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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 9/22-9/26
  • 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

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

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Shown in the photographs below are a top view and a side view of an aluminum rod with much of the rod inserted into a tank of water. The only problem with these photographs is that the part of the photograph showing the rod in the water has been masked out.

QOTW--027a

 The drawings below show three different versions of how the photographs might appear if the mask were not present. Your task is to identify which of the three options for each view is correct.

QOTW--027b

 

The correct top view of the rod in the water tank is:

  • (a) A
  • (b) B
  • (c) C

The correct side view of the rod in the water tank is:

  • (a) A
  • (b) B
  • (c) C 

 After September 25, 2014 click Read More for the answer

 

 

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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.

h3-12h3-12a

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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