# Properties of fluids

1. Mar 11, 2014

### spark693

1. The problem statement, all variables and given/known data

Which of the following phases is capable of permanently withstanding a force perpendicular to its surface?

A. Gas
B. Liquid
C. Solid
D. All of the above

2. Relevant equations

3. The attempt at a solution

I understand that the answer is D.
However, what does it mean to withstand a force? if a gas is in a cylinder with a moveable piston, and then a mass is placed on the piston, the gas would be compressed. So I'm not sure what an example of a gas withstanding a perpendicular force would be.

2. Mar 11, 2014

### paisiello2

Actually, technically all matter is and must be "compressed" in order to withstand a force. The molecules in a substance (whatever physical state it is in) need to transmit the force down to it's container or supports. This is achieved by squeezing the molecules closer together since the repulsive force between atoms increases as they get closer together.

A gas is just more compressible than a liquid which in turn is more compressible than a solid. Sometimes to simplify the mathematics we assume a gas or liquid or solid is "incompressible" which simply means that the change in volume is small enough that we can safely neglect it without losing accuracy.