# How to measure normal stress?

1. Oct 23, 2004

### chusifer

ok...my engineering professor just gave us this crazy hw problem. he wants use to design a tool to measure the normal stress in a biological tissue. now it doesnt have to be extremely detailed in any way, but...we dont even know how normal stress is measured. the problem asks for it to measure 1000 dyn/cm^2 up to 1,000,000 dyn/cm^2. now...this is obviously impossible, and he said that. he just wants us to think about the design.

so my question is....how does a tool measure normal stress?

2. Oct 23, 2004

### photon_mass

isn't that the question you're supposed to answer?

3. Oct 23, 2004

### enigma

Staff Emeritus
Measure the force. Measure the area. Divide the two.

4. Oct 23, 2004

### chusifer

well im suppose to...design some type of tool specific to a certain condition dealing with the body. im not really asking the specifics, im more just asking how does any device...like a scale, measure the normal stress?

5. Oct 23, 2004

### photon_mass

like how enigma said.
skin on a body is stretched. every seen before and after pictures of a very fat person before and after lipo? skin is stretched over the body. because it is stretched it subject to a restoring force. this is proportional to the amount your skin is stretched(clearly). how would you find out how much the skin is stretched on your body when it is still on your body?

6. Oct 23, 2004

### Pyrrhus

Well Stress is defined as

$$\sigma = E \delta D$$

where E is the modulus of elasticity of the body, and D is the deformation.

Also
$$\sigma = \frac{F}{A}$$

Do like enigma said.