1. External Pressure on Fluids

2. Pressure
Pressure is the amount of force applied over a given area on an object.
When pressure is applied to matter, compression can result.
Compression is a decrease in volume produced by a force.
The tennis racket applies a force to the ball. The resulting pressure causes the ball to compress.

3. Gases Are Compressible
A gas can easily be compressed because there is a large amount of space between its particles.
Gas that is trapped in a container and heated will increase in pressure
Heat causes the particles to move faster - these fast moving particles bounce off the sides of the container
The increased pressure could cause the container to explode.
Gas that is trapped in a container and cooled will decrease in pressure.
The decreased pressure could cause the container to implode.
This metal can has imploded as the gas inside is cooled.

4. Liquids and Solids Are Very Difficult to Compress
The particles of liquids and solids are already so tightly packed together that squeezing them together is almost impossible.
Solids and liquids are described as incompressible
When force is applied to the bottle, the gas particles move closer together. The gas is compressed into a smaller volume.
A bottle filled with liquid
When force is applied to the bottle, the liquid does not compress. There is no room for the liquid particles to move closer together
A bottle filled with gas

5. Compression and Deformation
Solids can appear to be compressed if the “air spaces” in the material are compressed.
An example would be squishing (compressing) a marshmallow.
Solids can also appear to be compressed when they are deformed
Deformation means to change shape without being forced into a smaller volume.
Example: a ball hitting a solid surface
As the ball deforms, it stores elastic energy which makes it bounce back upwards
The player’s face and the ball are temporarily compressed and deformed.

7. Comparing Pressure
Pressure depends on both the amount of force and also the area the force acts upon.

8. Fluids Under Pressure
Fluids always move from high pressure to low pressure
Fluids under pressure and compressed gases are used for a variety of everyday tasks
Air molecules pulled by gravity = atmospheric pressure
Air pressure increases as altitude decreases
The more air there is above, the more it
compresses the air molecules below
Therefore, air pressure is lower (less dense) at high altitudes.
When humans change altitude, our bodies try to equalize the pressure differences by having our ears “pop” (fewer air particles pressing from outside our body)

9. Pressure Differences
Fluids will always attempt to move from high pressure to low pressure
When we drink with a straw, we first remove air from the closed straw, which lowers the pressure inside. The atmosphere, having a higher pressure, then tries to get into the straw, and pushes the fluid up and out of the way to try to get into the straw!
This same idea is used for many purposes, including hydraulics, water rockets and dental tools.

10. Changing Temperature
Changing temperature of a fluid can affects its volume.
In general, as a fluid gets colder, it contracts (its volume decreases.)
As a fluid gets warmer, it expands (its volume increases.)
Changing the temperature of a fluid can also affect pressure.
As a fluid gets warmer, the particles move faster.
In a sealed container (where the volume cannot change), the particles hit the container walls with more force, and pressure increases.
When temperature drops, the particles move more slowly.
The particles collide less with the container, and the pressure decreases.