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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 9/15-9/19 with Answer
  • Question of the Week Archive
  • Question of the Week 9/1-9/5 with Answer
  • Question of the Week Summer Girls Special Part 4 with Answer

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

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Two rubber o-rings are tightly fitted onto a section of thin-walled aluminum tube, as shown in the photograph below. This device rests on a long strip of thin plastic, as seen in the photograph.

 

The plastic strip is held horizontal, along the surface of the table, and rapidly pulled out from under the cylinder. Because there is a lot of friction between the o-rings and the plastic sheet, pulling the sheet out from under the cylinder causes the cylinder to spin rapidly in the clockwise direction and to move from right to left in the picture above.

After the plastic sheet has been pulled out from under the cylinder, and the cylinder comes into contact with the table top, which of the following will happen?

  • (a) The cylinder will roll to the left.
  • (b) The cylinder will roll to the right.
  • (c) The cylinder will stop and remain near where it was when it left the plastic.

 After September 19, 2014 click Read More for the answer

 

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The "Rijke tube" seen in the photograph at the left below, sometimes called a "hoot tube," is a glass tube with a nichrome heating coil in the tube about one-quarter of the way up the tube, as seen in the photograph at the right below. When it is activated by passing electrical current through the nichrome wire, it responds by emitting a loud tone, which in this case is the fundamental frequency of the tube acting as an acoustical open tube. Click your mouse on the photograph at the left below to hear the nice sound.

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If we heat up the nichrome wire so that the Rijke tube emits its tone, then rotate the tube so that it is horizontal:

  • (a) How will the loudness of the tone change, if at all?
  • (b) How will the frequency of the tone change, if at all?

What will then happen when I return the tube to its initial upright orientation? 

After September 12, 2014, click Read More for the answer.

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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.

     

 

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.

   

 

 

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.

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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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