# Strain & pressure

1. Sep 17, 2006

### asdf1

What's the difference between strain & pressure? The formulas look the same...

2. Sep 18, 2006

### Staff: Mentor

3. Sep 18, 2006

### Staff: Mentor

Strain and pressure do not have the same units.

Pressure and stress do have the same units (Force/(unit area).

Strain is a dimensionless quantity which is the (change in length)/(unit length) and has units like in/in, ft/ft, m/m, and is often expressed in terms of percentage.

Strain is (L-Lo)/Lo,

Pressure = F/A and units are N/m2, psi (lbf/in2), . . . .

4. Sep 19, 2006

### asdf1

Ahhh!!! I must be going bonkers... Sorry! I mean stress and pressure...

Stress= Force/Cross-sectional Area
Pressure=Force/Area

5. Sep 21, 2006

### PerennialII

.... are you looking for the difference between "pressure stress" (hydrostatic stress, or well, pressure) and stress (a single component of stress) in general? The former being the average of the 11, 22 and 33 components.

6. Sep 21, 2006

Stress in a point placed inside the differential area $$\Delta A$$ is defined as $$\vec{\rho}=\lim_{\Delta A \rightarrow 0} \frac{\Delta \vec{F}}{\Delta A}$$. It can have any direction, while the direction of pressure is predefined mostly; air pressure, wind pressure, hydrostatic pressure, saturation pressure, etc. So, I would say that stress is a more general term.