Forces in FluidsPhysical Science

A fluid is any material that can flow and that takes the shape of its container. The particles of a fluid can overcome gravity and move past each other.

In science, a fluid is considered to be liquids or gases.

Fluids

All fluids can exert pressure, when the particles of the fluids are pressed against the sides of a container. Air pressure, water pressure and blood pressure are all examples of fluid pressure. The unit of measure for pressure is the Pascal (Pa).

Force 200 NPressure = Area 100 m2 = 2 Pa

Fluids and Pressure

When you blow up a balloon, what shape does it have? How about bubbles?

Fluids exert pressure evenly in all areas. When you blow up a balloon, the air particles spread out to the sides of the balloon and expand evenly in every direction.

When you add air to a tire, why do you only need one valve?

The Roundness of Fluid Pressure

Atmospheric Pressure: The atmosphere has layers of gases (nitrogen, oxygen, etc.) all the way up to about 150 m. 80% of all gases are found within 10 km of the Earth’s surface. Every layer of these gasses exert a pressure on the surface of the Earth.

Atmospheric Pressure

http://www.youtube.com/watch?v=nah0xPnbscY&feature=related&safety_mode=true&persist_safety_mode=1&safe=active

Atmospheric Pressure Experiment

There is so much pressure on the surface of the Earth that it registers at 101,300 Pa. That means there is 101,300 N of force on every square centimeter (cm2)! So how does your body handle so much pressure?

Because your own body fluids (blood, water) exerts equal pressure so you don’t even feel it (unless there is a dramatic change – see next slide!).

The Pressure’s On You!

Because the atmospheric gases are in layers, you will have less pressure when you go up into the atmosphere, and more pressure when you go down below the surface (below sea level).

Your body has to adjust to changes in pressure. Higher altitudes means “thinner” air, and your body may have too much pressure inside. How might that pressure escape?

Atmospheric Pressure Varies

Water pressure increases with depth due to gravity (just like air pressure).

The amount of water doesn’t cause the pressure, it is the depth that causes the pressure.

For example, no matter how much water is in the pool, river or ocean, a swimmer would feel the same amount of pressure at 5 m below the surface of the water.

Water Pressure

http://www.youtube.com/watch?v=oUK7agBG4KA&safety_mode=true&persist_safety_mode=1&safe=active

Water Pressure and Depth

Density is the amount of matter per unit of volume. Water is about 1,000 times more dense than air. This means layers of water weigh much more than layers of air, so there is lots more pressure.

Why is an pressurized chamber used with deep sea divers? What causes The Bends?

Water Density

Imagine getting onto a very crowded elevator.

What is it like? How do you feel when people get off at each stop?

Fluid pressure is similar. Fluids will always go where there is the least amount of pressure. This is why fluids (liquids and gases) spread out as much as they can.

Getting Away from the Pressure

http://www.youtube.com/watch?v=qbakZL62d6U&feature=related&safety_mode=true&persist_safety_mode=1&safe=active

Fluids moving to areas of less pressure

Imagine a glass of soda. The particles in the soda are close to each other in the glass. When you “suck” on a straw, you aren’t sucking up the soda, you are removing the air from the straw! This creates a vacuum tunnel for the soda particles to escape, because the straw has very little pressure compared to the pressure inside the glass!

How Do Straws Work?

http://www.youtube.com/watch?v=dfPUkCqQFn4&safety_mode=true&persist_safety_mode=1&safe=active

Would a straw work in outer space?

States that a change in pressure at any point in an enclosed fluid will be transmitted equally to all parts of that fluid.

Example: hydraulic devices. They use liquids to multiply forces. By pressing a lever, the liquids in the enclosed container will press evenly over a surface to create an even pressure across that surface. (car brakes, dentist chair, etc.)

Pascal’s Principle

http://www.design-your-homeschool.com/hydraulic-earth-mover.html

Hydraulic Earth Mover

Why do some things float on water, and some things sink? What happens when you take a floating object and try to force it under the water?

Buoyant force is the upward force that fluids exert on all objects.

Buoyant Force

Remember, fluids (liquids/gases) exert pressure evenly all over a surface. (Think about the bottom of a ship.

Because pressure in fluids increases in depth, then the pressure on the bottom of the object is greater than the pressure at the top of the object.

Causes of Buoyant Force

The pressure on the sides of the object are at the same depth, so they cancel each other out (net force of 0).

This means that the pressure on the bottom of the object (which has the greatest force) is exerting an upward force on the object and pushes the object up.

Causes of Buoyant Force

http://www.youtube.com/watch?v=2afDLk-JzEY&NR=1&safety_mode=true&persist_safety_mode=1&safe=active

http://www.youtube.com/watch?v=j3FKkZWZ0u8&feature=related&safety_mode=true&persist_safety_mode=1&safe=active

How Bouyancy Works

Remember our friend, Archimedes, who tested the gold crown to see if it was real gold?

The crown was made of denser material, so it had greater weight. When Archimedes put it into the water, the displaced water rose up higher on the sides of the pool. This means the heavier fake crown had less buoyant force so it sank deeper into the water.

Archimedes

An object in a fluid will float if it has a weight less than the weight of the fluid that is displaced.

An object in a fluid will sink if it has a weight greater than the weight of the fluid that is displaced.

Weight vs. Buoyant Force

Sinking Example: If you throw a rock that has a weight force of 75 N, it would displace 5 L of water. 5 L would weight force of about 50 N. Since the rock has a greater weight than the displaced water, it would sink (like a rock ).

Floating Example: Your rubber duck has a weight force of 4 N. The water that is displaced on the side of your tub has a weight of 6 N. So your rubber ducky would float. (what happens when you try to push it under the water).

Stages of Bouyancy

Why do parts of an object stay under water, while the top half floats?

Example: A real duck. If the duck gets into the water with a weight force of 9N. All he has to do is displace enough water that would also have a weight force of 9N. When a duck gets into the water, his feet, legs and stomach displace 9 N of water. That is all that is needed for the rest of the duck to float!

Half Sinking/Half Floating?

Objects that have more weight force means that they have more mass per unit volume, which is density.

Remember, the size of the object doesn’t matter. (The world’s largest rubber duck will still float). It is how dense it is, which makes it have a greater weight force.

Question: Why do helium balloons rise in the air?

Floating/Sinking due to Density

Because water is the universal solvent (and the most common of all liquids), scientists have a assigned it a density of 1 (fresh water is slightly less).

This means that the mass of water is equal to it’s volume.

Ex: 1 g of water has a volume of about 1 mL: 1 g 1 mL = 1 g/mL (density of water)Objects with a density of < 1 will floatObjects with a density of >1 will sink

Water Density

http://www.youtube.com/watch?v=aTcyACmiaho&feature=fvwrel&safety_mode=true&persist_safety_mode=1&safe=active

Dead Sea Density

Steel by itself is almost 8 times the density of water (steel density of 8 vs water density of 1)

It is the shape of the steel that makes it less dense. Instead of a solid piece of steel (which would sink), steel-hull ships are curved and hollow. This makes them have a density of less than 1!

(think about it – what is in the hull of a cruise ship?)

How Can a Steel Ship Float?

http://www.youtube.com/watch?v=G8ey_RBdxYM&feature=related&safety_mode=true&persist_safety_mode=1&safe=active

Why the Titanic Sank

How can a submarine sink and float? A submarine has compartments that allow sea water to come in. This added weight makes the submarine sink. When the submarine needs to surface, it uses compressed air to shoot the water out (and become less dense).

Fish can do the same thing with their swim bladder. Fish that don’t have a swim bladder (sharks), have to swim constantly to keep from sinking to the bottom of the ocean.

Changing Densities (Examples: Submarines and Fish)

We know that air pressure decreases as you go up layers of air. However, Daniel Bernoulli discovered that fluid pressure can decrease anywhere if you increase speed.

Bernoulli’s Principle states that, as the speed of a moving fluid increases, then the pressure around it decreases.

An object that is caught in moving fluids will stay there, because the pressure outside the moving fluid is usually greater.

Bernoulli’s Principle (Why Airplanes Fly)

http://www.youtube.com/watch?v=P-xNXrELCmU&safety_mode=true&persist_safety_mode=1&safe=active

Demos of Bernoulli’s Principle

So how can a large jet that has a weight force of 350,000 N fly? Not only can it fly, but it can go up 10,000 ft in the atmosphere?

If you have ever been on a plane, what does it do before it takes off the runway?

Bernoulli’s Principle

The momentum the plane is building as it moves down a runway allows the air above the curve of a wing to move faster than the air below the wing. Since there is less pressure above the wing, the air below the wings lift upward.

So what determines how quickly and how much lift a plane can get? The thrust and wing size determine lift of a plane.

Bernoulli’s Principle

http://www.youtube.com/watch?v=qbakZL62d6U&feature=related&safety_mode=true&persist_safety_mode=1&safe=active

Bernoulli’s Principle

Lift of the plane is determined by the plane’s thrust and wing size:

Thrust is the forward force produced by the plane’s engine. When the plane travels forward with more thrust, the air above the wings moves quickly, reducing more pressure. Planes with larger thrust force can use smaller wings, which keeps the plane’s weight low.

Lift of a Plane

Planes that cannot build up great speed because they don’t have the engines for great thrust make up for it in wing size. Large planes like gliders don’t need to build up great speed, but there is more moving air above the larger wings that allow them to lift off. The larger the wings, the smaller the engine that is needed. (What type of runway might you need for this?)

Lift of a Plane

Race cars travel very fast, so the air above them moves very fast. So why don’t they take off?

That is what spoilers are for. The spoiler on the back of a race car is an upside down wing. It allows the air below the spoiler to have less pressure, so that the air above the spoiler pushes down on the car and prevents it from “taking off”!

Race Cars

While moving air creates an area of low pressure, it also creates a force that opposes motion. The force that opposes or restricts motion in a fluid is called drag.

Example: Trying to walk down the street during a strong hurricane. Why is it difficult?

Planes and birds can adjust their wings to over come the drag of turbulence (unpredictable flow of air). Planes have flaps that move up and down to control drag during turbulence.

What a Drag

http://www.youtube.com/watch?v=cWJDUzUMqxA&safety_mode=true&persist_safety_mode=1&safe=active

Spoiler Test

Bernoulli’s principle doesn’t require wings. Baseball pitchers use this principle all the time. When they throw a baseball, the left side (inside) of the ball will have greater pressure than the right side (outside) of the ball. This is what creates a curve ball.

You’re Out!