke's law, stress, strain, deflection, Young's modulus and such? If not, we will need to start from first principles and notions of relevant proportionality.

Google can easily find a relevant parameter not given in the problem statement here.
 
jbriggs444 said:
In your course work, have they been talking to you about Hooke's law, stress, strain, deflection, Young's modulus and such? If not, we will need to start from first principles and notions of relevant proportionality.

Google can easily find a relevant parameter not given in the problem statement here.
Yes. We are using stress, strain, and Young’s modulus
 
haruspex said:
That's a mass, not a weight. Weight is a force.

What is 0.022? What is (m)2?
0.02^2 =0.4
 
brittanyr said:
0.02^2 =0.4
0.4*3.14= 0.1256 ... 0.1255^2 =0.2512? Correct?
 
0.2*0.2=0.04
The unit would be Meters
 
  • #10
brittanyr said:
0.2*0.2=0.04
Yes, but you want 0.02*0.02
brittanyr said:
The unit would be Meters
The metre is a unit of length. You are calculating an area. I gave you the hint in post #2. You had (0.02m)2. You (inaccurately ) squared the 0.02 but you left the m as just m.
 

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