Announcements 8/31/11 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)

Lab 1 starts Saturday The first homework assignment is due tonight My office hours are right after class in the underground

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

Chris (TA) has office hours MW 5:00 – 6:30 pm (changed!), F 3:30 – 5 pm, 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.

The Far Side

Thought question (ungraded):

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)

Reading quiz (graded):

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

Thought question

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

Reading 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

Thought 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

Thought 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

Thought question: Archimedes was charged with determining if

a crown was pure gold. One method he may have used: suppose he balanced the crown with a lump of pure gold outside water. After immersing, the balance tipped as shown. The crown must:

a. Be denser than goldb. Be less dense than gold

Thought questions: In the demo, what will happen to

the upper scale when the mass is submerged.

a. scale reading increasesb. scale reading decreasesc. nothing changes

What will happen to the lower scale?

a. scale reading increasesb. scale reading decreasesc. nothing changes Demo: scales

Thought question:

A cannonball is in a boat, which is in the middle of a small lake. (Frictionless, wave-less, etc.) A person tosses the cannonball from the boat into the lake. What happens to the overall water level of the lake (where the water touches the shore)?

a. It risesb. It fallsc. It stays the same