Lecture Outline Chapter 9 College Physics, 7 th Edition Wilson / Buffa / Lou © 2010 Pearson Education, Inc

Lecture Outline

Chapter 9

College Physics, 7th Edition

Wilson / Buffa / Lou

© 2010 Pearson Education, Inc.

Units of Chapter 9


9.2 Fluids: Pressure and Pascal’s Principle

• What is a fluid?

• Liquids and gases are referred to as fluids.

• Fluids have little or no elastic response to a force. Instead, the force merely causes the fluid to flow.


Pressure is defined as the force per unit area:

If the force is at an angle to the surface, the more general form (blue box) is used.



Unit of pressure: the Pascal (Pa)

Density is defined as mass per unit volume: (aka…measure of the compactness of matter)


Units???


The pressure in a fluid increases with depth, due to the weight of fluid above it.


1 atm = 101.3 kPa = 760 Torr

Ear Popping?? Scuba Diving?


• What is the total pressure on the back of a scuba diver in a lake at a depth of 8.00 cm?

• What is the force on the diver's back due to the water alone? (Take the surface of the back to be a rectangle 60.0 cm by 50.0 cm)


Pascal’s principle:

Pressure applied to an enclosed fluid is transmitted undiminished to every point in the fluid and to the walls of the container.

For an incompressible liquid, the change in pressure is transmitted instantaneously.



Hydraulic lifts take advantage of Pascal’s principle.

Using Pascal's Principle, it can be shown how such systems allow us not only to transmit force from one place to another, but also to multiply that force. What does this mean?

If, pi = po, then

Fi = Fo

Ai Ao



A garage lift has an input and lift (output) pistons with diameters of 10 cm and 30 cm, respectively. The lift is used to hold up a car with a weight of 1.4 x 104N.

a.) What is the magnitude of the force on the input piston?

b.) What pressure is applied to the input piston?



There are a number of methods used to measure pressure. [We will focus on 3 types]



• An IV is a type of gravity assist. Consider a hospital patient who receives an IV under gravity flow. If the blood gauge pressure in the vein is 20.0 mmHg, above what height should the bottle be placed for the IV blood transfusion to function properly?

9.3 Buoyancy and Archimedes’ Principle

• When an object is placed in a fluid, it will either sink or float.

• Why do things float??BuoyantThere must be an upward net force

acting on the object (greater than the downward weight force).

• This upward force is called the Buoyant Force.


A body immersed wholly or partially in a fluid experiences a buoyant force equal in magnitude to the weight of the volume of fluid that is displaced:

This is known as Archimedes’ PrincipleHow did he come up with principle?



• A container of water with an overflow tube, sits on a scale that reads 40 N. The water level is just below the exit tube in the side of the container. An 8.0 N cube of wood is placed in the container. The water displaced by the floating cube runs out the exit tube into another container that is not on the scale. Will the scale reading be:

1. exactly 48 N2. between 40 N and 48 N3. exactly 40 N4. less than 40 N


• Commonly said the helium and hot air balloons float because they are lighter than air. However, that is not technically correct. Why?


The buoyant force on an object that is completely submerged:



Examples:

1.It is the average density that matters; a boat made of steel can float because its interior is mostly air.

2.An object’s density may be changed; submarines fill tanks with water to submerge, and with air to rise.

3.Fish can control their depths by using their swim bladders or gas bladders.



• A uniform solid cube of material 10.0 cm on each side has a mass of 700 kg.

a.) Will the cube float in water? How do you know?

9.4 Fluid Dynamics and Bernoulli’s Equation

We prefer to think of fluids as being ideal. We like simple models. Ideal fluids have 4 characteristics of flow:

1.Steady

2.Irrotational

3.Nonviscous

4.Incompressible

Let’s discuss what each of these characteristics are.



1.Steady

1.Irrotational

1.Nonviscous

1.Incompressible


Equation of continuity


If there are no losses of fluid within a uniform tube, the mass of fluid

flowing in must be equal to the mass flowing out.


If the density is constant, then we call this the flow rate equation:


Or

Av = constant (flow rate)

Units?


• Blood flows at a rate of 5.00 L/min through an aorta with a radius of 1.00 cm. What is the speed of blood flow in the aorta?


• Bernoulli’s Principle in simple terms:

• As the pressure in the fluid decreases, the speed of the fluid increases (and vice versa).

• What type of relationship is this?


• Because of the shape and orientation of an airfoil (wing), the air streamlines are closer together, and the air speed is greater above the wing than below. By Bernoulli’s Principle, the resulting pressure difference supplies part of the upward force called the lift.


Bernoulli’s equation is a consequence of the conservation of energy.


I don’t think you will need this on your AP. The objective is a little unclear. So I want you

to at least know what the variables stand for…and I will quiz you on this formula for

future formula quizzes. Just in Case!!!

9.5 Surface Tension, Viscosity, and Poiseuille’s Law

Molecules of a liquid exert small attractive forces on each other. These attractive forces are called van der Waals forces.

There is an inward pull of surface molecules causing the surface of the liquid to contract and to resist being stretched or broken under tension.


Let’s try this: Carefully placing a

sewing needle on top of water in a cup.

9.5 Surface Tension and Viscosity

• Viscosity: an internal resistance to flow

• Like friction

• Honey is very viscous

• Viscosity varies with temperature. “Slow as molasses in January”


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Lecture Outline Chapter 9 College Physics, 7 th Edition Wilson / Buffa / Lou © 2010 Pearson Education, Inc