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 3/23 - 3/27 With Answer
  • Question of the Week 3/16 - 3/20 With Answer
  • Question of the Week 3/9 - 3/13 With Answer
  • Question of the Week 3/2 - 3/6 With Answer

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

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A small amount of water (a few millilitres) of water is heated in a 12-ounce soda can (photograph at left below) until the water boils, producing steam as evidenced by the condensation coming out of the can (photograph at the center below). The steam-filled can is then grabbed by a pair of tongs and quickly placed upside down on a dish of room-temperature water, as seen in the photograph at the right below, taken just before the top of the can reaches the water. You can see how the demonstration is set up in an mpeg video by clicking your mouse on the photographs, or on this link.

i4 19a i4 19b i4 19c

The question this week involves exactly what happens after the can hits the water.

When the can reaches the surface of the water:

  • (a) The can will RAPIDLY implode.
  • (b) The can will SLOWLY implode.
  • (c) Water will be pulled up into the can, filling the can with water.
  • (d) Nothing will happen.

On or after 3/27, click Read More for the answer.

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This question involves the beam of a laser, expanded into a horizontal line using a cylindrical lens, as shown in the picture at the upper right below. A side view of the apparatus is seen in the photograph at the upper left below.

l4 22  l4 22c 
 l4 22d  l4 22e























The picture at the upper right above is a photograph of the apparatus taken from behind the apparatus, showing the laser beam expanded into a horizontal line by the cylindrical lens. The picture at the lower left above, and the close-up (with the laser turned off) at the lower right, show the apparatus from the front, looking backwards; you can see the laser, the cylindrical focusing lens in front of the laser, and a thin, taut heating wire running along the optic axis of the system immediately below the laser beam. The two posts holding the taut wire can be identified by the laser light skimming over them, rendering them visible with a slight red laser glow.


These two photographs below show the laser beam striking a screen about ten feet (three meters) in front of the laser without the cylindrical lens, and the expanded laser beam striking the screen after the cylindrical lens (seen mounted on the laser beam exit port) has been inserted.

l4 22f l4 22g







So, what will happen when the wire gets hot?

An electric current is than passed through the wire, having enough magnitude to make the wire very hot to the touch but not red hot to the eye. The question this week involves how this affects the expanded laser beam.

 Shown below are the original expanded laser beam, labelled (a), and four other possible photographs of the laser beam after the wire has been heated by passing a large electrical current, labelled (b), (c), (d), and (e). (The original expanded laser beam is shown for comparison with the possible laser beams affected by the heated wire.) Surveying the possibilities, heating the wire may cause the entire bright line to move higher (b) or to move lower (c), it might cause only a small part of the bright line to move higher (d) or to move lower (e), or it might have no effect, so the line stays the same (a).

al4 22gg bl4 22hh cl4 22ii dl4 22jj el4 22kk


After March 20, click Read More for the answer.

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A flask containing dyed green water is resting in a styrofoam bucket of icewater, as seen in the photograph at the left below. A plug seals the top of the flask so that there is no air on top of the water. Inserted into the water are a thermometer and a capillary tube, as seen in the close-up photograph at the right. Some of the water from the flask extends into the capillary tube.

i1 21  i1 21a 

If the water in the flask were to expand, the green water level would rise higher in the capillary tube. On the other hand, if the water in the flask were to contract, the green water level would fall lower in the capillary tube. The flask used is made of quartz glass, which has a very small coefficient of thermal expansion, so if the flask and the water change temperature the predominant effect will be due to the expansion or contraction of the water.

Suppose that the flask is now removed from its icewater bath and allowed to slowly warm up. What will the water level in the capillary tube do as the water in the flask warms up? Will it:

  • (a) rise.
  • (b) fall.
  • (c) rise for a bit and then fall.
  • (d) fall for a bit and then rise.
  • (e) remain the same.

After March 13, click Read More for the answer.

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The device in the photograph above is called a "fan cart" because of the electrical fan mounted on it. The fan blows air toward the right in the photograph, so with the sail removed and the fan ON the cart will move to the left (click here for video).

On the other hand, with the fan OFF and the sail mounted on the cart, you can blow into the sail from the left with your breath or a larger stationary fan and make the cart move to the right (click here for video). Notice that the propeller is not rotating, and the wind from the stationary fan at the left causes the sail to bend to the right.

When the fan is turned ON with the sail mounted on the cart, if you hold the cart still, you can again notice the sail bending due to the air from the cart fan blowing into the sail (click here for video).

Suppose that the sail is mounted in place as shown in the photograph, the cart fan turned ON, and the cart released to move if it wants to. What will the cart do?

  • (a) It will move to the left.
  • (b) It will move to the right.
  • (c) It will remain motionless.

Click Read More for the answer after March 6.

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