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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 10/27-10/31 With Answer
  • Question of the Week 10/13-10/17 with Answer
  • Question of the Week 10/6 -10/10 with Answer
  • Question of the Week 9/29-10/3 with Answer

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

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A vertical tube is filled to 170 cm with blue water, as seen in the photograph at the left below. The photograph at the center below shows the bottom end of the tube, connected to a much narrower capillary tube mounted horizontally as seen. A cap covers the end of the tube; when the cap is removed the water will begin to flow out of the tube into the pan beneath. A timer can be used to measure how long water takes to flow out.

q032-1 q032-2 q032-3

The timer, initially set to zero in the picture at the left, is started at the same time the cap is removed and the water begins to flow out of the tube. The time taken for half of the water to flow out of the tube (the level has gone down from 170 cm to 85 cm) is about 150 seconds, as read on the timer in the photograph at the right.

The question this week regards how the water will continue to flow out of the tube as time continues to pass. For example, what will happen in another (approximately) 150 seconds?

  • (a) All the water will be gone before an additional 150 seconds passes.
  • (b) All the water will be gone just as an additional 150 seconds is reached.
  • (c) Almost, but not quite, all of the water will be gone after an additional 150 seconds.
  • (d) Only about half of the remaining water will be gone after an additional 150 seconds.
  • (e) The water will stop flowing shortly after the initial 150 seconds.

 

For the answer, click Read More after October 31st.

 

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A box containing two spring-loaded cannon hangs by a magnet within a large vertical frame, as seen in the photograph below.

 p1-02

Each cannon is lined up with holes in a plastic sheet about half way across the box, and a cloth receptacle covering a second hole, also aligned with the cannon, at the left side of the box.

 p1-02a

The cannons will fire large ball bearing projectiles if the appropriate switch is pushed. Click cannon #1 to see the front cannon fired, cannon #2 to see the rear cannon fired, or both cannons to see both cannons fired simultaneously. Short videos from the point of view of an observer in the reference frame are also available for cannon #1 and cannon #2. Notice that the projectiles do not make it through the holes in the plastic sheets at the center of the box; gravity has the nasty habit of pulling things downward, and they are not fired with very much speed.

Now suppose that the box is released by the magnet and allowed to fall freely with the acceleration of gravity. Further suppose that immediately after the box begins to fall the cannons will be fired, so the projectiles are moving from right to left across the box while the box is accelerating downward. What will the balls do? They might hit below their respective holes, as in the case when the box is at rest. They might go through the first hole, but not through the second hole and into the receptacle. They might go through the first hole and into the receptacle. Or, they might hit the center plastic sheet above the hole. Notice that the second cannon travels at a much larger upward angle than the first cannon.

When the balls are fired in the accelerating reference frame:

Ball one:

  • (a) will hit below the hole of the first plastic sheet.
  • (b) will go through the first plastic sheet but not through the second.
  • (c) will go through both plastic sheets into the receptacle.
  • (d) will hit the first plastic sheet above the hole.

Ball two:

  • (a) will hit below the hole of the first plastic sheet.
  • (b) will go through the first plastic sheet but not through the second.
  • (c) will go through both plastic sheets into the receptacle.
  • (d) will hit the first plastic sheet above the hole.

Why?

 

After October 17, click Read More for some answers.

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The teacup shown in the photograph at the left below can be struck lightly around its brim to create pleasant tones corresponding to standing wave vibrations of the brim of the beaker like the standing waves shown in the figure at the center below. The solid line represents the circular brim of the cup at rest, and the dashed lines represent the maximum excursion of the oscillation of the brim of the teacup after it is struck by the spoon; each "n" identifies a nodal point in the motion of the beaker brim.

   

 QOTW 30

 

A more complete drawing of the beaker brim, showing the handle, is given at the right above, because the handle might have some part in how the standing wave develops. The handle is shown at "12 o'clock," and other points on the rim are labelled correspondingly by clock numerals. Arrows have been inserted at the diagonals between the four major clock numbers. The standing waves shown in the center figure above occur when the brim of the teacup is struck at the points numbered 3, 6, 9, and 12, with the nodal lines at the diagonals between these numbers. Click your mouse on the drawing at the right above to hear the tone obtained when the cup is struck at the position of the handle (marked 12 o'clock).

When the rim of the beaker is struck at the various places(the numbers and the arrows at the diagonals) indicated in the drawing at the right above, which of the following statements are true:

  • (a) The tones produced by striking the brim of the beaker at 3, 6, 9, and 12 have the same frequency.
  • (b) The tones produced by striking the brim of the beaker at the four arrows have the same frequency.
  • (c) The tones produced by striking the brim of the beaker at all eight of these points have the same frequency.
  • (d) The tones produced by striking the brim of the beaker at the numbered points are higher in frequency than those obtained when the points identified by arrows are struck. 

After October 10th, 2014, click Read More for the answer.

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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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