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

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

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Two identical masses are suspended from a fixed point using three identical strings, as seen in the photograph below. The top string, connected to the fixed support, holds the upper mass; the middle string connects the upper mass to the lower mass; and the lower string connects the lower mass to a rod that can be grasped in order to pull downward on the lower string.

 c3 03new

When the rod is pulled downward, exerting a force on the lower string, which of the three strings will break?

The three possibilities are that, when the lower string is pulled downward:

  • (a) the upper string will break.
  • (b) the middle string will break.
  • (c) the bottom string will break.
  • (d) it's more complicated than that.

 

Click Read More for the answer after February 20th, 2015.

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This week's question focuses on a little toy duck, shown in the photo below.

q320

If you click on the photo, a video will load, and you will see that when knocked over, the duck wobbles back and forth in a remarkable manner.

For this to happen, there must be a hollow cavity inside the duck, with a ball rolling around in it.

The question: What shape is the cavity?

q320a  q320c  q320d  q320e
 a   b   c   d 

The arrow on the drawing shows which way is up - ie the head of the duck.

Click Read More for the answer after February 14

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The figure below shows a 1000-milliliter dewar flask filled with water, with thermal probes at the top and the bottom. The temperature of the water is 26oC (around room temperature), as measured by both probes and seen on the large digital displays at the right in the photograph. An electric heater is positioned near the top of the water. When the heater is turned on, it begins to heat the water. As time goes by, measured in seconds by the digital timer at the left in the photograph, we can measure the temperature of the water at the top and the bottom of the dewar, displaying the temperature of the water at the top and the bottom of the dewar as a function of elapsed time. Click on the photograph below to see a video of the action (or lack thereof) for the first few seconds.

Perhaps it is appropriate to share a few numbers: the volume of the dewar is 1000 ml; the heater draws a little over 1 ampere at 110 VAC (60 Hz) so that it produces about 125 Watts of heating.

 q314

The question this week involves how the temperature of the water near the top and near the bottom of the dewar will change with time as the heater heats the water near the top of the dewar.

Part 1: After about thirty minutes (1800 seconds) the temperature of the water near the top of the dewar will be:

  • (a) less than 26oC.
  • (b) about 26oC.
  • (c) about 30oC.
  • (d) about 50oC.
  • (e) about 75oC.
  • (f) almost 100oC.

Part 2: After about thirty minutes (1800 seconds) the temperature of the water near the bottom of the dewar will be:

  • (a) less than 26oC.
  • (b) about 26oC.
  • (c) about 30oC.
  • (d) about 50oC.
  • (e) about 75oC.
  • (f) almost 100oC.

On or after February 13, click Read More for the answer.

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A pulley system can be used to lift heavy objects using a smaller force. A typical one is shown in the photo below.

In this case, a heavy object is placed on the hook below the lower pulley, and a presumably lighter force is applied to the hook at the left just inside the left support on the frame.

b3 12b

The question this week is how much force applied to the rope at the left is required to balance a weight put onto the hook below the movable pulley. Note that this system is in equilibrium; the small black block hanging above the hook on the rope at the left exactly balances the weight of the movable pulley.

Two kilograms of mass placed on the hook at the left will exactly balance how much mass hanging from the hook below the pulley?

  • (a) 1 kg.
  • (b) 2 kg.
  • (c) 4 kg.
  • (d) 8 kg.
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