G.C. Sloan: Physics 2305, Exam 4

Physics 2305: Exam 4 Results

Answer keys are below As usual, letter grades assigned to exams are for comparative purposes only; they aren't used to determine final course grades.

How do you get your exam back?

You have three options

Come by my office between now and the end of May to pick it up. It's not a bad idea to email me in advance or call to make sure I'm in.

Send a stamped, self-addressed envelope with 55 cents postage to:

     G. Sloan--Exam 4
     Physics Department
     Virginia Tech
     Blacksburg, VA 24061-0435

Pick your exam up from Prof. John Simonetti (Robeson 319) in the summer or fall.

What about normalization?

I added 3 points to the scores for forms A and C to correct for a lower average compared to forms B and D. This is the first time this semester that such a step was necessary.

Results




Students:	235
Mean:	69.5
Std Dev:	17.7



A	29
B	69
C	105
D	15
F	17

Answers to multiple-choice questions

	Form A	Form B	Form C	Form D
1.	D	D	B	C
2.	C	B	A	B
3.	B	C	D	A
4.	A	A	D	C
5.	D	A	D	C
6.	D	A	B	A
7.	D	B	C	C
8.	C	B	C	C
9.	B	C	C	C
10.	B	C	B	C
11.	C	A	D	B
12.	A	B	A	B
13.	B	A	D	A
14.	C	C	C	C

Answers to free-form questions

The gravitron
a) v = 2 pi r / T = 12.6 m/s (Forms A,C) or 9.8 m/s (Forms B,D);
b) Your diagram should have mg down, force of friction up, and the normal force pointing to the center of the gravitron; there should be no force labelled "ma" or "mv²/r";
c) mu = gr/v² = 0.19 (A,C) or 0.25 (B,D).

The bowling ball
a) K = (7/10) mv²;
b) L = (7v²)/(10 g sin theta) = 0.78 m (Form A) or 1.06 m (Form B).

The nickel in the bowl
a) K = (3/4) mv²;
b) v = (4gh/3)^1/2 = 1.5 m/s (Form C) or 1.8 m/s (Form D).

H₂O
a) Q_V = 11.3 MJ (Form A) or 9.02 MJ (Form C) leaving 0.52 MJ (A) or 2.8 MJ (C) for the rest of the problem;
b) T_f = T_i + Q_left/n C_p = 427 K (A) or 749 K (C) (note that C_p = 4R);
c) W = Q_left - DE_int = n C_p DT - n C_v DT = 0.125 MJ (A) or 0.70 MJ (C) (C_v = 3R).

N₂
a) n = (p_iV_i) / (RT_i) = 12.15 moles (Form B) or 15.0 moles (Form D);
b) T_f = T_i + Q_left/n C_p = 330 K (B) or 324 K (D) (C_p = 3.5 R);
c) W = Q - DE_int = n C_v DT = 3.0 kJ (both forms) (C_v = 2.5 R).

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