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Physics Question of the Week

Each week, we highlight one of our many demonstrations by asking a physics question relevant to the topic designed to be illustrated. This not only challenges audiences to carefully examine their previously held views on physics, but also provides suggestions for teachers looking to use these demonstrations in their own classrooms.

Be sure to check back each week for the solutions to the previous weeks' questions and for newest question of the week.

  • Question of the Week Archive
  • Question of the Week Summer Girls Special Part 1
  • Question of the Week 5/12-5/16 with Answer
  • Question of the Week 5/5-5/9 with Answer
  • Question of the Week 4/28-5/02 with Answer

A candle flame is positioned immediately in front of a loudspeaker, as seen in the photograph below. This view is "along the axis" of the loudspeaker.

 h1-02

An audio oscillator/amplifier connected to the loudspeaker is turned on at the resonant frequency of the loudspeaker, about 36 Hz, so that the loudspeaker produces a large sound wave. The input to the speaker is several watts of power, and its efficiency is over ten percent at its resonant frequency, so it is probably putting out nearly one watt of audio.

The question this week involves what the candle flame will do, if anything, when the audio input is turned up to a high level. In particular, will the flame move, and if so, how?

When the loudspeaker is operated at a large power level at its resonant frequency, the candle flame will:

  • (a) oscillate in the direction along the axis of the sound wave as it leaves the speaker, forming a long bright line when viewed across the speaker axis (from left to right in the photograph above).
  • (b) oscillate in the direction perpendicular to the axis of the sound wave as it leaves the speaker, forming a long bright line when viewed looking toward the speaker along its axis (the view in the picture above).
  • (c) oscillate in both directions, forming a large bright region when viewed from any direction.
  • (d) not move at all, because of the nature of the sound wave.

 

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A steel mass hangs by a spring from the inside of an upside-down teflon beaker, as seen in the photographs below. The photograph at the right is looking up into the beaker to see how the spring is attached.

c4-51 c4-51a

 

The beaker will be held in the air and released from rest. Of course, the entire system will fall vertically downward. But the question this week involves what the steel weight will do relative to the beaker immediately after it is released.

 

The beaker might act like a parachute and catch the air, so the weight would then extend the spring a bit more after it is released. On the other hand, perhaps because the teflon beaker is less dense than the steel weight, the beaker will accelerate faster and the weight will move up into the beaker. Or perhaps the entire system will simply fall downward in the same configuration - that is, with the weight extending out the same distance that is shown in the still photograph. Or maybe the logic here is entirely dorked up.

 

When the beaker system is released, the weight will:

 

  • (a) extend the spring further.
  • (b) move into the beaker.
  • (c) remain in the same configuration as it falls.

 

After May 16, 2014, click Read More for the answer

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A light dowel rod hangs by a string from a fixed point. The bottom end of the dowel rod rests on a frictionless surface, in this case a small piece of very light foam, that supports the end of the dowel on the surface of the water.

c6-21a

 

The drawings above illustrate three possible resting positions for the dowel. When the dowel is placed in one of the three positions and released, where will it finally come to rest?

  • (a) with its upper end beneath the support point.
  • (b) with its center of mass beneath the support point.
  • (c) with its lower end beneath the support point.

After May 9, 2014, click Read More for the Answer.

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A V-shaped wooden base is constructed so that the point of the V is lower than the open end, as seen in the photographs below. When a wooden cylinder is placed at the center of the V and released, it rolls downward, toward the point of the V, as seen in an mpeg video by clicking your mouse on the photograph at the left.

b1-07b  b1-07c

 

A double cone is then positioned at the center of the V. When it is released, will it:

 

  • (a) roll toward the point of the V?
  • (b) roll toward the open end of the V?
  • (c) remain where it was released?

 

After May 2, 2014, click Read More for the Answer

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