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Physical Science II. Fluid Physics. Fluids. Question : What are the three common states of matter?. Fluids. Three common states of matter: (1) Solid (2) Liquid (3) Gas

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Physical Science II

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Physical Science II

Fluid Physics


  • Question:

    • What are the three common states of matter?


  • Three common states of matter:

    (1) Solid (2) Liquid (3) Gas

  • Thus far, we have focused primarily on solids. In this unit, we will be investigating the physics of liquids and gases…


  • Fluid – a nonsolid state of matter in which the atoms or molecules are free to move past each other—includes both liquids and gases:

    • Solids have a definite volume and shape.

    • Liquids have a definite volume but not a definite shape—take the shape of the container they are placed in

    • Gases do not have a definite volume or shape—spread out when placed in a larger container



  • Which is more comfortable to sit on?

    • A recliner or a bicycle seat

  • The recliner is obviously the more comfortable choice, but why?

  • The recliner is more comfortable because it reduces the pressure acting on your body.


  • Pressure – the force per unit area acting on a surface:

  • In terms of the bicycle seat and the recliner, the recliner is more comfortable because it has a larger surface area.

  • The larger surface area causes the pressure to be lower.


  • Pressure is measured in Pascals (Pa), where 1 Pa = 1 N/m2


  • Question

    • What do you feel when you dive to the bottom of a deep pool?


  • As you travel deeper in a fluid, the pressure increases.

  • The only two factors that determine pressure in a fluid are depth and the type of fluid.

  • For example:

    • If we climb to 3000 feet above sea level, we don’t notice the change in air pressure.

    • If you dive only 8 feet below the surface of water you notice a much larger pressure acting on your body.

Air Pressure

  • We live at the bottom of a vast quantity of air.

  • The Earth’s atmosphere, what we call air, is made up of many different types of gasses.

  • The air pressure at sea level is approximately 100,000 Pascals.

  • Just as pressure increases with depth under water, the same is true of air pressure.

Air Pressure

  • If you’ve ever felt your ears pop while driving up a mountain or flying in a plane you’ve experienced your bodies reaction to a change in air pressure.

  • You may be surprised to hear that right now there are 1000N of force pushing down on your head from the atmosphere.

  • The only reason you survive this pressure is because your body is exerting it’s own pressure against the atmosphere.

Air Pressure

  • Air pressure is measured with an instrument known as a barometer.

  • The barometer is a device commonly used by meteorologists to help determine the weather forecast.

  • A rising barometer usually indicates clear weather with low humidity.

  • A falling barometer indicates the approach of a storm system.

Pascal’s Principle

  • Pascal’s Principle –a pressure applied to a fluid in a closed container is transmitted equally to every point of the fluid and to the walls of the container:

    • Example 1 – When air is added to a tire, the pressure increases at all points inside the tire by the exact same amount.

Pascal’s Principle

  • Example 2 – In a hydraulic lift, when a small force F1 is applied to a small piston of area A1, a larger force F2 is transmitted to a larger piston of area A2:

    Thus, if A2is greater than A1, then F2is greater than F1!

Pascal’s Principle

  • Hydraulic lifts operate by utilizing Pascal’s Principle.

  • In an auto body shop, hydraulic lifts allow vehicles to be elevated in order to be inspected and repaired.

  • Large construction vehicles use hydraulics to operate shovels.

Pascal’s Principle

Pascal’s Principle

  • The advantage of hydraulics is the ability to move large forces by only exerting a small force.

  • This is exactly like the operation of a simple machine.

  • Like a simple machine, if you increase your output force, you will also decrease the output distance.


  • Have you ever stood in a pool and tried lifting a friend while submerged in the water?

  • If you have, you probably noticed how easy it was to lift your friend.

  • So, why is lifting your friend easier in the pool?


  • Buoyant Force – an upward force that acts on an object in a fluid.

  • Every object in a fluid experiences a buoyant force.

  • When you and your friend are in a pool, the buoyant force from the pool water acts against the gravitational force on your bodies,

  • This makes it seem as if you and your friend weigh less.


  • Apparent Weight – in a fluid your apparent weight is equal to your actual weight minus the buoyant force.

  • There are several methods to find the buoyant force acting on an object, but the most important came from a Greek mathematician named Archimedes.

Archimedes’ Principle

  • Archimedes discovered an important property of buoyancy.

  • If you make the mistake of filling a bathtub too high, the water is going to overflow when you get into the tub.

  • The reason for this is because water is displaced to make room for your body to fit in the tub.

Archimedes’ Principle

  • Archimedes’ Principle – the buoyant force on an object is equal to the weight of the fluid that is displaced by that object.

  • To measure the buoyant force you can collect the water that is displaced by an object and measure the weight of that water.

  • The weight of the water is equal to the buoyant force.

Density and Buoyancy

  • Whether or not an object will float is based on density.

  • If an object is more dense than the liquid it is placed in, it will sink.

  • If an object is less dense than the liquid it is placed in, it will float.

Density and Buoyancy

  • The buoyant force acting on a floating object is exactly equal to the weight of that object.

  • Floating objects have an apparent weight of zero Newtons.

  • Also, since the buoyant force is equal to the weight of water displaced, floating objects must displace an amount of water that weighs as much as the object.

Density and Buoyancy

  • Examples:

    • When a 1500 N boat is floating in water, it is displacing 1500 N of that water.

    • If a 500 N person is floating in a pool, they are displacing 500 N of water.


Principles of Flight

  • In some of the earliest records of human history, studies of flight have been discovered.

  • The ancient Greeks studied birds in great detail and the Italian inventor Leonardo Da Vinci designed several flying machines.

  • For centuries humans had been attempting to fly with all sorts of wild creations, but it was the Wright brothers who eventually succeeded in controlled flight.

Principles of Flight

  • Since the Wright brothers’ historic flight there have been many advancements in the design of aircraft.

  • However, all flight is based on a series of physics principles.

Principles of Flight

  • Flight is possible because it occurs within the atmosphere of the Earth.

  • Earth’s atmosphere is a fluid that is made up of several different types of gasses most notably, nitrogen, oxygen and carbon dioxide.

  • The ability to fly through this fluid is based on four forces:

    • Lift, Thrust, Drag and Weight


  • One of the most important aspects of flight is lift.

  • Lift – the upward force acting on an airplane.

  • The cause of lift is a difference in air pressure on the top and bottom of the wing, which is explained by Bernoulli’s Principle.

  • Bernoulli’s Principle explains that when the speed of a fluid increases, the pressure within the fluid decreases.


  • The wing of an airplane is shaped so that the air moving over the top of the wing travels faster than the air moving around the bottom of the wing.

  • A lower pressure is created on top of the wing by the faster moving air.

  • The higher air pressure on the bottom of the wing provides the upward force that lifts the airplane.


  • While lift allows the airplane to get off the ground, it would not exist without any thrust.

  • In order for lift to be generated, the air must be moving around the wing at a high velocity.

  • The engines of the airplane provide the force that pushes it to speeds that are great enough to generate the lift.


  • Thrust is the force that is provided by the engines to move the airplane.

  • In order for the plane to get off the ground, the engines must provide enough thrust to move the air around the wings with enough speed to generate lift.

  • As long as the thrust is strong enough, the airplane will experience lift.


  • It is important to remember that air is made up of many different molecules and therefore it creates friction.

  • Air resistance is the friction that is created by the Earth’s atmosphere.

  • In terms of flight, air resistance produces an effect called drag.

  • Drag is simply the force of the air resistance that is acting against the forward motion of an airplane.


  • The drag forces that act against the motion of an airplane reduce fuel efficiency.

  • The more drag there is, the less efficient the aircraft.


  • The last of the forces involved in flying is weight.

  • Remember that weight is the force of gravity that is acting on an object.

  • In order for an airplane to get off the ground and to stay in flight, the lifting force must overcome the weight of the plane.

Forces of Flight

  • All in all, the success of flight is dependent on all four forces acting together.

  • If thrust and drag are equal, the plane will move at a constant speed.

  • If weight and lift are equal, the plane will fly at a constant altitude.

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