Physical Science

Physical Science Unit: Forces in Fluids

What is Pressure? Related to the word press Earth’s gravity pulls downward Due to gravity, your feet exert a force on the surface of Earth over an area the size of your feet

Pressure = Force / Area Pressure: a force pushing on a surface Pressure = Force / area Unit of measure for Pressure is the Pascal: 1Pa = 1N/m2 Remember 1 N = 1kg m/ sec2

Pressure and Area • The amount of pressure you exert depends on the area over which you exert force. • Larger area = less pressure exerted

Pressure Can be calculated by dividing the force exerted by a fluid by the total area over which the force acts When force is measured in Newtons (N) and area is measured in square centimeters (cm2), pressure is measured in Newtons per square centimeter (N/cm2)

Calculating Pressure Pressure = Force Area Measured in Pascals – 1 N/m²

Pressure The pressure a fluid exerts is due to the fact that the fluid is made of particles that have weight and motion. The weight of the particles in a fluid cause them to push against objects

Pressure Problems What is the force water applies to your hand when you dive to bottom of a swimming pool? Assume your hand has an area of 0.008 m2 and the water pressure is 120,000 Pa. Solution: P = F/A, F = P ·A, A = F/P, F = (.008) x (120,000) = 960 N

Pressure Problems Your brand new car weighs 16,170 N and you need to get your first oil change. You pull it onto the hydraulic lift, which has an area of 5005 cm2. The mechanic applies a force to a small piston that has an area of 6.5 cm2. What force does he apply to lift your car? Solution: P = F/A, F = P ·A, A = F/P,

Solution Divide larger area by smaller 5005/65 = 770 new car weighs 16,170 N an area of 65 cm2 an area of 5005 cm2 Divide power by area 16170/ 770 = 21 N

Fluid Pressure • Fluid is a material that can easily flow • Liquids • Gases • Air and helium

Fluid Pressure Fluid is a material that can easily flow. All Liquids and Gases are fluids. In a fluid, all of the forces exerted by the individual particles combine to make up the pressure exerted by the fluid.

Fluid Pressure Pascal's Vase - demonstrating that depth, not shape, determines fluid pressure... • Fluid is a substance that can flow easily. • Scientifically liquids & gases are considered “fluids” • In fluids, molecules are constantly moving in all directions • . • Pressure = Force / Area

Fluid Pressure As a molecule moves and collides w/ a surface, it exerts a force on that surface All of the forces exerted by the individual molecules are added together to make up the pressure exerted by the fluid

What Causes Fluid Pressure In a fluid, all of the forces exerted by the individual particles combine to make up the pressure exerted by the fluid.

Air Pressure Air exerts pressure because it has mass. Because the force of gravity pulls down on the mass of air, the air has weight. Weight of the air is the force that produces air pressure or atmospheric pressure.

Air Pressure 1 in x 1 in square column of air weighs 14.7 lbs at sea level Air exerts a balanced force when fluid is NOT moving: the pressure pushing down on your hand is balanced by the pressure pushing up on your hand • Air pressure is the result of the weight of a column of air pushing down on an area. • 14.7 lbs/inch2 ( 10.13 N/cm2) • 1013.25 millibars • 1013.25 hPa (hectoPascals) • 29.92 inches of Hg

Balanced Pressure When you hold out your hand, you are holding up air. 1,000 Newtons of air, which is about the same weight as that of a large washing machine! .

What keeps air pressure from crushing you? The fluids in your body exert pressure The air pressure (outside your body) and your body pressure are equal

At high Altitudes There are fewer particles of air in a given area So air pressure decreases

Unbalanced Pressure What happens when air pressure becomes unbalanced?

Variations in Fluid Pressure Elevation – the distance above sea level. As altitude increases  Air pressure decreases As air pressure decreases, so does density. As water depth increases  water pressure increases Water 800x more dense than air, so pressure increases dramatically w/ depth – every 33 ft in depth adds 1 “atmosphere “ of pressure

Suction Is the result of unequal air pressure Ex operating a vacuum cleaner

Variations in Atmospheric Pressure and Elevation. Popping in ear is caused by changing air pressure

Measuring Pressure Barometer measures atmospheric pressure

Archimedes Principle The buoyant force on an object is equal to the weight of the fluid displaced by the object. The buoyant force is opposite (pushes up) to the force of gravity (pulls down)

Archimedes Principle Can be stated in terms of density: an object will float in a fluid if the density of the object is less than the density of the fluid

Density Comparing densities, you can predict whether an object will float or sink in a fluid. If object is more dense than fluid - it sinks (obviously!) If object is less dense than fluid - it floats. (no, duh...) An object with a density equal to that of the fluid floats at a constant depth.

Density • Density = Mass / Volume • Density of water = 1.0 • Changing density can explain why an object (like a submarine) floats or sinks.

Density of hot air Balloon The density of hot air is less than that of cold air, which is why hot air balloons can rise

Buoyancy Buoyancy is the ability to float.

Buoyant Force Buoyant force is when water and other fluids exert an upward force. The buoyant force acts in the direction opposite to the force of gravity, so it makes an object feel lighter. The less dense the object is, the greater the buoyant force it experiences.

Buoyant Force

Buoyancy Ice cube float in water because the buoyant force on it is equal to the weight of water displaced Same is true for submarines When an object is placed in a fluid, it displaces some of the fluid, the amount displaced equals the volume of the object

Pressure Problems An object weighs 36 g in air and has a volume of 8.0 cm3. What will be its apparent weight when immersed in water?Solution:When immersed in water, the object is buoyed up by the mass of the water it displaces, which of course is the mass of 8 cm3 of water. Taking the density of water as unity, the upward (buoyancy) force is just 8 g.The apparent weight will be (36 g) – (8 g) = 28 g.

Transmitting Pressure in a Fluid • In the 1600s, Blaise Pascal developed a principle to explain how pressure is transmitted in a fluid • Fluid exerts pressure on any surface it touches.

Pascal’s Principle When force is applied to a CONFINED FLUID, an increase in pressure is transmitted equally to ALL parts of the fluid.

Pascal’s Principle • Pascal’s principle states that pressure increases by the same amount throughout an enclosed or confined fluid • When force is applied to a confined fluid, the change in pressure is transmitted equally to all parts of the fluid.

Pascal’s Principle at Work

Pascal’s Principle and Hydraulics Hydraulic devices use the principle that pressure is transmitted equally in all directions throughout a liquid.

Hydraulic Systems • Hydraulic system uses liquids to transmit pressure and multiply force in a confined fluid. • 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 the confined fluid, which pushes on a larger surface area. • Because hydraulic systems use fluids to transmit pressure, hydraulic systems have few moving parts that can jam, break or wear down.

Hydraulic Lifts

Bernoulli’s Principle The pressure exerted by a moving stream of a fluid is less than the pressure of the surrounding fluid. The faster the fluid moves, the less pressure it exerts on the surface of the object

Physical Science