Sucking or blowing? Drawing breath Sucking on a straw Vacuuming Wind and pressure

You probably know what it's like to be under pressure - being pushed to do something, or maybe pulled in a direction you don't want to go. A force is a push or a pull, so pressure has something to do with forces. In this module you will investigate forces involved with pressure.

Sucking or blowing?
It's often said that a vacuum cleaner sucks up dirt, or that you suck a drink through a straw. Is there such a force as "sucking"? The direct answer is "no". All changes in the movement of gases, liquids and solids are the result of the application of unbalanced forces. "Sucking" in, or blowing out, requires a means of unbalancing competing forces. A common example is breathing.

Drawing breath
"I've hardly had time to draw breath" is a common phrase when someone is under pressure; what does it mean to breathe in and to breathe out? Stop to take a few breaths and observe what is happening. You will have found that as you breathe in muscles draw down on your diaphragm and your chest expands. Your ribs may move upwards and outwards, especially with a deep breath. The chest cavity gets larger and, as it is a sealed unit, air pressure inside the lungs is reduced. Air pressure from outside then forces air into the lungs. When breathing out, the lung cavity contracts and air is forced from the lungs because the pressure inside is greater than the pressure outside.

Air pressure and the lungs

Sucking on a straw
Liquid travelling up a straw uses the same process as breathing. The only difference is that the liquid is forced into the mouth by air pressure at the liquid surface. In this case the mouth is sealed by your lips around the straw. By expanding your mouth volume air pressure inside your mouth is reduced and the external air pressure pushes liquid up through the straw and into your mouth. Try it to experience the pressure changes.

Sucking on a straw

Vacuuming
A vacuum cleaner is not correctly named - strictly speaking a vacuum is a space where there are no atoms or molecules, but it does sound better than a "Low Pressure Cleaner"! A vacuum cleaner works on the same principle as you experience when breathing, except that an electric motor is used to reduce pressure inside the cleaner's bag, by spinning a fan with blades that move air out of the bag. Air pressure from outside forces air into the cleaner bag, carrying small bits and pieces from the floor or carpet, into the bag. Air passes through the filter cloth or paper and the larger dirt particles get trapped, so the dirt is safely stored and clean air expelled from the bag.

Wind and pressure
Air pressure is constantly changing in the atmosphere, and these atmospheric forces control our weather. A high pressure system often means good weather and a low brings rain or more cloudy conditions. It is when there is a change in pressures that we notice the wind the most. Where a high pressure system is followed closely by a low pressure region the air will begin to move quickly causing strong winds. The greater the difference in pressure the stronger the winds. An easy way to remember which way the winds will blow is "the winds will blow from high to low". You experience this if you have a puncture in your tyre, or if a valve leaks!

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