Phys. 121: Tuesday, 18 Nov. ● HW 9 returned: please pick up. HW 10 will likely come back Thurs. morning; I have posted solutions to HWs 10 and 11 outside.

Phys. 121: Tuesday, 18 Nov.● HW 9 returned: please pick up. HW 10 will likely come back Thurs. morning; I have posted solutions to HWs 10 and 11 outside my office.● HW 12: due by Wed. before break (26th).● Mast. Phys.: Assign. 10 due by midnight. More ex. credit is available. Next assign. is the LAST one!!● Reading: Read chs. 18 and 19 for first week of Dec.● Exam 2: Test corr. through the OSL are available on problems 3, 4, 10, and 14; return them to me with your original exam by this evening.● Exam 3: will be Thursday, over chapters 9, 12, 13, and sect. 10.7. All the usual stuff is available online. Don't forget your calculator and formula sheet!

Clickers:

A mass oscillates on a horizontal spring with period T 2.0 s. If the amplitude of the oscillation is doubled, the new period will be

A. 1.0 s.

B. 1.4 s.

C. 2.0 s.

D. 2.8 s.

E. 4.0 s

Slide 14-51

Notice that ω (and therefore also T and f)is NOT adjustable; it's always the same fora given mass and spring. In contrast, bothA and φ depend upon the particular motion(initial conditions).

Clickers:

A block of mass m oscillates on a horizontal spring with period T 2.0 s. If a second identical block is glued to the top of the first block, the new period will be

A. 1.0 s.

B. 1.4 s.

C. 2.0 s.

D. 2.8 s.

E. 4.0 s.

Clickers:

A. /2 rad.

B. 0 rad.

C. /2 rad.

D. rad.

E. None of these.

This is the position graph of a mass oscillating on a horizontal spring. What is the phase constant 0?

QuickCheck 14.2

A. /2 rad.

B. 0 rad.

C. /2 rad.

D. rad.

E. None of these.

This is the position graph of a mass oscillating on a horizontal spring. What is the phase constant 0?

Initial conditions:x = 0vx > 0

Clickers:

A block oscillates on a very long horizontal spring. The graph shows the block’s kinetic energy as a function of position. What is the spring constant?

A. 1 N/m.

B. 2 N/m.

C. 4 N/m.

D. 8 N/m.

E. I have no idea.

Internal Energy and Temperature

Besides kinetic and potential energy, objects made up of

smaller particles (atoms, molecules) can also have internal

energy. Internal energy is simply the kinetic energy of the

smaller particles bouncing against each other randomly

(and not that due to the net motion of the large object).

When two such objects touch each other, their molecules

can now bounce against each other at the interface, and so

internal energy can be transferred. If no net transfer occurs,

we say the objects have the same temperature.

Temperature and the Zeroth Lawof Thermodynamics

If internal energy has a net flow from object A

to object B when they come into contact, we say

that object A has a higher temperature than object

B. If there is no net flow, they have the same

temperature. Different scales of temperature exist

(see next slide).

Zeroth Law of Thermodynamics: if objects A

and C are at the same temperature, and so are

objects B and C, then objects A and B are also

at the same temperature.

Conversions:T_C = T_K – 273.15T_F = (9/5) T_C + 32

Clickers: At what temperature do theFahrenheit and Centigrade temperaturescales agree with one another?

a) Never b) At 0 degrees c) At 100 degrees d) At -40 degrees e) At -215 degrees

Clickers: What is heat?

a) A fluid which raises temperature when added to an object (and lowers temp. when removed) b) Potential energy which is bound in molecules c) Potential energy which is pure rotational d) Kinetic energy of the center of mass of large objects, plus its net rotational K.E. e) Kinetic energy whose details are hard to keep track of

Clickers: Benjamin Thompson, aka CountRumford, found (while boring cannon duringthe American Revolution) that heat kept comingout (and so heat wasn't a fixed liquid). He also...

a) was born in America b) invented thermal underwear c) invented the double boiler (for cooking) d) fought for the British during the war e) all of the above

Mechanical Equivalent of Heat Heat is internal energy which is being

put into or taken out of an object. Changes

in heat go with changes in temperature.

Because of this, temperature changes can

be made into energy changes! The ratio of

energy added or taken out (in Joules) to the

temperature change (in degrees of any kind)

is called the heat capacity of the object.

(But, heat capacity increases with object size!)

Mechanical Equivalent of Heat (Joule'sexperiment):

Specific Heat c is heat capacity perunit mass, and depends only on whatthe object is made of... not how largeit is!

Clickers: Your oven is at 400 degrees F.Which of these objects inside couldyou safely grab with your hands?

• a) A steel frying pan• b) A potato• c) A thin sheet of aluminum foil• d) An angry lobster (who is still alive)

that you're trying to bake for dinner

Clickers: A cheeseburger with 500food calories (Cal) has enough energy togive how much mass a speed of 1 m/s?

• a) 1 gram• b) 0.5 kg• c) 1 kg• d) 100 kg• e) 4.2 million kg

Example: How long should youmicrowave 330 mL of water at 10˚ C (in a 900 W oven)so that it just reaches the boiling point?

Hot and Cold coming to equilibrium: if the twoobjects are insulated from the surroundings, set theheat (ΔQ) lost by the hot object equal to the heatgained by the cold one, and set the final temperaturesto be equal. The heat capacities determine the rest.

Clickers: if a hot rock of mass 250 gramsis dropped into an equal mass of cold water,

whose temperature changes the most?

a) The rock

b) The water

c) They will have equal temperature changes.

d) Need more information to tell.

Example: A 1.1 kg ironhorseshoe at 550˚ C is plunged into a bucketfull of 15 kg of water at 20˚ C. Find the finalequilibrium temperature.

Heat capacity is NOT well defined during a phasechange.Notice the constant temperature during the changes!

Instead, a fixed amount of heat per mass isneeded to make the phase change (and thetemperature remains constant during this).It's called the latent heat or the heat oftransformation.

L depends upon the substance, and whether it's thefreezing/melting point (L_f for “fusion”) or theboiling/condensation point (L_v for “vaporization”).Table 17.3 on p. 482 lists a few of these.