Transcript Lecture 2 - Archimedes' Principle

Announcements 1/7/11
Prayer

Two signup sheets going around (again).
a. Check your info / Add your info

If you weren’t here on Monday:
a. Read the syllabus, talk to students who were here last
time
b. Look at website:
http://www.physics.byu.edu/faculty/colton/courses/phy12
3-winter11/
(especially the “How to get started” section, and lecture
notes from Wednesday’s class)

Lab 1 starts tomorrow
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The first homework assignment is due tonight!

Remember: my office hours are right after class, MWF, in
the underground lab under the skylight; TA’s office hours
are 4 – 5:30 pm, MWF, same place

Thought question (ungraded):

For a longer canyon behind the dam
(red arrow length), the dam…
a. can be weaker
b. must be stronger
c. can be the same
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. yes
b. no
Demo
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Bed of nails
Reverse tug of war
Barometer
(closed)
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P1=0

vacuum
unknown
P2
Demo: Barometer
How to read?
Straws
Thought question

How long can a straw be (and still function)?
a. 0 – 0.5 m
b. 0.5 m – 2 m
c. 2 m – 5 m
d. 5 m – 20 m
e. No height limit
Reading quiz:

The buoyant force on an object submerged in
water always equals…
a. the weight of the object
b. the net force on the object
c. the weight of the water that would
otherwise occupy the object's space
FB
Archimedes’ Principle
FB = “weight of the displaced fluid”
mg
Proof: imaginary plastic bag
filled with water
FBuoyant = B = mdisplaced fluid  g
= rfluidVobjectg
Demos
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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. ironwood
b. walnut
c. same buoyant force
ironwood
walnut
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).
rlead > rsteel > rironwood. The bouyant force is
greatest on the _______ cube
a. lead
b. steel
c. wood
d. 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 gold
b. 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 increases
b. scale reading decreases
c. nothing changes
 What will happen to the lower scale?
a. scale reading increases
b. scale reading decreases
c. nothing changes
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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 rises
b. It falls
c. It stays the same