Fluids

Pressure in Liquids A liquid exerts a pressure against the bottom of its container P = Force Area But it also exerts a force against the container’s side, - At any point within a liquid, the forces that produce pressure are exerted equally in all directions

How Pressure Within a Fluid Varies In a still fluid, as you go deeper in the fluid, more is on top of you so the pressure increases with depth. In a still fluid, as you go deeper in the fluid, more is on top of you so the pressure increases with depth. Compare the pressure of two different fluids at the same depth : The pressure is greater in the more dense fluid – so density of the fluid affects the pressure it exerts Compare the pressure of two different fluids at the same depth : The pressure is greater in the more dense fluid – so density of the fluid affects the pressure it exerts

Density Density if the “puffyness” of an object. Density if the “puffyness” of an object. Density = Mass Volume If an object is submerged in a fluid and it is less dense than the fluid, it will float on top, if more dense, it will sink, and if it is the same density it will stay within the fluid where ever you put it If an object is submerged in a fluid and it is less dense than the fluid, it will float on top, if more dense, it will sink, and if it is the same density it will stay within the fluid where ever you put it

Bouyancy Submerge something under a liquid and the object pushes the liquid out of the way (displaces it), the VOLUME of the displaced liquid = the VOLUME of the object. Submerge something under a liquid and the object pushes the liquid out of the way (displaces it), the VOLUME of the displaced liquid = the VOLUME of the object. But notice something else: The object seems to weigh less in the liquid than on land – WHY? Because the fluid exerts an upward force (called the Buoyant Force) on the submerged object that helps you lift if against gravity But notice something else: The object seems to weigh less in the liquid than on land – WHY? Because the fluid exerts an upward force (called the Buoyant Force) on the submerged object that helps you lift if against gravity

Archimedes’ Principle An immersed object is buoyed up by a force equal to the weight of the fluid it displaces An immersed object is buoyed up by a force equal to the weight of the fluid it displaces It does NOT (except in a special circumstance) equal the weight of the object It does NOT (except in a special circumstance) equal the weight of the object The Special Circumstance - The Principle of Floatation: A Floating object displaces a weight of fluid equal to its own weight OR when an object is immersed and the weight of the fluid displaced just equals the weight of the object, the object will sink no further and just float The Special Circumstance - The Principle of Floatation: A Floating object displaces a weight of fluid equal to its own weight OR when an object is immersed and the weight of the fluid displaced just equals the weight of the object, the object will sink no further and just float

THINK ! If a 5 kg box with a volume of 3 L is submerged in water, how much VOLUME of water does it displace? _____ If a 5 kg box with a volume of 3 L is submerged in water, how much VOLUME of water does it displace? _____ If water has a density of 1, ( 1 kg = 1 L) how much does this displaced water WEIGH? _____ If water has a density of 1, ( 1 kg = 1 L) how much does this displaced water WEIGH? _____ What is the buoyant force on the box ________? What is the buoyant force on the box ________? What is the apparent weight of the box ( Apparent weight = weight on land (F) - buoyant force (F) _________ What is the apparent weight of the box ( Apparent weight = weight on land (F g ) - buoyant force (F B ) _________

Pascal’s Principle Changes in pressure at any point in an enclosed fluid at rest are transmitted undiminished to all points in the fluid and act in all directions. Changes in pressure at any point in an enclosed fluid at rest are transmitted undiminished to all points in the fluid and act in all directions. Example is the hydraulic lift or press Example is the hydraulic lift or press

Our Atmosphere Is made of 78 % N 2 21 % O 2 1 % Argon 1 % Argon.03 % CO 2 90% of the air exists below 20 km (12 miles)

Atmospheric Pressure Gasses like those in our atmosphere ARE compressible, so the density of a gas (and our atmosphere) varies with depth Gasses like those in our atmosphere ARE compressible, so the density of a gas (and our atmosphere) varies with depth Air density at sea level is much higher than on top of a mountain. Air density at sea level is much higher than on top of a mountain. SO – if density is greater at the bottom of the atmosphere, what about air pressure? It is greater too. SO – if density is greater at the bottom of the atmosphere, what about air pressure? It is greater too.

Atmospheric Pressure is significant! The air above us has mass and weight. It pushes down at 100,000 N /m 2 (mat demo) The air above us has mass and weight. It pushes down at 100,000 N /m 2 (mat demo) We don’t notice it because we were born here but it can crush things (can demo). We don’t notice it because we were born here but it can crush things (can demo). This significant pressure is why straws, pumps in wells, and barometers work. This significant pressure is why straws, pumps in wells, and barometers work.

Measuring pressure Pressure can be measured with a barometer or a pressure gage. They work differently and measure different things! Pressure can be measured with a barometer or a pressure gage. They work differently and measure different things! The fluid in a barometer is pushed upward (Science never sucks) by the downward force of the atmosphere. The fluid in a barometer is pushed upward (Science never sucks) by the downward force of the atmosphere. A gage (like a tire gage) measures the DIFFERENCE between the internal pressure in the container and the atmospheric pressure outside the container. A gage (like a tire gage) measures the DIFFERENCE between the internal pressure in the container and the atmospheric pressure outside the container.

The GAS Laws Boyle’s P1V1= P2V2 Boyle’s P1V1= P2V2 CharlesV1 = V2 CharlesV1 = V2 T1T2 Gay-LusaccP1=P2 Gay-LusaccP1=P2 T1T2

Bernoulli’s Principle Pressure within a MOVING fluid changes with speed – as the speed of the fluid increases the internal pressure drops Pressure within a MOVING fluid changes with speed – as the speed of the fluid increases the internal pressure drops Explains lift forces like airplane wings. The air moving over the top of the wing is going faster, it has less pressure. This lets the slower moving air below the wing (which has more pressure) to push up on the bottom of the wing – giving it lift. Explains lift forces like airplane wings. The air moving over the top of the wing is going faster, it has less pressure. This lets the slower moving air below the wing (which has more pressure) to push up on the bottom of the wing – giving it lift.