1. Chapter 4: Forces and Fluids

2. Lesson 1: Pressure and Density of Fluids

3. 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.

4. Pressure of Fluids
Pressure 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)

5. 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?

6. Pressure and Depth
When you dive under water, what happens to your ears?
Pressure increases with depth
Atmospheric Pressure
The ratio of weight of all the air above you to your surface area
Page 127 FIGURE 4
Atmospheric pressure increases as you hike down a mountain toward lower elevation. It decreases as you hike up.

7. 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 depth
Why is a dam’s wall thicker at the bottoms than at the top?

8. Density of Fluids Why 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= g/cm3

9. Density
The 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.

10. Lesson 2: The Buoyant Force

11. 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.

12. 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

13. Buoyant Force and Pressure
FIGURE 7 on page 133
A fluid applies pressure to all sides of an object
Horizontal forces are equal while vertical are not
Pressure 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.

14. Buoyant Force and Depth
Does the buoyant force on a diver change as she dives deeper?
Does not except with great changes in depth
Because 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.

15. Archimedes’ Principle
The volume of the water displaced by the object is equal to the volume of the object.
Archimedes’ Principle
The 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?

17. Sinking or Floating
When 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.

18. Buoyant Force and Density
If 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.

19. Metal Boats
Figure 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 water
The total weight of the boat and the water inside is greater than the water displaced/buoyant force= it sinks

20. 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.

21. Balloons Helium 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.

22. Lesson 3: Other Effects of Fluid Forces

23. Fluid Forces Forces are beneficial- hose or ketchup
They are also dangerous- air pushes against cars that move, floods, tornadoes, and hurricanes

24. Pascal’s Principle
Blaise 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.

25. Pushing on a Fluid The principle is applied to fluid power systems.
Using a pressurized fluid to transfer motion
Use Figure 12 on page 141
Piston on the left moves down; right moves up
Surface 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.

26. F= P x a Why 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 d
No the work done is equal

27. Hydraulic Lifts
An hydraulic lift is an example of a fluid power system that uses a liquid for the fluid.
Oil-filled hydraulic
Narrow tube & wider tube under the car
Pushing 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

28. Examples of Pascals
Copy this in to your notebook

29. Bernoulli’s Principle
A garden hose illustrates the principle that describes the relationship between speed and pressure in fluids.
Bernoulli’s principle
States that the pressure of a fluid decreases when the speed of the fluid increases
As 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.

30. 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.

31. 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.

32. Damage from High Winds Roof 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

33. 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.

34. Soccer Kicks A player puts a spin on the ball
This 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

35. Drag Forces
Drag 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 shape
Greater surface area= greater drag
And increases with density
Swimming in water vs. walking in air