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Informal sessions:

o Tu 3:30-4:20pm, RAS 218;

o Fri 4-4:50pm, ECJ 1.204.

Lecture 34(iq30)

· SH-Oscillator (SHM): d²x/dt + w²x = 0. (1)

· Damped Oscillator: d²x/dt + w²x + (b/m) dx/dt = 0. (2)

o Resistive force R = - bv.

o Q factor: Q=2pE_env/|DE_env| = (km)^1/2/b.

· Driving a damped oscillator: d²x/dt + w²x + (b/m) dx/dt = F, (3)

o F=F₀ cos wt

o At resonance frequency: w=w₀: x=A sin w₀t, A=F₀/(bw₀).

Reminders:

· HW12 is relatively easy and short. Please start working on it asap. Bring your questions to today’s informal session for more discussion on HW12.

· For midterm3, please work through all the problems you missed. Make sure you understanding the physics of each problem.

· Concerns? A good time to see me is after MWF’s class and after TuF’s informal sessions. If you want to discuss your standing in the class, please bring your quest-record with you when you come. If you would like to see me in my office, please let me know ahead of time.

Your exam letter grade

Definition:

§ Scaled score = (Rawscore – Avg) x 100/std +500, where

§ Avg=class average(rawscore), Std=standard deviation.

§ Avg and Std are posted at the bottom of “my answer” page in your account.

For each exam, you have made two letter-grades.

§ The first letter grade is based on the percent-score-cutoffs.

They are: 40%, 55%, 75% and 90% for D, C, B and A respectively;

§ The second letter-grade is based on the scaled-score-cutoffs.

They are: 400, 450, 525, and 600 for D, C, B and A respectively.

If these two letter-grades are not the same, the better of the two is what you have made.