Chapter 4: Forces and Fluids

Lesson 1: Pressure and Density of Fluids

What is a fluid?A fluid is any substance that can flow and take the shape o the container that holds it.

When you pour milk into a glass, the milk in the glass occupies the same volume it occupied in the carton- its just a different shape.

Gases fill the shape of its container but do not occupy the same volume in different containers. A gas fills its entire container no matter the size of the container.

Pressure of FluidsPressure is the amount of force per unit area applied to an object’s surface.

All fluids apply pressure.Pressure applied on an object by a fluid is related to the weight of the fluid.

Pressure (P)= force applied to the surface (F) divided by the surface area (A)

Pressure is measure in Pascal (Pa)

The Direction of Pressure

A fluid applies pressure perpendicular to all sides of an object in contact with the fluid.

Pressure decreases when the surface area over which a force is applied increases

Pressure increases when the surface area decreases

Pushing a box with a single finger or the whole hand?

Pressure and Depth

When you dive under water, what happens to your ears?Pressure increases with depth

Atmospheric PressureThe ratio of weight of all the air above you to your surface area

Page 127 FIGURE 4Atmospheric pressure increases as you hike down a mountain toward lower elevation. It decreases as you hike up.

Underwater Pressure

Underwater pressure depends on the sum of weight of the air above and the weight of the water above.

As a diver dives farther down, the air column stays the same but the water column increases in height and it weighs more.

Pressure increases with depthWhy is a dam’s wall thicker at the bottoms than at the top?

Density of FluidsWhy is there more pressure under water?

If the volume of two fluids is the same, the fluid that weighs more is denser.

Water is denser.Density (D) = mass (m) / volume (V)Measured in g/cm3

Water = 1.0 g/cm3

Air= 0.001 g/cm3

DensityThe density of a material is determined by the masses of the atoms or the molecules that make up the material and the distances between them.

A penny is made of zinc which has a greater mass than a water molecule. Zinc atoms are also closer together so penny is denser.

Most solids are denser than liquids or gases.

Lesson 2: The Buoyant Force

What is a buoyant force?

A buoyant force is an upward force applied by a fluid on an object in a fluid.

The buoyant force balances the weight of this person and his raft, so it floats.

There are buoyant forces in liquids and in the air.

Buoyant Forces in liquid and air

Why is it easier to lift someone in water than when you are not in water?

Buoyant forces act on floating objects and submerged objects.

The buoyant force from the air keeps a helium balloon up even though a gravitational force pulls downward.

It does not need to float in air for there to be a force.The buoyant force acting on you is less than

gravitational force= you don’t float

Buoyant Force and PressureFIGURE 7 on page 133A fluid applies pressure to all sides of an object

Horizontal forces are equal while vertical are notPressure increases with depth

Pressure pushing up is greater than pressure pushing down

The difference between the upward and downward force from the pressure on each diver is the buoyant force.

Always in an upward direction because the pressure is always greater at an object’s bottom.

Buoyant Force and Depth

Does the buoyant force on a diver change as she dives deeper?Does not except with great changes in depthBecause the pressure on top and on bottom the diver increase by the same amount

The depth of a submerged object has little effect on the buoyant force.

Archimedes’ PrincipleThe volume of the water displaced by the object is equal to the volume of the object.

Archimedes’ PrincipleThe weight of the fluid that an object displaces is equal to the buoyant force acting on the object.

The object that displaces more water has a greater buoyant force acting on it.

Which diver displaces more water?

Sinking or FloatingWhen the weight of the water displaced by an object equals the weight of the object, the object floats.Tennis ball and leaf

When an object’s weight is greater than the buoyant force, the object sinks.

The water displaced by the sunken billiard ball weighs more than the water displaced by the floating tennis ball.

Buoyant Force and DensityIf an object weighs more than the water it displaces, the object sinks.

If has a greater density than water.If an object is more dense than the fluid it is placed in, then the buoyant force on that object is less than the object’s weight, and the object will sink.

Metal BoatsFigure 10: the boats are the same size and made of the same material

Boat on the left is filled with air.the weight of the displaced water equals the weight of the boat plus the air inside it= it floats

Boat on the right is filled with waterThe total weight of the boat and the water inside is greater than the water displaced/buoyant force= it sinks

Explain in terms of density

The boat on the left is less dense than water so it floats and the boat on the right is more dense therefore, it sinks.

The buoyant force is greater on the right boat because it displaces more water.

BalloonsHelium is less dense than oxygen or nitrogen in the air.

Therefore, the buoyant force is greater than the weight of the balloon.

Over time, the balloon shrinks so the volume of the balloon decreases and the density increases.

The balloon’s density is greater than the air.The buoyant force is less than the balloon’s weight, and it falls to the ground.

Lesson 3: Other Effects of Fluid Forces

Fluid Forces

Forces are beneficial- hose or ketchupThey are also dangerous- air pushes against cars that move, floods, tornadoes, and hurricanes

Pascal’s PrincipleBlaise Pascal, a French physicist, studied the pressure of fluids in closed containers.

A fluid cannot flow into or out of a closed container.

It states that when pressure is applied to a fluid in a closed container, the pressure increases by the same amount everywhere in the container.

If you push on a ketchup bottle, the pressure increases equally throughout the bottle.

Pushing on a FluidThe principle is applied to fluid power systems.Using a pressurized fluid to transfer motion

Use Figure 12 on page 141Piston on the left moves down; right moves upSurface area of the input piston is half that of the output piston

Any input force applied to the input piston results in an output force that is doubled by the output piston.

F= P x aWhy is the OF greater than the IF?

Using the formula, if the pressure is always equal throughout the system, when you increase the surface area of the output piston, the force will result in being larger.

Does the output piston do more work than the input piston?Remember: w = f x dNo the work done is equal

Hydraulic LiftsAn hydraulic lift is an example of a fluid power system that uses a liquid for the fluid.

Oil-filled hydraulicNarrow tube & wider tube under the carPushing down on a piston in the narrow tube creates an upward force on the larger piston to lift the carhttp://science.howstuffworks.com/transport/engines-equipment/elevator1.htm

Examples of Pascals

Copy this in to your notebook

Bernoulli’s PrincipleA garden hose illustrates the principle that describes the relationship between speed and pressure in fluids.

Bernoulli’s principleStates that the pressure of a fluid decreases when the speed of the fluid increasesAs the water flows through the hose, it applies

a constant pressure on the sides of the hose, and comes out of the hose at a constant speed.

What happens when you pinch the hose?The water speeds up in the pinched part.After it travels through the pinched part, it returns to its normal speed.

Pressure of the water on the sides of the hose in the pinched part is less than it is elsewhere in the hose.

How do Airplanes work? Bernoulli's principle works on the idea that as a wing passes

through the air the its shape make the air travel more over the top of the wing than beneath it. This creates a higher pressure are beneath the wing than above it. The pressure difference cause the wing to push upwards and lift is created.

There are several things that effect the amount of lift created. The first is speed, the faster the wing moves through the air the more air is forced over and under the wing, therefore the more lift is created. Another thing that effects the amount of lift created is the density of the air. The denser the air is the more lift is produced. This is why planes climb better in the winter, the colder air is denser. The final thing that can change the amount of lift created by the wing is the shape of the wing. Certain wings produce more lift.

Damage from High WindsRoof blown off during a windstorm

The strong winds blow over the house at high speeds lowered pressure

Air inside the house as no speed The pressure outside the house is lower than the pressure inside the house

Air pressure pushing down on the house is less than the air pressure pushing up

Upward force inside the house is greater than the combined downward forces outside (including gravity), the roof begins to rise

After the roof lifts…The wind continues to blow. Its speed is not the same below and above the roof. Therefore, the air pressure is the same also.

The force from the air pressure (upward & downward) on the roof is now balanced.

The force of gravity now adds to the downward air pressure.

When the combined downward force is greater than the upward, the roof crashes back down.

Soccer KicksA player puts a spin on the ballThis makes the speed of the air on one side of the ball greater than the other side.

The side with the lower speed has a greater air pressure acting on it.

The side with the higher speed has a lower air pressure acting on it.

Air moves from areas of high to low pressure the ball spins toward the side with low pressure

Drag ForcesDrag force

A force that opposes the motion of an object through a fluid.

As the speed increases, the drag force also increases

Faster the runner= greater drag= harder he or she has to work

Drag force also depends on size and shapeGreater surface area= greater drag

And increases with densitySwimming in water vs. walking in air