Forces in Fluids: P R E S S U R E Chapter 12 。１

What is Pressure? The word pressure is related to another word. Do you know what it is?The word pressure is related to another word. Do you know what it is? PRESSurePRESSure Example – When you lean against a wall you exert pressure against it.Example – When you lean against a wall you exert pressure against it.

Force and Pressure Why don’t you sink into snow when you are wearing snow shoes?Why don’t you sink into snow when you are wearing snow shoes? Why does it hurt so much when someone with high heels steps on your foot?Why does it hurt so much when someone with high heels steps on your foot? The amount of pressure is determined by the area over which the weight (force) is distributed.The amount of pressure is determined by the area over which the weight (force) is distributed.

Calculating Pressure Formula:Formula: Pressure = Force __________ Area F PA

Calculating Pressure Pressure (Defined):Pressure (Defined): The force on a surface divided by the total area over which it is exerted.The force on a surface divided by the total area over which it is exerted.

Calculating Pressure How is force measured?How is force measured? How is area measured?How is area measured? The unit for pressure is: pascal (Pa)The unit for pressure is: pascal (Pa) 1 Pa = 1N __________ 1m 2

TIME OUT! Let’s Review Area… Area is a measure of a surface.Area is a measure of a surface. Area is found by multiplying the length times the width.Area is found by multiplying the length times the width. Example: Compute the area of a rectangle that is 2 meters wide by 3 meters long.Example: Compute the area of a rectangle that is 2 meters wide by 3 meters long. Answer = 6 mAnswer = 6 m 2

Calculating Pressure A lower pressure can be produced by…A lower pressure can be produced by… … increasing the area a force acts on.… increasing the area a force acts on. Or you can…Or you can… … produce higher pressure by decreasing the area a force acts upon.… produce higher pressure by decreasing the area a force acts upon. Examples?Examples?

Let’s try an example together A solid block of wood has a square base 1m x 1m and weighs 5000 N. How much pressure does it exert on the floor underneath?A solid block of wood has a square base 1m x 1m and weighs 5000 N. How much pressure does it exert on the floor underneath? Pressure = Force/AreaPressure = Force/Area Pressure = 5000N / (1m x 1m)Pressure = 5000N / (1m x 1m) Pressure = 5000N/ 1m 2Pressure = 5000N/ 1m 2 Pressure = 5000 N/m 2 or 5000 PaPressure = 5000 N/m 2 or 5000 Pa

Try this one on your own… A baseball strikes a catcher’s mitt with a force of 200N. If the area the ball strikes is 0.04 m, what is the pressure on the mitt?A baseball strikes a catcher’s mitt with a force of 200N. If the area the ball strikes is 0.04 m 2, what is the pressure on the mitt? Pressure = Force/AreaPressure = Force/Area Pressure = 200N/0.04 m 2Pressure = 200N/0.04 m 2 Pressure = 5000 PaPressure = 5000 Pa

Fluid Pressure A fluid can be either a liquid or a gas.A fluid can be either a liquid or a gas. A fluid is a substance that can easily flow and is able to change shape.A fluid is a substance that can easily flow and is able to change shape. Examples?Examples?

Fluid Pressure All of the forces exerted by the individual molecules…All of the forces exerted by the individual molecules… … in a fluid add together to make up the pressure exerted by the fluid.… in a fluid add together to make up the pressure exerted by the fluid.

Fluid Pressure All Around The pressure exerted by air is…The pressure exerted by air is… …referred to as air pressure or atmospheric pressure.…referred to as air pressure or atmospheric pressure. How does atmospheric pressure relate to weather?How does atmospheric pressure relate to weather? The atmospheric pressure determines whether it is a sunny day or rainy day.The atmospheric pressure determines whether it is a sunny day or rainy day.

Fluid Pressure All Around Air exerts pressure because…Air exerts pressure because… … it has mass.… it has mass. Each cubic meter of air around you has a mass of about 1 kilogram.Each cubic meter of air around you has a mass of about 1 kilogram.

Fluid Pressure All Around Average air pressure at sea level is…Average air pressure at sea level is… … N/cm.… N/cm 2. Given that the surface area of your hand is about 100 cmGiven that the surface area of your hand is about 100 cm 2, the total pressure on your hand is about 1000 N or 225 lbs! How can your hand possibly support the weight of the atmosphere when you don’t feel a thing?How can your hand possibly support the weight of the atmosphere when you don’t feel a thing?

Balanced Pressures In a fluid that is not moving…In a fluid that is not moving… … pressure at a given point is exerted equally in all directions.… pressure at a given point is exerted equally in all directions.

Balanced Pressures Pressure inside your body…Pressure inside your body… …balances the air pressure outside your body.…balances the air pressure outside your body.

Balanced Pressures This pressure comes from…This pressure comes from… … fluids inside your body.… fluids inside your body. Some parts of your body contain gases (lungs and sinus) and others contain liquids (blood and cells).Some parts of your body contain gases (lungs and sinus) and others contain liquids (blood and cells).

Variations in Fluid Pressure Pressure and Elevation Air pressure decreases as elevation increases. This is because there is less air/weight on top of you.This is because there is less air/weight on top of you. Why is the cabin of an airplane pressurized?Why is the cabin of an airplane pressurized?

Variations in Fluid Pressure Pressure and Elevation Your ears “pop” (as you go up in altitude) because…Your ears “pop” (as you go up in altitude) because… … for a moment, the air pressure behind your eardrums is greater than it is outside.… for a moment, the air pressure behind your eardrums is greater than it is outside.

Variations in Fluid Pressure Pressure and Depth Fluid pressure depends on depth.

Variations in Fluid Pressure Pressure and Depth Water pressure…Water pressure… … increases as depth increases.… increases as depth increases. Why are submarines able to submerge so far underwater?Why are submarines able to submerge so far underwater?

Variations in Fluid Pressure Pressure and Depth At greater depths, there are more fluid molecules, greater mass, and thus more pressure.At greater depths, there are more fluid molecules, greater mass, and thus more pressure. In the deepest parts of the oceans, pressure can be 1000 times that of sea level!In the deepest parts of the oceans, pressure can be 1000 times that of sea level!

Barometer An instrument called a barometer is used to measure atmospheric pressure.An instrument called a barometer is used to measure atmospheric pressure. In this type of barometer, the height of the liquid column increases as atmospheric pressure increases.In this type of barometer, the height of the liquid column increases as atmospheric pressure increases. High atmospheric pressure indicates sunny weather.High atmospheric pressure indicates sunny weather. Low pressure means rainy weather.Low pressure means rainy weather.

Fluid Movements Two common fluid pressures are air and water.Two common fluid pressures are air and water. Air pressure decreases as you go to higher elevations.Air pressure decreases as you go to higher elevations. Water pressure increases the deeper you go.Water pressure increases the deeper you go. Fluids will move from areas of high pressure to low pressure.Fluids will move from areas of high pressure to low pressure.

Why Do Objects Float? Chapter 12 – Section 2 Pages

The Buoyant Force When you pick up an object underwater, does it feel lighter or heavier than air? Why? Water exerts a force called the buoyant force that acts on a submerged object. The buoyant force acts in the upward direction, against the force of gravity, so it makes an object feel lighter.

What Causes the Buoyant Force? Remember – a fluid exerts pressure on all surfaces of a submerged object. Since the pressure in a fluid increases with depth, the upward pressure on the bottom of an object is greater than the downward pressure on the top.

Floating & Sinking Over 2000 years ago, a Greek mathematician named Archimedes studied buoyancy and the concept of displacement. He found that a submerged object displaces, or takes the place of, a volume of fluid equal to its own volume. Archimedes Principle states that the buoyant force on an object is equal to the weight of the fluid displaced by the object.

Floating & Sinking If the weight of water displaced is MORE than the weight of the object - it will FLOAT! If the weight of the water displaced is LESS than the weight of the object – it will SINK. If the weight of the water displaced is EQUAL to the weight of the object – it will be SUSPENDED

Density By comparing the density of an object to the density of a fluid, you can decide if it will float. Density = Mass / Volume What is the density of water? 1 g/mL or 1 g/cm 3 M DV Density of Lead = 11.3 g/cm 3 Density of Lead = g / 16 cm 3

Buoyancy & Density Which of these substances will float?

Changing the Buoyant Force Changing the density of an object can make it float or sink. The density of a submarine is decreased when water is pumped out of floatation tanks. Since its volume remains the same, its density decreases when its mass decreases.

Buoyancy & Density Another way of changing density is to change volume. Why are large ships made of steel able to float? 1) Boats have a large volume that is filled with air that has little mass. 2) The shape of boats allows for a large amount of water to be displaced creating added buoyant force.

Practice Problem #1 o An object has a mass of 500 grams and occupies a volume of 40 cm 3. What is the density of the object? If it is submerged in water, will the object float? o Density = Mass / Volume o Density = 500 grams / 40 cm 3 o Density = 12.5 g/cm 3 o No – the object will sink

Practice Problem #2 o If an object has a density of 0.9 g/cm 3 and it occupies of volume of 3.5 cm 3, what is the mass of the object. Will the object float? o Mass = Density * Volume o Mass = 0.9 g/cm 3 * 3.5 cm 3 o Mass = 3.15 grams o Yes – the object will float

Chapter 12 – Section 3 Doing Work with Fluids Pascal’s Principle Bernoulli’s Principle Pages

Fluid Pressure In the 1600s, Blaise Pascal, a French Mathematician discovered that pressure increases by the same amount throughout an enclosed or confined liquid. This is known as Pascal’s Principle – When force is applied to a confined fluid, an increase in pressure is transmitted equally to all parts of the fluid. Blaise Pascal (1623 – 1662)

Fluid Pressure Hydraulic Systems are designed to take advantage of Pascal’s principle. A hydraulic system multiplies force by applying the force to a small surface area. The increase in pressure is then transmitted to another part of a confined fluid, which pushes on a larger surface area.

Hydraulic Lift greater force Pressure from a small piston (smaller surface area) creates a greater force to lift a car) P = F/A P = 20N / 5m 2 P = 4 Pascals F = P * A F = 4 Pa * 10m 2 F = 40 Newtons << 4 Pascals

Bernoulli’s Principle Daniel Bernoulli In the 18 th century, a Swiss scientist named Daniel Bernoulli discovered that fluid pressure changes depending on the speed of the fluid. when the speed of a fluid increases, the pressure exerted by the fluid decreases. This is known as Bernoulli’s Principle – when the speed of a fluid increases, the pressure exerted by the fluid decreases. Daniel Bernoulli (1700 – 1782)

Chimney’s and Bernoulli’s Principle In a fireplace, hot air that is less dense rises upward. Air moving across the top of the chimney lowers the air pressure. The air travels from high pressure in the chimney to low pressure at the top of the chimney.

Damage from High Winds Why do people nail boards outside of their windows before a hurricane comes? High winds from hurricanes cause a drop of pressure outside of the house. If wind is blowing fast enough, the difference between the higher pressure (inside) and lower pressure outside can cause the windows to shatter and roofs to blow off.

Roof Damage from Storm

Tornadoes Because the wind velocities of tornadoes are so high, extremely low pressures are found in the center of a tornado and hurricanes.

Wings and Flight Faster moving air on the top of an airplane wing causes low pressure. Slower moving air on the bottom causes high pressure. The air (fluid) flows from high to low pressure. This creates lift.