What is Pressure? Pressure is related to the word press Pressure refers to a force pushing on a surface. For example, when you lean on a wall, you push against the wall and exert pressure on it. When you stand on the ground , the force of gravity pulls you downward. So the soles of your shoes push down on the ground and exert pressure on it.

Force and Pressure Look at Figure 1 on page 78 and lets read together how force and pressure are related. Why does the lady sink in the snow when she is not wearing snowshoes? Why doesn’t she sink when she is wearing the snowshoes? The reason is because the size of the area over which your weight is distributed is different. Your weight is distributed over a smaller area when you are not wearing snowshoes. Your weight is distributed over a larger area when you wear the snowshoes.

Calculating Pressure The relationship between force, area, and pressure are summarized in the following formula: Pressure = Force Area * Pressure is equal to the force exerted on a surface divided by the total area over which the force is exerted.

Pascal Force is measured in Newtons Area is measured in square meters The SI unit of pressure is the newton per square meter or (N/m ) This unit of pressure is also called the pascal (Pa): 1N/m = 1 Pa. When area is measured in smaller units, we use squared centimeters instead of squared feet.

Lower and Higher Pressure You can produce a lower pressure by increasing area a force acts on. You can produce higher pressure by decreasing area a force act on. For example, the blades of ice skates have a very small surface area so they exert much higher pressure on the ice than ordinary shoes do.

Fluid Pressure A fluid is a substance that can easily flow. As a result fluid is able to change shape. Both liquids and gases have this property. Examples are air, helium, water, and oil

What are fluids made up of? Fluids exert pressure against the surfaces they touch. Like all matter, fluids are made up of molecules. These molecules are tiny particles that are much too small to see with the naked eye or even a good microscope. One liter of water contains 33 trillion, trillion molecules or 33 followed by 24 zeros. In fluids, molecules are constantly moving around at high speeds. They are constantly colliding with each other and other surfaces they come into contact with.

Pressure exerted by Fluids As each molecule collides with a surface, it exerts a force on the surface. All of the forces exerted by the individual molecules in a fluid add together to make up the pressure exerted by the fluid. Fluid pressure is the total force exerted by the fluid divided by the area over which the force is exerted.

Fluid Pressure All Around If you hold out your hand in front of you, palm up, you are actually holding up a weight equivalent to a washing machine. You are surrounded by a fluid that presses down on you all the time. This fluid is the mixture of gases that make up the Earth’s atmosphere. The pressure exerted by the air is usually referred to as air pressure or atmospheric pressure.

Balanced Pressures How could your hand possibly support the weight of the atmosphere when you don’t feel a thing? In a fluid that is not moving, pressure at a given point is exerted equally in all directions. Air is pushing down on the palm of your hand with 10.13 newtons per centimeter squared. It is also pushing up on your hand with the same amount of pressure so the two pressures balance each other out and you don’t feel a thing.

Pressure inside your body Why aren’t we crushed even though the air pressure outside your body is so great? The reason is because the pressure inside your body balances the air pressure outside your body. Where does the pressure come from? The pressure inside your body comes from fluids within your body . Some parts of your body, lungs, sinus cavities, and inner ears contain air. Other parts contain cells and your blood vessels contain liquids.

Take a look… Look on page 82 at Figure 5. What does the first figure show you? What does the second figure show you? What happened to the metal can in the second picture? Why?

Variations in Fluid Pressure Does the pressure in fluids ever change? What happens to pressure as you move up to a higher elevation or down to a deeper depth within a fluid. Think about this… have your ears ever popped as you went up in an elevator? The reason for this is that air pressure decreases as elevation increases. Remember that air pressure at a given point results from the weight of air above that point. At higher elevations, there is less air above and therefore less weight of air to support.

Why do your ears pop? The reason your ears pop is because if air pressure decreases as you move up in elevation, the air pressure outside your body changes. The air pressure inside will adjust, but more slowly. For a moment, the air pressure behind your eardrums is greater than it is outside, so your body releases this pressure with a “pop” so that the pressures are once again balanced.

Pressure and Depth Fluid pressure depends on depth. The deeper you swim, the greater the pressure you feel. Water pressure increases as depth increases. As with air, you think of water pressure as being due to the weight of the water above a particular point. In addition, air in the atmosphere also pushes down on the water. Therefore the total pressure at a given point below the water results from the weight of the water as well as the air above it. In the deepest parts of the ocean, the pressure is more than 1000 times the air pressure you experience every day.