Goal: To understand liquids and gasses Objectives: 1)To understand Pressure 2)To understand Buoyancy 3)To learn about Hydraulics 4)To learn about Surface Tension 5)To understand Phase Transitions 6)To apply this to The atmosphere 7)To learn about Adiabatic tendencies and Boyle’s Law 8)To learn about Bernoulli’s moving air Principle 9)To explore Plasma

Pressure When understanding fluids one of the keys is Pressure. Pressure is a measure of the force a fluid exerts per area. Pressure = Force / Area Or, Force = Pressure * Area

Earth example On the surface of the earth the atmosphere exerts a pressure of 16 pounds per square inch. Why aren’t we crushed by this pressure?

What causes pressure? Pressure in reality is the weight of the stuff above you pressing down on you. So, if you weighed a segment of air 1 inch by 1 inch which went to the top of the atmosphere that air would weigh 16 pounds.

Off the Deep End You dive into the bottom of the deep end of a swimming pool. What happens and why?

Pressure underwater Liquid Pressure = the pressure on the surface of the liquid + weight density * depth What about for water? The surface has about 1 bar of pressure (the air pressure at sea level). Every 10 meters is about 1 more bar of pressure. So, Pressure = 1.0 * 10 5 Pascal + Density * Depth * g *1 m 2

Pressure Blow out. You are probably familiar with the fact that winds go from high pressure areas to low. However, do you understand why? Why does this happen?

Water under the dam Imagine a dam which is 100 m high. What will the pressure on the bottom of the dam be on the water side? How about on the air side? What will the pressure halfway up the dam be? Suppose you poked holes in the dam at those two positions. What would the result be and why would the two holes react differently?

Net force As we see, the forces in difference places (well different depths) are different. So, what about the NET force? Suppose we have a crate which is 1 cubic meter and is a perfect cube. What is the net force in the sideways direction (hint how will the forces on each side compare)?

Buoyancy What is the force on the bottom if the crate is set up so that the top is on the exact surface of the water? What direction is this force? What is the force on the top of the crate? What direction is the force. Now, find the net force on the crate. This force is called Buoyancy.

What are the only things that the Buoyancy force depends on? No I have not given some exact equation, but look at what you have done here and think about how that would affect it.

Depends on Volume of object Density of medium you are in NOTICE: the mass of the object is NOT a dependency. So, a cubic meter of ice and rock have the same Buoyancy force!

Um, wait a minute hold the phones – stop the presses! How can a rock and ice for the same volume have the same buoyancy force? Clearly they have different masses and therefore different weights. How can this be? Well, look at the net force of everything (add the gravity force and the buoyancy force). The mass of a cubic meter of ice is about 700 kg. For rock it is about 3000 kg. What direction will this net force be for ice vs. rock?

Sink like a rock? If the force of buoyancy is less than the gravitational force, the object will sink. However, it won’t fall as fast as if you dropped it. Its acceleration will be slowed. If the force of buoyancy is exactly the SAME, you will just float where you are at. At what density do you think this will be the case?

Floatation Device If buoyancy exceeds gravity you go UP! However, what happens when part of the crate exits the water. That is, how does the upwards and downwards pressure forces change (if at all) as the crate pushes up?

Some change ends change The force on the top stays the same – pretty much. It is moving into air, which is far less dense than water, so the pressure barely changes. The bottom pressure decreases, so that force decreases. So, your buoyancy decreases. When will the buoyancy stop decreasing – and when does that occur?

Archimedes’ Principle The buoyancy stabilizes once the buoyancy force equals the weight of the object. At this point, it is good to note that the buoyancy force is the weight of the liquid you are displacing (i.e. the weight of the water for the volume that the object takes up of the water). So, the amount of water displaced is equal to the weight of the crate. Why is that useful?

Float vs sink If you float, you move water equal to your weight. If the object sinks, it moves water equal to its volume. How can we use this to find the mass and density of an object?

You ship is in a lock. Your ship has an iron anchor. You toss the anchor into the water. Will the level of water in the lock rise or fall?

Hydraulics Image from wikipedia The pressures for each side are the same. F1 / A1 = F2 / A2 So, a force over a big area can be held up by a small force over a small area. Note though that the works are the same.

Surface tension In an infinite liquid at every point you have liquid pushing against you from every direction. However, when you have a surface, you press the liquid against that surface, but nothing pushes back, or liquid doesn’t. This causes the surface to become more adhesive or film like.

Atmosphere Air is like a fluid – but one that is not very dense. As you get higher, the air gets thinner (less dense). For every 5.6 km you go up, the atmospheric pressure decreases by half (meaning that half of the air is below you).

Buoyancy Works the same way, but now it is based off of the density of local air instead of water. If you are less dense than air, your buoyancy is greater than weight, so you fly! This is how hot air balloons work.

Tire pressure If you have a closed surface, you can add a lot more of a gas. This makes it have a higher pressure (pressure is stuff running into you, so if you have more of it, then you have more pressure).

Boyle’s Law P1V1 = P2V2 Meaning that if you have air inside a closed object and you make it bigger, the pressure inside that object decreases. If you shrink it, the pressure increases.

Pressure of moving fluids This applies to either air or water. If it moves, the pressure decreases. So, the pressure of the water in a moving river is less than the pressure of water that is not moving. Note this leads to a problem for swimmers…

Don’t swim in moving water… Imagine the typical river. Lets say the water is flowing at 5 m/s in the center. What is the velocity of water at the edge? Because of this, imagine you were trying to swim to shore from the center of the river. Since things get pulled to the region of lowest pressure, how will this affect you getting to shore? If there were 2 boats 5 meters from each side of the center of the river that were unanchored what would happen to those two boats?

Airplanes! Airplanes are set up such that the air velocity on the top of the wing is much higher than the bottom. How do the pressures of air on the top wing and bottom wing compare? What is the direction of the resulting force? Why would the magnitude of this force depend on the aircraft’s velocity?

Curve ball The velocity is relative to the surface. When you spin a ball, one side of the ball has lower pressure than the other. This creates a force which moves the ball perpendicular to the direction of motion. The direction of the force just depends on the direction of spin (so you can get it to curve either direction, down, up, or even at some weird angle as is the case of a knuckleball).

Plasma Plasma is the 4 th form of matter (solid, liquid, gas, plasma). Plasma is a super heated gas in which electrons are stripped from every atom. Stars are made of 100% plasma.

Conclusion Today we have seem how fluids (liquids and gasses) affect the world around us. We have examined Buoyancy and pressure.