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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 12/8-12/12 with Answer
  • Question of the Week 12/1 - 12/5 with Answer
  • Question of the Week 11/24 - 11/28 With Answer
  • Question of the Week 11/17 - 11/21 With Answer

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

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A temperature probe is placed in a container of icewater, as seen in the photograph below. At the left is a digital timer, measuring elapsed time in seconds, in the center is the thermal probe, measuring the temperature of the icewater bath in degrees Celcius, and at the right is the 1000 ml beaker of icewater with the thermal probe.

q278

Now 50 ml of salt is poured into the icewater, the timer started, and the thermal probe used to mix the salt into the icewater, as seen in an mpeg video by clicking your mouse on the photograph above.

The question is: What will happen to the temperature of the icewater as time goes on?

  • (a) The temperature of the icewater will increase.
  • (b) The temperature of the icewater will decrease.
  • (c) The temperature of the icewater will remain about the same

To see the answer, click Read More on 12/12/14

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Three identical magnets are stuck to a magnetic stainless steel disc as shown in the photograph below. (The magnets are the orange rectangles.)

d1-38

The disc will now be rotated, slowly at first, and then at a faster and faster angular speed.

In what order will the magnets fly off the disc?

  • (a) The outer magnet will come off, then the middle magnet, then the inner magnet.
  • (b) The inner magnet, then the middle one, then the outer one.
  • (c) They will all fly off at the same time.

After December 5, click Read More for the answer.

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Suppose that you are in a boat in a closed pond, so no water can flow in or out of the pond. In your boat you have a large rock. A friend marks the water level on the side of the pond, then you throw the rock out of the boat into the pond, and it quickly sinks to the bottom. A model of this situation is shown below, where the boat floats in an aquarium with a lead weight rock; pieces of black tape mark the water level.

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Which of the following statements most accurately describes what happens to the water level in the pond after the rock has been taken out of the boat and allowed to sink into the pond?

  • (a) The water level in the pond will be higher.
  • (b) The water level in the pond will be lower.
  • (c) The water level in the pond will remain the same.

After November 28, click Read More for the answer.

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Shown in the photograph at the left below is a physical pendulum, with a bicycle wheel mounted at the end of the pendulum arm. The bicycle wheel can be free to rotate about its axis, as seen by clicking the photograph at left below, or it can be tied down, as seen in the photograph at the right, so that it cannot rotate at all.

d2-51 d2-51b

If the wheel is tied down, so that it cannot rotate about its axis, it oscillates with some period, as seen in a video by clicking your mouse on the photograph at the right above. The pendulum is allowed to make ten oscillations while the clock runs, so the period is the final measurement on the clock divided by ten.

Now suppose that the cord tying the wheel is released, allowing the wheel to rotate about its axis, if it wants to. The pendulum will be pulled to the side as in the case above and again released from rest, but with the wheel free to rotate about its axis.

With the wheel free to rotate, which of the following statements are correct?

  • (a) The period of oscillation will be greater with the wheel free.
  • (b) The period of oscillation will be less with the wheel free.
  • (c) The oscillation will immediately damp out because the wheel will begin to rotate.
  • (d) A coupling resonance will occur between pendulum oscillations and wheel rotations.

For the answer, click Read More after November 21.

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