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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 9/1-9/5
  • Question of the Week Summer Girls Special Part 4 with Answer
  • Question of the Week Summer Girls Special Part 3 with Answer
  • Question of the Week Summer Girls Special Part 2 with Answer

Missed one of our older Question of the Weeks? Click here to access our archive.

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The photograph at the left below shows two short focal length parabolic concave mirrors. The mirror at the right has a heat source (a match) at its focus, and the mirror at the left has a thermal probe at its focus, seen in the photographs at the center and right. The match has not yet been lit in the photograph at the left, so the probe is at room temperature, 21.2o C, as read by the meter in the picture.

 i2-12  i2-12b i2-12c 

 

When the match is lit, as shown in the photograph at the left below, infrared radiation from the flame is reflected by the mirror at the right into a parallel beam, then focused by the mirror at the left onto the thermal probe. This causes the temperature to rise to 23.7o C, as seen in the photograph. This temperature took a couple of minutes and a couple of matches, but virtually any type of small heat source will work.

 i2-12a i2-12d 

 

 

Now suppose that the heat source is replaced by a copper cylinder that has been cooled to the temperature of liquid nitrogen, seen in the photograph at the right below. What will happen? You must say why your predicted behavior occurs. If you suggest an unusual answer you must justify it with some type of "physics" logic.

With the cooled copper cylinder at the focus of the mirror at the right:

  • (a) the temperature will become higher.
  • (b) the temperature will become lower.
  • (c) the temperature will remain the same.

 After September 5th, 2014, click Read More for the answer.

 

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It's about time, so we are finally off the topic of sound and on to other important issues.

The photograph below shows a compass, with the red end of the compass pointing toward the north, inside a coil. Notice that Dan is about to connect a wire to the second contact of the coil so that about ten amperes of current will flow in the coil. The direction of the (positive) current in the coil is shown by the arrow on the coil. If it matters, the coil contains 10 turns of wire, and is about 7cm in radius.

q329

When Dan connects the coil, it might cause the needle to deflect in a different direction, if the field is strong. On the other hand, if the magnetic field of the coil is a different type of field from that of the earth, perhaps nothing will happen. Or maybe the needle will just deflect slightly from where it is with no current in the coil.

When Dan connects the coil, what will happen?

  • (a) The compass needle will point in the upward direction in the picture.
  • (b) The compass needle will point in the downward direction in the picture.
  • (c) The compass needle will point to the left in the picture.
  • (d) The compass needle will point to the right in the picture.
  • (e) The compass needle will remain where it is after the coil is connected.
  • (f) The compass needle will deflect only slightly from where it is before the coil is connected (Specify which direction.)

Enough information has been given above so that a simple calculation can be made, if it is actually appropriate.

After July 18th, 2014, click Read More for the answer.

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Everyone knows that when you heat something up it expands. Well, almost everything, except water around the freezing point and a few other weird substances. One example of something that expands when heated is the brass plate shown in the photograph below.

 i1-11

The question this week involves the hole.

When the brass plate is heated up by the gas burner shown in the photograph, the hole will:

  • (a) get bigger.
  • (b) get smaller.
  • (c) remain the same size.

 Click Read More for the asnwer.

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Shown in the photograph below is an open tube about 50 cm long. If you blow at an angle into one end of the tube you can make a sort of musical tone.

 q259

Click your mouse on the photograph above to hear the fundamental frequency of the tube on an mpeg video.

Shown in the photographs below are (top to bottom) a closed tube the same length as the original tube, an open tube with half the length of the original tube, and a closed tube with half the length of the original tube.

(a) tube a: 
(b) tube b: 
(c) tube c: 

What is the relationship of fundamental frequency of each of the three tubes above to the frequency of the original tube?

Case (a): Relative to the original tube, the frequency of tube (a) is:

  • (a) higher in frequency by a factor of two.
  • (b) lower in frequency by a factor of two.
  • (c) the same frequency.

Case (b): Relative to the original tube, the frequency of tube (b) is:

  • (a) one octave higher.
  • (b) one octave lower.
  • (c) the same frequency.

Case (c): Relative to the original tube, the frequency of tube (c) is:

  • (a) one octave lower.
  • (b) two octaves lower.
  • (c) the same frequency.

Click Read More for the answer

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