Forces in FluidsForces in Fluids

What are fluids?What are fluids?

Any substance that can Any substance that can change shape or flow easily.change shape or flow easily.

(Liquid or Gas)(Liquid or Gas)

PressurePressure

• Pressure decreases as the area over Pressure decreases as the area over which a force is distributed increases.which a force is distributed increases.

• Pressure tells how much force pushes Pressure tells how much force pushes on each part of a surface. on each part of a surface.

• The formula for pressure is:The formula for pressure is:•Pressure = Force ÷ AreaPressure = Force ÷ Area

• The unit for pressure is the Pascal (Pa).The unit for pressure is the Pascal (Pa).•1 Pa = 1 N/m1 Pa = 1 N/m22

Fluid PressureFluid Pressure• All of the forces exerted by the individual All of the forces exerted by the individual

particles in a fluid combine to make up the particles in a fluid combine to make up the pressure exerted by the fluid. pressure exerted by the fluid.

• A fluid is a gas, such as air, or a liquid, A fluid is a gas, such as air, or a liquid, such as water.such as water.

• The tiny particles in a fluid move all the The tiny particles in a fluid move all the time.time.

• The particles push on everything around thThe particles push on everything around them.em.

• Fluid pressure is the force exerted by the Fluid pressure is the force exerted by the particles of a fluid. particles of a fluid.

• Air is a fluid.Air is a fluid.• Air pushes down on everything on Earth.Air pushes down on everything on Earth.• Air pressure is one kind of fluid pressure.Air pressure is one kind of fluid pressure.

Variations in Fluid PressureVariations in Fluid Pressure

• As your elevation increases, air As your elevation increases, air pressure decreases. pressure decreases.

• There are different amounts of There are different amounts of air pressure in different places. air pressure in different places.

• There is less air pressure in There is less air pressure in high places, such as on a high places, such as on a mountain top. mountain top.

• There is more air pressure in There is more air pressure in lower places, such as in a lower places, such as in a valley. valley.

• Air pressure is measured with a Air pressure is measured with a barometer.barometer.

• Elevation vs. PressureElevation vs. Pressure

Variations in Fluid Pressure Variations in Fluid Pressure 22• Water pressure Water pressure

increases as depth increases as depth increases. increases.

• Water also exerts Water also exerts fluid pressure.fluid pressure.

• The deeper you go in The deeper you go in the water, the more the water, the more water pressure water pressure pushes on you. pushes on you.

Floating or Sinking Floating or Sinking (Buoyancy)(Buoyancy)• The buoyant force acts in the The buoyant force acts in the

direction opposite to the force of direction opposite to the force of gravity, so it makes an object feel gravity, so it makes an object feel lighter. lighter.

• Water and other fluids push up on Water and other fluids push up on objects.objects.

• This upward push is called the This upward push is called the buoyant force.buoyant force.

• It makes objects in fluid feel It makes objects in fluid feel lighter. lighter.

• If an object’s weight is more than If an object’s weight is more than the buoyant force, the object will the buoyant force, the object will sink.sink.

• If the object’s weight is equal to If the object’s weight is equal to the buoyant force, the object will the buoyant force, the object will float. float.

DensityDensity • By comparing densities, you By comparing densities, you can predict whether an object can predict whether an object will sink or float in a fluid. will sink or float in a fluid.

• Density tells how much mass Density tells how much mass an object has for its volume. an object has for its volume.

• To find density, you can use To find density, you can use the formula:the formula:

• Density = Mass ÷ VolumeDensity = Mass ÷ Volume• If an object is more dense If an object is more dense

than a fluid, the object will than a fluid, the object will sink in that fluid. sink in that fluid.

• If an object is less dense than If an object is less dense than a fluid, the object will float on a fluid, the object will float on that fluid. that fluid.

• What does it mean if the What does it mean if the object is suspended in the object is suspended in the fluid?fluid?

Floating or Sinking Floating or Sinking (Buoyancy)(Buoyancy)

• When the object is placed in When the object is placed in a fluid, it takes up space.a fluid, it takes up space.

• Some of the fluid needs to Some of the fluid needs to move to make room for the move to make room for the object. object.

• The weight of the fluid that The weight of the fluid that needs to move is equal to needs to move is equal to the buoyant force. the buoyant force.

• A big object takes up more A big object takes up more room than a small object. room than a small object.

• So a big object is acted on So a big object is acted on by a greater buoyant force by a greater buoyant force than a small object. than a small object.

Pascal’s PrinciplePascal’s Principle• When force is applied to a When force is applied to a

confined fluid, the change in confined fluid, the change in pressure is transmitted pressure is transmitted equally to all parts of the equally to all parts of the fluid. fluid.

• A fluid pushes against its A fluid pushes against its container.container.

• This is called fluid pressure.This is called fluid pressure.• When a container of fluid is When a container of fluid is

squeezed, the fluid pressure squeezed, the fluid pressure increases.increases.

• Pascal’s principle says that Pascal’s principle says that when force is applied to a when force is applied to a fluid in a closed container, fluid in a closed container, pressure increases all pressure increases all through the fluid. through the fluid.

Pascal’s Principle (Transmitting Pascal’s Principle (Transmitting Pressure in a Fluid)Pressure in a Fluid)

• You can see Pascal’s principle You can see Pascal’s principle with a water balloon.with a water balloon.

• When you push in on one part When you push in on one part of the balloon, other parts of of the balloon, other parts of the balloon bulge out.the balloon bulge out.

• A hydraulic device contains A hydraulic device contains fluid.fluid.

• Force is applied to one part of Force is applied to one part of the device.the device.

• The change in fluid pressure The change in fluid pressure can be used to multiply the can be used to multiply the force. force.

Hydraulic SystemsHydraulic Systems

• A hydraulic system multiplies force by A hydraulic system multiplies force by applying the force to a small surface area.applying the force to a small surface area.

• The increase in pressure is then The increase in pressure is then transmitted to another part of the confined transmitted to another part of the confined fluid, which pushes on a larger surface fluid, which pushes on a larger surface area.area.

• Hydraulic systems use fluids to transmit Hydraulic systems use fluids to transmit pressure.pressure.

• Hydraulic systems multiply force. Hydraulic systems multiply force.

Hydraulic SystemsHydraulic Systems

• When a hydraulic system is used, a force When a hydraulic system is used, a force is applied to a small area.is applied to a small area.

• The pressure is transmitted through the The pressure is transmitted through the fluid.fluid.

• The fluid pushes on a larger area.The fluid pushes on a larger area.• The pressure stays the same, but the force The pressure stays the same, but the force

is multiplied. is multiplied. • The lifts used in car repair shops, and the The lifts used in car repair shops, and the

chairs in barber shops and beauty salons, chairs in barber shops and beauty salons, use hydraulic systems.use hydraulic systems.

Bernoulli’s Principle (Pressure Bernoulli’s Principle (Pressure in Moving Fluids)in Moving Fluids)• Bernoulli’s principle states Bernoulli’s principle states

that as the speed of a that as the speed of a moving fluid increases, the moving fluid increases, the pressure within the fluid pressure within the fluid decreases. decreases.

• Bernoulli’s principle says Bernoulli’s principle says that the faster a fluid that the faster a fluid moves, the less pressure it moves, the less pressure it exerts. exerts.

• Fluid moves from places Fluid moves from places with high pressure to places with high pressure to places with low pressure.with low pressure.

• When you suck on a drinking When you suck on a drinking straw, you make an area of straw, you make an area of low pressure in the straw.low pressure in the straw.

• This causes the fluid in the This causes the fluid in the cup to move up the straw.cup to move up the straw.

Applying Bernoulli’s Applying Bernoulli’s PrinciplePrinciple

• Wind moves air over a chimney.Wind moves air over a chimney.

• The air pressure is lower at the top of The air pressure is lower at the top of the chimney than at the bottom.the chimney than at the bottom.

• Smoke moves up the chimney Smoke moves up the chimney because of the different air pressure.because of the different air pressure.

Applying Bernoulli’s PrincipleApplying Bernoulli’s Principle

• Bernoulli’s principle helps explain how planes fly.Bernoulli’s principle helps explain how planes fly.• It also helps explain why smoke rises up a chimney, how an It also helps explain why smoke rises up a chimney, how an

atomizer works, and how a flying disk glides through the atomizer works, and how a flying disk glides through the air.air.

• Airplane wings are curved so air moves faster over the top.Airplane wings are curved so air moves faster over the top.• There is less pressure on top of the wing. There is less pressure on top of the wing. • Fluid pressure pushes the airplane wing upward. Fluid pressure pushes the airplane wing upward. • Lift is an upward force due to different air pressures above Lift is an upward force due to different air pressures above

and below an object. and below an object.