Comments on UMD Physics Questions
The
`Physics question of the week'
from UMD is a
fantastic web site. A really fun way to learn Physics.
These are some comments on the questions.
Also, I have categorized the problems for
ease of reference.
 Statics
 Dynamics
 Scaling
 Fluids
 Estimation
 Circuits
 Electricity, Magnetism
 Light
 Geometry
 Other

 Statics
 torques: 1,
 bouyancy: 3, 10, 16, 17, 32, 40,
56,
 springs: 26, 31, 72,
 friction: 23,
 multicomponent systems: 5, 25,
 Surface tension: 5,
 Dynamics
 particles: 2, 6, 7, 8, 47, 49, 54,
 rolling: 11, 20, 41, 50, 52, 59, 62, 73, 87,
 springs: 31, 35,
 rigid bodies: 53, 85,
 angular momentum: 85,
 Pendulums: 38, 39,
 multicomponent systems: 25, 30, 41, 42, 43, 45, 52,
 Normal modes: 69, 70,
 Bouyancy: 69, 70, 71,
 rotation: 24, 30, 41, 44, 76, 82, 87, 88, 89,
 Elasticity: 5, 9, 48, 57, 81,
 Friction: 20, 24, 73,
 Impulses: 20, 42, 66, 67, 77,
 Collisions: 29,
 Fluid jets: 84,
 in rotating frame: 87, 88,
 Scaling arguments: 48, 57, 64,
 Fluids: 5, 46, 61, 64, 66, 67, 71, 78, 79, 84, 89,
 Air pressure: 81,
 Air flow: 86,
 Weather patterns: 89,
ms
 Estimation: 14, 37, 64,
 Circuits: 21, 22, 27,
28, 36, 37, 60, 74, 75,
 transformers: 55,
 inductors: 90, 91,
 Electromagnetism, Electricity, Magnetism; see also Circuits
 Microwaves: 15,
 Motors: 36, 37, 80,
 Inductors: 80, (see also circuits) 90,
 Lefthandrule: 58,
 Right hand rule: 63, 68,
 Plain Magnetism: 83,
 Light
 General: 12, 18, 19,
 Lenses: 4,
 Refraction+dispersion: 13,
 Diffraction: 65,
 Geometry: 33,
 Other: 51, 92,
link that was
http://www.physics.umd.edu/deptinfo/facilities/lecdem/QOTW/questions.html
is now
http://www.physics.umd.edu/lecdem/outreach/QOTW/active/questions.htm
but I can't get the links below to get the correct pages!
(q001 is intended to be correct)
 Q1 (Super balance). Topic: Statics;torques
Good question, leads into discussion of how balances are made
in shops and bathrooms.

Q2 (Ball race with dip). Topic: Dynamics;particles
Great question.

Q3 (Ball oil water). Topic: Statics;bouyancy;advanced

Q4 (Rays and lens). Topic: Light;lenses

Q5 (Balloons). Topic: Elasticity, Surface tension, Fluids

Q6 (Ball and buggy). Topic: Dynamics;particles;elementary

Q7 (Ball and accelerating buggy). Topic: Dynamics;particles;elementary

Q8 (Ball and buggy on slope). Topic: Dynamics;particles;

Q9 (Falling slinky). Topic: Dynamics;elasticity

Q10 (Finger in beaker). Topic: Statics;bouyancy
Great question.

Q11 (Rolling race). Topic: Dynamics;rolling
Great question.

Q12 (Chromatic rod). Topic: Light;general
Doesn't teach much Physics.

Q13 (Spectrum). Topic: Light;refraction
I think this problem needs a figure showing the setup, to make
clear where the collimated light comes from.

Q14 (Sphere filling). Topic: Estimation;volume.
Good one, but doesn't teach a great deal, apart from caution.

Q15 (Microwave polarization). Topic: EM;microwaves
Good.

Q16 (Barometer). Topic: Statics;bouyancy
This is a bit of a Rube Goldberg setup, but a good question
nonetheless; just takes a moment to understand
what is going on.

Q17 (Floating boat and rock). Topic: Statics; bouyancy.

Q1819: (Reflections of Gwen and transparency).
Topic: Light
Not very profound.

Q20: (pulling out the rug). Topic: Dynamics;rolling+friction+impulses.
This is a fantastic question.
The result is precisely correct whatever assumptions
are made about friction! I love it.
The only problem is the photo on the first
page shows the hand in a misleading position.
(The actual experiment has the sheet held horizontal,
as stated in the caption.) [Pulling as in the
photo would make the cylinder roll right.]

Q21: (interlaced circuits 1). Topic: Circuits.
OK question.

Q22: (interlaced circuits 2). Topic: Circuits.
OK question, but it depends heavily on the unstated
assumption that the batteries have ZERO internal resistance.
I'd like to see an ammeter recording the current flowing
in the right hand circuit  For real batteries, I bet it is not zero!

Q23: (sliding meter). Topic: Statics;Friction.
Good question.

Q24: (rotating magnets). Topic: Dynamics;Rotation+Friction.
Good question, but the answer movie is a but rushed.

Q25:
(floating wonky dowel), Good one. Topic: Dynamics+Statics.

Q26:
(string and two scales). I found this question a bit confusing
to understand on a first reading. Topic: Statics;springs.

Q27:
(Bulbs: series/parallel) . Topic: Circuits
This question is not ideal since (a) the parallel picture
does not clearly show the larger bulb burning brighter,
so this might dent the confidence of the student;
(b) the question is not clear about whether we
are to compare the brightnesses before'n'after
or to compare the bulbs with each other;
(c) the answer
could depend on the nonlinear properties
of bulbs  the resistance of a bulb is not
constant, so a cautious student could
justifiably say "OK, the resistance of the
two bulbs are R(large) < R(small) when they
are in the initial configuration, but
when we put them in series, the current
will drop, and who knows which bulb will then
have the larger resistance? I know that both
bulbs will get dimmer, but I really can't say by how much."
I am not happy with the given answer which
boldly, implicitly,
asserts that the bulbs have a constant resistance.

Q28:
(Three plate capacitor). Topic: Circuits
The answer given is incorrect, and
you can see by looking at the dial of
the meter!
The correct answer is that the human
cannot possibly insert the plate
perfectly straight, so the
effective distance between
the plates is reduced and
the capacitance increases, so the
voltage decreases. You can see
this effect (about 5%) in
the answer.
A better version of this question
would include the human
waggling the third plate around
and ask what happens to the
reading during the waggling:
(a) sometimes higher; (b) sometimes
lower; (c) can be higher or lower;
(d) stays absolutely constant.

Q29:
(Newton's cradle) Topic: Dynamics; collisions
nice question, but not
very striking demonstration.

Q30:
(Keys and matchbook) Topic: Dynamics; rotation + coupled systems
nice question, though
the "correct" answer depends
rather heavily on the intervention
of nonideal friction forces.
The answer (a) could be defended
by someone who thought that
Gwen's finger and the string
were both of very low friction.

Q31: (Connected Springs) Topic: Dynamics+statics;springs
A beauty.

Q32:
(Ballast styrofoam). Topic: Statics; bouyancy
The question needs to say that the black line
is exactly at the halfway point of the
block. This is a great question, which could lead
to all sorts of followup questions like "what if the
polystyrene block has nonvertical sides?"
 Q33,
Q34:
(3D Projections) Topic: Geometry.
Not much physics in these! (I don't think their "answers" are very
helpful either. The 3d object could have an enormous number of shapes!)
 Q35: (dynamic spring on weighingmachine) Topic: Dynamics;springs
A very good problem. A little hard to see in the picture
that the object at the bottom is simply a hook for attaching the
spring, and that the tray that that hook is attached to is a rigid
part of the hanging assembly.
 Q36: (two handcranked generators) Topic: EM;Motors
A fun problem, though more details of the insides of the generators are needed. Are
they standard DC motors with brushes? Potentially very educational.
 Q37: Topic: Circuits
Series bulb and motor.
An unanswerable question, in my opinion, unless viewed as a
tricky estimation question. The answer given is unconvincing.
 Q38: Topic: Dynamics;pendulums
An excellent question. (Though the use of large pendulum
amplitude may cause confusion.)
 Q39: Topic: Dynamics;pendulums
A nice question, but 38 is the one with the big mental impact.
 Q40: Topic: Statics;bouyancy
Fantastic question!
 Q41: (Trike) Topic: Dynamics;rolling
Great question.
 Q42: (masses and breaking string) Dynamics; impulses
I like this question a lot.
 Q43: (Toppling chimney) Topic: Dynamics; multicomponent systems
Fantastic question, very appropriate after double pendulum has been
studied. [In addition to the torque answer given, it is
Nice to answer it using the idea of exponentially growing normal modes. The second mode has the higher imaginary frequency.]
 Q44: (ball bearing in spiral tube.) Dynamics; rotation.
Elementary question.
 Q45: (3 pulleys) Dynamics; multicomponent systems
Great question.
 Q46: (fan cart) Dynamics; fluids.
A contrived question, unanswerable.
 Q47: (falling balls) Dynamics; particles.
Good very elementary problem.
 Q48: (block notes) Dynamics; elasticity.
Good question (1B).
 Q49: (cart on slope) Dynamics; particles.
Good question (1B).
 Q50: (Cable drum) Topic: Dynamics;rolling
Good question (1B). [Equivalent to trike.]
 Q51: Other.
 Q52: Dynamics; multicomponent systems.
Takes a long time to read.
An excellent question.
 Q53: Dynamics; rigid bodies.
A good question (1B) though I think the restricted range
of answers give the answer away.
 Q54: Dynamics; particles
Silly question, unanswerable; need to know the walking speed. I don't
see how anything can be learnt from this.
 Q55: Circuits; transformers.
Takes some time to read.
An interesting question. I don't understand it!
 Q56: (inflated balloon) Statics; bouyancy.
Great question.
 Q57: (thermal expansion) Elasticity;
I am not sure this question gives insight.
Is it not possible to make objects for which the other answer is
correct? Is linear thermal expansion correct?
 Q58: EM; left hand rule
Good (year 0).
 Q59: Dynamics; rolling.
Classic 1B question. But are their two objects really the same mass? :)
 Q60: (5 bulbs) Circuits.
Good year 0 question.
 Q61: (Feynman's sprinkler) Fluids;
Classic question, wonderful!
 Q62: (cones on slope) Dynamics; rolling
Not very deep question.
 Q63: (hanging coils) EM; right hand rule
V. Nice year 0 question.
 Q64: (coffee filters) Fluids/Dynamics/Scaling
Wonderful (1B) question; shame that the preamble gives it all away.
 Q65: Light; Diffraction
Good (1B)
 Q66: (hourglass) Fluids;
Nice (1a/1B) question.
 Q67: (hourglass2) Fluids;
Very interesting. I don't get it!
 Q68: EM; right hand rule
year 0/1a.
 Q69: Dynamics; normal modes + bouyancy
Good 1B question. It caught me!
 Q70: Dynamics; normal modes + bouyancy
Good 1B question.
 Q71: (balloon in box) Dynamics; bouyancy
Classic question. (1B) Really deserves a careful discussion of
pressure gradients in the box.
 Q72: Statics;springs
Good year 0 question.
 Q73: (car with locked rear wheels) Dynamics; friction
A very interesting question, but the answer given seems
completely unconvincing.
 Q74: Circuits.
Good elementary (year 0) question.
 Q75: Circuits.
Good elementary (year 0) question.
 Q76: Dynamics, circular motion, good one  year 1/2.
The answer mpeg does not show the answer very clearly but the question is grand.
 Q77: Dynamics, collisions, excellent year 2 question. I think their answer, talking about "transfer of linear momentum", may be found confusing, since the amount of momentum "transferred" can be more than the initial amount possessed by the ball.
 Q78: Fluids. Very good, thoughtprovoking, year 2.
 Q79: Fluids. Very good, thoughtprovoking, year 2.
 Q80: Electricity: inductors and motors. A hard year 1 question. Good.
 Q81: A very interesting challenging question, leaving you with lots more questions. Probably good for year 2, 3, 4, 5, 6!
 Q82: Rotational dynamics. Like 44.
 Q83: Magnetism. Year 0/1.
 Q84: Fluids and dynamics.
A good thoughtprovoking question. To answer it exactly
requires calculation (more calculation than is normal in
these questions) and careful reading of the question. Year 1.
 Q85: Gyroscope. Grand year 1 question. Nicely filmed.
 Q86: Air flow. Misleading diagram shows air bouncing off sail like a mirror. Good question, apart from that. Year 1/2.
 Q87: Dynamics. coriolis forces, rolling...
too much going on in this question in my opinion. A rolling body on
a turntable is nontrivial. Their answer neglects the forces
associated with rolling contact.
 Q88: Dynamics. coriolis forces, rolling...
too much going on in this question in my opinion.
They do mention the problem raised in the previous question
(i.e. neglecting the rolling forces) but they fluff it, in
my opinion. The whole thing should have been done with the ball
thrown through the air, not touching a surface!
 Q89: Rotation, fluids  I don't have a clue what this question is on about. The setup is not clear. The distances of the two flags from the centre of rotation are actually the same. (I thought one of them was
further out than the other; I also thought the rotation speed might be big enough to cause a big height difference).
I am sceptical about their creation of a "high" and "low" pressure
area by pumping fluid from one to the other.
I don't buy the analogy, but the experiment might be interesting. Rather
hard to see what is going on in the answer video. Year 2.
 Q90: Electricity, inductors. Year 1. Good.
 Q91: Electricity, inductors. Year 1. Great followup question!
 Q92: Radiation, reflectivity. Bizarre question. Requires a better screen than mine, perhaps!
 Q93: Radiation, reflectivity. An impossible question, but the answer is instructive.
 Q94:
 Q95:
 Q96: It's hard to see what "suspended at centre of mass" means. There must be a bolt hole through the centre of mass, I guess. Poor year 1 question.
 Q97:
 Q98:
 Q99:
 Q100: Optics, 1st year question, easy, but leaves an uneasy feeling
 Q101:
 Q102:
 Q103:
 Q104:
 Q105:
 Q106:
 Q107:
 Q108:
 Q109: Nice elementary dynamics qu.
David MacKay <mackay@mrao.cam.ac.uk>
Last modified: Wed Nov 20 19:48:33 2002
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