FLUIDS AND PRESSURE

PRESSURE

• Pressure is the force per unit area that is applied over a surface of an object.

When you walk in sand, your feet sink and walking can be difficult. If you rode your bike with narrow tires over the sand, the tires would sink deeper than your feet. Why do your tires sink deeper than your feet?What would happen if you gained weight? Would your feet sink deeper than before?

•Why is it easier to walk in snow when you are wearing skis or snowshoes than when you are wearing winter boots?

• Skis and snowshoes increase the area in contact with the snow, reducing the pressure so you don’t sink as far into the snow when you stand or walk.

CALCULATING PRESSURE

• Pressure Equation

Pressure (in pascals) = force area

• SI unit for pressure = pascal (pa)

CALCULATING PRESSURE

• A water glass sitting on a table weighs 4N. The bottom of the water glass has a surface area of 0.003m². Calculate the pressure the water glass exerts on the table.

P=f/a P=4/0.003 P= 1,333 Pa

FLUID

What is a fluid?Fluid – a substance that has no definite shape and can flow

examples: water and air

• Even though each graduated cylinder contains the same volume of liquid, the pressure exerted by the higher column of liquid is greater.

PRESSURE INCREASES WITH DEPTH

• Pressure increases with depth. Why? -The deeper you go, the more water is above you. The pressure is exerted by the weight of the water above you.

FINDING THE TITANIC

• What was a factor in locating the Titanic?• The depth of the ocean where it sank! The sonar

technology that was being used to find wrecked ships did not work very well or at all after certain depths were reached. ROVs and submarines had to be created to withstand the pressure or they would literally be crushed under water.

GOING HIGHER

• As you go higher in the atmosphere, atmospheric pressure decreases as the amount of air above you decreases. This means air pressure decreases.

WHY DO OBJECTS FLOAT?

• The buoyant force is an upward force that is exerted by a fluid on any object in the fluid. • The buoyant force pushes an object in a fluid upward, but

gravity pulls the object downward. If the weight of the object is greater than the buoyant force, the net force on the object is downward and it sinks.

BUOYANT FORCE AND SHAPE

• Buoyant force does depend on the shape of the object. The fluid exerts upward pressure on the entire lower surface of the object that is in contact with the fluid. • If the surface is made larger, then more upward pressure

is exerted on the object and the buoyant force is greater.

NOTICE THE SHAPES OF THE MATERIAL

ARCHIMEDES’ PRINCIPLE

• The buoyant force on an object is equal to the weight of the fluid it displaces. http://www.youtube.com/watch?v=ijj58xD5fDI

FLOATING AND DENSITY

• Any object with a density less than the fluid it is placed in will float.

• http://www.youtube.com/watch?v=y0SnFCs9z1g

DENSITY EXPIRAMENT

• http://www.youtube.com/watch?v=9gI8db3jPGE&feature=player_embedded

PASCAL’S PRINCIPLE

• According to Pascal’s principle, when a force is applied to a fluid in a closed container, the pressure in the fluid increases everywhere by the same amount.

PRESSURE ON FLUIDS

• Suppose a watertight, movable cover, or piston, is sitting on top of a column of fluid in a container. If you push on the piston, the fluid can’t escape past the piston, so the height of the fluid in the container doesn’t change. As a result, the piston doesn’t move. But now the force exerted on the bottom of the container is the weight of the fluid plus the force pushing the piston down. Because the force exerted by the fluid at the bottom of the container has increased, the pressure exerted by the fluid also has increased.

HYDRAULIC SYSTEM

• A hydraulic system uses a fluid to increase an input force. • The fluid in a hydraulic system transfers pressure

from one piston to another.

HYDRAULIC SYSTEM

• On the basis of Pascal’s law, various hydraulic devices are used to multiply a force. The brakes in your car are a good example of a basic piston-driven hydraulic system. When you depress the brake pedal in a car, it is pushing on the piston on a cylinder filled with hydraulic.

HYDRAULIC SYSTEM• Oxygen-bearing liquid pumped

into the inadequately developed lungs of premature babies is being used as a way to deliver oxygen but also to help the lung tissueexpand and develop.

HYDRAULIC SYSTEM

• http://www.youtube.com/watch?v=c9-g2pTrH_U

BERNOULLI’S PRINCIPLE

• According to Bernoulli’s principle, when the speed of a fluid increases, the pressure exerted by the fluid decreases. • Higher velocity = lower pressure

• http://www.youtube.com/watch?v=IV3juayaVEw

• http://www.youtube.com/watch?v=Y_iqjtK74tc

BERNOULLI’S PRINCIPLE

• Hurricanes sometimes can blow roofs from houses. When wind blows across the roof of a house, the pressure outside the roof decreases. If the wind outside is blowing fast enough, the outside pressure can become so low that the roof can be pushed off the house by the higher pressure of the still air inside. http://www.youtube.com/watch?v=L-qzOF3E_DU

BERNOULLI’S PRINCIPLE

• Because of this principle is how airplanes can fly!

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DRAG ON TRACTOR TRAILERS