Chapter 3 – Forces in Fluids

Section 1

Fluids and Pressure What is a fluid?

Fluid – any material than can flow and that takes the shape of its container.

What are some examples?

Pressure What types of pressure are there?

› Air pressure› › Pressure – the amount of force exerted on a

given area.

Pressure = force/area

SI unit for pressure is Pascal.

Pressure = force/area

The force of air in a balloon is 2N and the area of the balloon is 2m2. What is the pressure exerting on the inside of the balloon?

Why are bubbles round? The air in the bubble exerts pressure

evenly in every direction, so the bubble expands in every direction.

Atmospheric Pressure The weight of the atmosphere due to gravity

pulling it to the earth.

Atmosphere is made up of: Nitrogen, Oxygen, and other gases.

The atmosphere exerts a pressure of approximately 101,300 N on every square meter.

Also known as 101,300 Pa

So that means:

10N (the weight of pineapple) is applied on every square centimeter (roughly the area of the tip of your little finger) of your body.

Why doesn’t this hurt?

Because the fluids inside your body also exert pressure.

Atmospheric Pressure Varies

Depending on your elevation atmospheric pressure changes.

On top of Mount Everest the atmospheric pressure is 33,000 PA.

The pressure increases as the atmosphere gets “deeper.”

If you travel to higher or lower points in the atmosphere the fluids in your body have to adjust to maintain equal pressure.

Just like when your ears “pop” in an airplane. Small pockets of air behind your eardrums

contract or expand as atmospheric pressure increases or decreases.

“pop” occurs when air is released due to these pressure changes.

Imploding Pop Can

http://www.wfu.edu/physics/demolabs/demos/avimov/bychptr/chptr4_matter.htm

Review1. How do particles in a fluid exert

pressure on a container?

2. Why are you not crushed by atmospheric pressure?

3. Explain why dams on deep lakes should be thicker at the bottom than near the top.

Water Pressure Water pressure increases with depth

because of gravity. The greater a diver goes in water the

greater the pressure becomes because more water above the diver is being pulled by Earth’s gravitational force.

Bill nye video – Fluid Pressure

DensityIs the amount of matter in a certain volume.Water is more dense than air.Water exerts more pressure than air.

If you climb up a tree 10 meters and then you dive 10 meters underwater. Where would you have the greater pressure exerted on you?

Denser Pop Video!http://www.wfu.edu/physics/demolabs/demos

/avimov/bychptr/chptr4_matter.htm

Fluids Flow from High Pressure to Low Pressure

When drinking through a straw:› You remove the air from the straw and

reduce the pressure in the straw. What you are drinking has the greater pressure so the liquid moves up the straw. The outside pressure forces the liquid up the straw along with the suction you are creating in the straw.

When breathing:› When you inhale a muscle increases the

space in your chest, giving your lungs room to expand. This expansion lowers the pressure in your lungs so that it becomes lower than the outside air pressure.

› Air then flows into your lungs from higher pressure to lower pressure.

Pascal’s Principle Blaise Pascal, 17th century French

scientist.

Pascal’s Principle – a change in pressure at any point in an enclosed fluid will be transmitted equally to all parts of that fluid.

Putting Pascal’s Principle to Work

Hydraulic – devices that use liquids to transmit pressure from one point to another.

Ex. Hydraulic Brakes: Driver’s foot exerts pressure on a cylinder of

liquid. Pascal’s principle tells you that this pressure is transmitted equally to all parts of the liquid-filled brake system.

This liquid presses a brake pad against each wheel, and friction brings the car to a stop.

Review1. Explain how atmospheric pressure helps

you drin k through a straw.

2. What does Pascal’s principle state?

3. When you squeeze a balloon, where is the pressure inside the balloon increased the most? Explain your answer in terms of Pascal’s principle.

Section 2 Buoyant Force BrainPOP | Buoyancy | Quiz

Buoyant Force Buoyant force- The upward force that

fluids exert on all matter.

Buoyant Force Is Caused by Differences In Fluid Pressure

Water exerts fluid pressure on all sides of an object.

The side pressures are equal thus they cancel each other out. The only fluid pressures affecting the object are at the top

and bottom. The bottom is greater because pressure increases with depth. Therefore the water places an upward force on the object known as buoyant force.

Determining Buoyant Force Archimedes a Greek mathematician discovered

how to determine buoyant force.

Archimedes Principle- states that the buoyant force on an object in a fluid is an upward force equal to the weight of the volume of fluid the object displaces

Displace= take the place of

Determining Buoyant Force Continued

Archimedes Principle.flv

The weight of an object has nothing to do with the buoyant force. Only the weight of the displaced fluid determines the buoyant force of an object.

Weight –vs Buoyant Force An object will sink if it has a weight

greater than the weight of the fluid that is displaced.

An object will sink if its weight is greater than the buoyant force acting on it.

An object floats when it displaces a volume of fluid that has a weight equal to the objects weight.

An object will float if the buoyant force of the object is equal to the objects weight.

Weight Vs. Buoyant force cont.

Sinking ( Rock) The buoyant force is equal to the weight of the displaced water. The rocks weighs more than the displaced water therefore it sinks.

Floating- The fish weighs 12 N It displaces a volume of water that has a weight of 12N Therefore the fish floats

Weight Vs. Buoyant Force Cont

Buoying up- When the buoyant force on an object is greater than the objects weight the object is buoyed up ( pushed up) out of the water until what's left underwater displaced an amount of water that equals the objects entire weight.

An Object Will Float or Sink Based on Its Density.

Density= Mass per unit volume. The rock sunk because it was more

dense than water. The duck floats because it is less dense than water. The fish is equal to the density of water.

More Dense than air. Why does the rubber duck float on

water but not in the air? Because it is less dense than water but

not less dense than air

Less Dense Than Air One substance that is less dense than

air is helium. Helium displaces a volume of air that is much heavier than itself so it floats.

How to Calculate Density. Density =Mass Volume

What is the density of a 20 cm sample of liquid with a mass of 25g?

1.25g

A 546 g fish displaces 420 cm of water. What is the density of the fish?

1.3g

The Mystery of Floating Steel

Scan pic page 71

Steel is almost 8 times more dense than water.

How does a ship Float? The secret is the shape of the ship. The amount of steel in the block is the

same as the ship. But the hollow shape increases the volume of he ship.

Because density is mass per volume an increase in the ship’s volume leads to a decrease in the density.

Most ships are built to displace more water than necessary in order to hold extra people and cargo.

Density on the move.

Lab scientific method

a. Independent Variable:- The independent variable experimental factor that is changed by the experimenter.

b. Dependent Variable:- the quantity that changes because of the variation in the independent variable.

c. Hypothesis:-Educated guess