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
  • torques: 1,
  • bouyancy: 3, 10, 16, 17, 32, 40, 56,
  • springs: 26, 31, 72,
  • friction: 23,
  • multi-component 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,
  • multi-component 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,

• 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 set-up, 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 set-up, but a good question nonetheless; just takes a moment to understand what is going on.
• Q17 (Floating boat and rock). Topic: Statics; bouyancy.
• Q18-19: (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 non-linear 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 non-ideal 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 half-way point of the block. This is a great question, which could lead to all sorts of follow-up questions like "what if the polystyrene block has non-vertical 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 weighing-machine) 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 hand-cranked 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; multi-component 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; multi-component 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; multi-component 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, thought-provoking, year 2.
• Q79: Fluids. Very good, thought-provoking, 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 thought-provoking 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 set-up 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.