Motion Due to Pressure: Explanation & Examples

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Learn how fluids move due to differences in pressure. Discuss some examples of this motion in real life. See how much your knowledge has moved with a quiz.

What is Fluid Motion Due to Pressure?

Everything around you is either a solid, liquid, or gas. Like the pan you use to cook with is a solid, the water in it a liquid, and (if you boil it) the steam is a gas.

Solids tend not to move very much - they have a definite shape and generally stay where they're put. Liquids and gases on the other hand are a lot more inclined to move - they're fluid. That's why a fluid is either a liquid or gas. Fluids move for many different reasons. If you pour a bucket of water over your best friend's head, it moves because of gravity. But fluids can move without any help from us at all.

Fluids move because of differences in pressure. Pressure is related to the number of molecules moving through an area. If you imagine gas molecules inside a container, the more times those molecules hit the edges of the container the higher the pressure. So the pressure is higher when:

  • if the molecules are moving faster in the container
  • if there are more molecules inside the container

So pressure is also related to density - how tightly packed the molecules are. Densely packed fluid molecules will be at higher pressure.

If a low density (low-pressure) fluid is near to a high density (high-pressure) fluid, the high density fluid will move toward the low density fluid. This will happen until the two areas have equal density - until they are balanced out. This is because molecules like to spread out and fill the available area. But this effect can lead to some interesting phenomena in the universe. In this lesson we're going to talk about a couple of examples of fluid motion due to pressure.

Example: Breathing

Breathing is an example of fluid motion due to pressure. We think of breathing as sucking the air into our bodies. This is easy to imagine because we feel ourselves breathing in. But that's not how it really works.

Lungs work by pressure

When you breathe in, what you're really doing is lowering the diaphragm at the bottom of your lungs. This increases the amount of space inside your lungs. Any air that is leftover inside your lungs spreads out to fill the larger space. This produces a low-pressure (low density) area inside them. The air outside of your body is higher pressure, and that causes the air to be pulled into your lungs. The high-pressure fluid moves to fill the space available in the low-pressure area inside your lungs. All your body actually does is move the diaphragm up and down.

Example: Weather

Another example of fluid motion due to pressure are weather currents. Predicting the weather is all about looking at air currents and how they move. The Earth's atmosphere has low-pressure and high-pressure areas. As air heats up the molecules move further apart, reducing the density of the air and creating a low-pressure area. So hot-air tends to be low-pressure, and cold air tends to be high-pressure. This can lead to air currents moving towards and away from each other.

Keep in mind that this is different than when a fluid is in a container. A container with higher temperature will have higher pressure because the molecules are contained and bouncing off the walls at a higher rate. The molecules in a free fluid however, like in the air, will simply move farther apart when heated, reducing density and pressure.

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