## Homework 5

Grinsekatze
Beiträge: 35
Registriert: 09.05.2012, 18:59

### Homework 5

Hi there,

Has anyone solved example no. 2 yet? I am a little lost on the first situation (vertical force at an angle of 0° from the axis). Is it correct that there cannot be a moment (because of the force parallel to the neutral axis)? This would mean that there is no stress at all if SC lands straight on his legs (perpendicular to the floor).

steliflo
Beiträge: 1
Registriert: 25.02.2016, 08:52

### Re: Homework 5

Hey, it is true that there is no bending but there is stress in the normal direction. This stress can be calculated by the formula sigma=F/A

Grinsekatze
Beiträge: 35
Registriert: 09.05.2012, 18:59

### Re: Homework 5

Thank you! I thought so, as there has to be some effect on the bone.

smefix
Beiträge: 83
Registriert: 23.10.2012, 18:54

### Re: Homework 5

This is my 5th HW assignement.
I have now updated everything and now it should be fine.

In the previous version I forgot to write $I_2 \cdot {\alpha}_2$ at the very end, but I have used it for the calculations, so the value was correct, I just forgot to copy it.

I also corrected the signs of the stresses.

Any suggestions are welcome
Du hast keine ausreichende Berechtigung, um die Dateianhänge dieses Beitrags anzusehen.
Zuletzt geändert von smefix am 15.01.2017, 14:01, insgesamt 4-mal geändert.

randomuser
Beiträge: 2
Registriert: 07.11.2016, 16:45

### Re: Homework 5

Hey thanks for your solution. I couldn't attend the last tutorial so I'm not sure about this: shouldn't "I" be transformed by Steiner's theorem to the knee respectivley hip? Or is the usage of the radius of gyration sufficent?

kr

Grinsekatze
Beiträge: 35
Registriert: 09.05.2012, 18:59

### Re: Homework 5

I don't think we need Steiner's theorem as we consider the equilibrium at the COM.

I have another question: Shouldn't we use the generalised Hooke's Law to calculate the strain(s)? I don't think it is correct to superpose the two strains (as one is in longitudinal and the other one in transverse direction)....

smefix
Beiträge: 83
Registriert: 23.10.2012, 18:54

### Re: Homework 5

Grinsekatze hat geschrieben:I don't think we need Steiner's theorem as we consider the equilibrium at the COM.

I have another question: Shouldn't we use the generalised Hooke's Law to calculate the strain(s)? I don't think it is correct to superpose the two strains (as one is in longitudinal and the other one in transverse direction)....
Look at the slides 46+ from 09-Def_Mech_Pt2.pdf. In this example you can see, that the bending of a straight beam results in stresses which are parallel to the axis of the beam, since the beam gets compressed and elongated in its axial direction.
In the tutorial we had the same problem. We also had a bending force and a compressive force, which were perpendicular to each other. However, the resulting stresses are parallel and so we could add them.

But I forgot to draw the momentum curve. Which should be a straight line with the value $F \cdot l$ at the upper end of the bone and $0$ at the end of the rod

Grinsekatze
Beiträge: 35
Registriert: 09.05.2012, 18:59

### Re: Homework 5

Thank you for this great explanation!
You are absolutely right.