Announcements 83111 Prayer n Signup sheet going around

Announcements 8/31/11 Prayer n Signup sheet going around again n If you weren’t here on Monday: – Read the syllabus, talk to other students – Look at website: n http: //www. physics. byu. edu/faculty/colton/courses/phy 123 -fall 11/ n n (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

n. A word about exams: – Note cards will likely NOT be allowed. – 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. – A sample first page is provided on the class web site, covering the entire course. The Far Side

Thought question (ungraded): n For a longer canyon behind the dam (red arrow length), the dam… – can be weaker – must be stronger – can be the same (Think of the pressure exerted by the water against the dam)

Reading quiz (graded): n 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? – yes – no

Demos n Bed of nails n Reverse tug of war

Barometer (closed) vacuum P 1=0 unknown P 2 Demo: Barometer n How to read? n Straws

Thought question n How long can a straw be (and still function)? – 0. 5 m – 2 m– 5 m – 5 m – 20 m – No height limit

Reading quiz: n The buoyant force on an object submerged in water always equals… – the weight of the object – the net force on the object – 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 = rfluid. Vobjectg

Demos n Coke n Aluminum n Beads

Thought question: n 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 – ironwood – walnut – same buoyant force ironwood walnut

Thought question: n 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 buoyant force is greatest on the _______ cube – lead – steel – wood – same buoyant force

Thought question: n 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: – Be denser than gold – Be less dense than gold

Thought questions: In the demo, what will happen to the upper scale when the mass is submerged. – scale reading increases – scale reading decreases – nothing changes n What will happen to the lower scale? – scale reading increases – scale reading decreases – nothing changes n Demo: scales

Thought question: n 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)? – It rises – It falls – It stays the same