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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 11/24 - 11/28
  • Question of the Week 11/17 - 11/21 With Answer
  • Question of the Week 11/10 - 11/14 With Answer
  • Question of the Week 11/3-11/7 with Answer

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

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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.

f2-05

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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When a certain calibrated laboratory mass is placed a the pan balance as seen in the photograph at the left below, we see that its weight is approximately 10 newtons, as read on the scale. If the same weight is suspended from an identical spring scale, we see in the picture in the center below that it requires about 10 newtons of force for the upper spring scale to suspend the mass.

 

b2-02  b2-02b 

b2-02c 

Now suppose that while the lab mass is resting on the lower scale we start to lift it with the upper scale.

When the upper scale reads 6 newtons, as seen in the picture at the right, the lower scale will read:

  • (a) 10 newtons.
  • (b) 8 newtons.
  • (c) 6 newtons.
  • (d) 4 newtons.
  • (e) 2 newtons.
  • (f) 0 newtons

After November 14, click Read More for the answer.

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Shown in the center image below is a matted photograph of some flowers supported by a string passing over two pegs. At the left and the right of the picture are close-up views of the string as it winds its way over the pegs.

 a2-71L a2-71M  a2-71R 

For this experiment we will slip the support string over the end of one of the pegs at a time. While we do so we will hold a finger over the end of the other peg so that the string cannot slip over the end of that peg at the same time. The question involves what will happen.

When holding the left peg and slipping the string over the end of the right peg,

  • (a) the picture will remain suspended on the left peg.
  • (b) the picture will remain suspended on the right peg.
  • (c) the picture will fall freely into the box below.

When holding the right peg and slips the string over the end of the left peg,

  • (a) the picture will remain suspended on the right peg.
  • (b) the picture will remain suspended on the left peg.
  • (c) the picture will fall freely into the box below.

For answers, click Read More after November 7, 2014.

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