Announcements 8/29/12 Prayer Signup sheet going around again If you weren’t here on Monday:

a. Read the syllabus, talk to other studentsb. Look at website:

http://www.physics.byu.edu/faculty/colton/courses/phy123-fall11/

(especially the “How to get started” section, and lecture notes from Monday’s class)

The first homework assignment is due tonight Don’t forget warm-up questions before each lecture My office hours are right after class in the underground

lab under the skylight. You can follow me there if you like.

Clement (TA) has office hours MWF 3 – 4:30 pm also in the underground lab

A word about exams:a. Note cards will likely NOT

be allowed.b. Instead, I will give you the

hardest equations (in my opinion) on the first page of the exam. All other equations you will be expected to have memorized, or be able to derive if needed.

c. A sample first page is provided on the class web site, covering the entire course. Recommendation: print out that page and use it while working the homework!

The Far Side

Warmup 1 question 1 Imagine taking a thumbtack between your

thumb and forefinger, with the sharp end on your thumb and the tail end against your forefinger (even though each end of the tack exerts the same pressure). If you squeeze, it's immediately painful on your thumb but not your finger. Why?

a. Um...I think this question is worded incorrectly... Doesn't each end of the tack exert the same force, but the pressure exerted on your thumb is greater due to the smaller area at the pointy part of the tack?

Warmup 1 question 2 Is it possible for two fluids to exert a

different pressure on a submerged object (submerged object in the same location, only the fluids are changed)? Explain why or why not?

a. Yes. The pressure on a submerged object due to the fluid depends on the density of the fluid. If the two fluids have different densities, the pressure on the object will be different.

Warmup 1 question 3 Was there anything from the reading

assignment that you would like me to spend extra time on?

a. Can we talk about compressible fluids yet? b. Could you explain Pascal's law using

examples and go over the proof behind it?–A change in the pressure applied is

transmitted undiminished to every point of fluid and to the walls of the container

Warmup 1 question 4 Other comments:

a. I am simultaneously taking Physics 123 and Physics 220. I heard that if I purchase the Physics 220 it will work for both classes. Is this true?

b. The sinking fishing bob is unsettling.

Clicker question:

For a longer canyon behind the dam (red arrow length), the dam…

a. can be weakerb. must be strongerc. can be the same

(Think of the pressure exerted by the water against the dam)

Clicker question:

Ralph measures the pressure in his flat tire with a standard automotive pressure gauge. The gauge reads zero. Is the pressure inside the tire 0 atm?

a. yesb. no

Demos

Bed of nails Reverse tug of war

Barometer

How to read? Straws

unknown P2

P1=0

(closed)

vacuum

Demo: Barometer

Clicker quiz:

How long can a straw be (and still function)?

a. 0 – 0.5 mb. 0.5 m – 2 mc. 2 m – 5 md. 5 m – 20 me. No height limit

Warmup 2 question 1 An apple is held completely submerged just

below the surface of water in a container. The apple is then moved to a deeper point in the water. Compared with the force needed to hold the apple just below the surface, is the force required now bigger or smaller than at first? Explain your answer. (Assume that the density of water is the same at all depths.)

a. The force required to hold the apple under water will be the same at both depths. The pressure on the bottom of the apple will be larger as the depth increases, but so will the pressure on the top of the apple. The net difference will be the same independent of the depth. So the force will also be the same. (Also: buoyant force is weight of displaced fluid, so as long as density is not changing the buoyant force won't change.)

Clicker quiz:

The buoyant force on an object submerged in water always equals…

a. the weight of the objectb. the net force on the object c. the weight of the water that would

otherwise occupy the object's space

Archimedes’ Principle

FBuoyant = B = mdisplaced fluid g = fluidVobjectg

FB

mg

FB = “weight of the displaced fluid”

Proof: imaginary plastic bag filled with water

Demos

Coke Aluminum Beads

Clicker question: Two cubes of the same size and shape are

made out of wood. The ironwood cube sinks, but the walnut cube floats. The bouyant force is greatest on the _______ cube

a. ironwoodb. walnut c. same buoyant force

ironwoodwalnut

Clicker question:

Three cubes of the same size and shape are put in water. They sink. One is lead, one is steel and one is a dense wood (ironwood). lead > steel > ironwood. The buoyant force is greatest on the _______ cube

a. leadb. steel c. woodd. same buoyant force

Warmup 2 question 2 A wooden block floats in water, and a steel

object is attached to the bottom of the block by a string. If the block remains floating, which of the following statements are valid. (Choose all the correct statements.)

a. The buoyant force on the steel object is equal to its weight.

b. The buoyant force on the block is equal to its weight.

c. The tension in the string is equal to the weight of the steel object.

d. The tension in the string is less than the weight of the steel object.

e. The buoyant force on the block is equal to the weight of the volume of the water it displaces.