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 9/29-10/3
  • Question of the Week 9/22-9/26 with Answer
  • Question of the Week 9/15-9/19 with Answer
  • Question of the Week Archive

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

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A small flashlight bulb, which approximates a point source, is placed in a stand on an optical rail, as seen on the left side of the photograph at left below. The light bulb is energized by a DC power supply.

20 centimeters from the bulb is a photometer sensor, shown on the right side of the same photo.

With the room lights turned off, and the light bulb illuminated, the output of the photometer reads 4 units, as seen in the photo at right below.


q316b q316

Suppose we now move the sensor another 20 cm further away, thereby doubling its distance from the light bulb. At this point, what will the photometer read?

  • (a) The same, roughly 4 units.
  • (b) One-half as much, roughly 2 units.
  • (c) One-fourth as much, roughly 1 unit.
  • (d) No reading at all, 0 units.


 After October 3rd, 2014, click Read More for the answer.


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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.


 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.



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.


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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