FLUID PRESSURE Or feeling a bit stressed lately???
Are you under a lot of pressure? Do you feel a lot of pressure? PRESSURE! Right now, 1000 N of pressure are pushing on the top of your head! (Enough to crush a can so why aren’t you crushed by air pressure?)
What is pressure? The result of a force distributed over an area Pressure = force = Newton area meters 2 SI unit of pressure is the pascal (Pa) 1 Pa = 1 newton/m 2
Atmospheric Pressure The atmosphere is the thin layer of nitrogen, oxygen, and other gases that surrounds Earth. Atmospheric pressure is the pressure caused by the weight of the atmosphere. Atmospheric pressure is exerted on everything on Earth, including you. Examples: at sea level - 14.7 psi or 101.3 kPA or 1 bar or 29.9 inches of Hg. 1000 N is the weight of the atmosphere.
Atmospheric Pressure The air inside this balloon exerts pressure that keeps the balloon inflated against atmospheric pressure.
In the image below, the force of the air particles hitting the inner surface of the tire creates pressure, which keeps the tire inflated.
Fluids Exert Pressure, continued Pressure and Bubbles Soap bubbles get rounder as they get bigger because fluids exert pressure evenly in all directions. Since air is a fluid, adding air to an air bubble causes it to expand in all directions at once.
Fluids Exert Pressure A fluid is any material that can flow and that takes the shape of its container. Fluids include liquids and gases. All fluids exert pressure, which is the amount of force exerted per unit area of a surface.
Fluids Exert Pressure, continued Calculating Pressure Pressure can be calculated by using the following equation: pressure force area The SI unit for pressure is the pascal. One pascal (1 Pa) is the force of one newton exerted over an area of one square meter (1 N/m 2 ).
Examples If the area of a box touching the ground is 1.5 square meters, and its weight is 2700 newtons, what pressure does the box exert on the ground? A gymnast standing on one hand (area 0.02 m 2 ) pushes down on the ground with a force of 600 N. How much pressure does the gymnast exert on the ground?
So. …….. The greater the area the force is distributed over, the less the pressure
For example: sharp knife vs dull knife Sharp knife exerts a LARGE FORCE over a small area FORCE/ area = PRESSURE Dull knife exerts a small force over a LARGE AREA Force/AREA = pressure
You do the math Sharp knife: area = 1 mm 2 Dull knife: area = 5 mm 2 Force applied to each knife = 5 N Sharp knife = 5/1 = 5N/m 2 Dull knife = 5/5 = 1N/m 2
How about? Soft Bed vs. hard floor Bed exerts force upward equal to your weight Mattress exerts force upward over your whole body Floor only pushes on pressure points which are touching the floor
Another example: Ice skating (or roller blading) A shoe distributes the force (body weight) over a much larger area than the skate So the skate exerts a much higher pressure on the ice than the shoe does The skate pushes into the ice and Voila! You can skate!
So what are fluids? Substances that flow and have no definite shape Examples: gases and liquids
What are some characteristics of pressure in fluids?
Standard atmospheric pressure = 14.7 pounds per square inch What happened to this can?
Depth effects pressure As depth increases, water pressure increases (directly proportional)
Altitude effects pressure As altitude increase, air pressure decreases (inversely proportional)
Pressure Differences and Fluid Flow Just by drinking through a straw you can observe an important property of fluids: Fluids flow from areas of high pressure to areas of low pressure.
Fluids want to... Move from areas of high pressure to areas of low pressure Everything wants to be equal
Pressure Differences and Tornadoes The air pressure inside a tornado is very low. Because the air pressure outside of the tornado is higher than the pressure inside, air rushes into the tornado. The rushing air causes the tornado to be like a giant vacuum cleaner.
Everything wants to be equal! Pressure on the outside >>>> pressure on the inside So when the can is heated, the can is crushed! Why?????