Date post: | 19-Dec-2015 |
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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