## 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. Also, be sure to visit our discussion forum for discussion of many physics demonstrations, including links to Questions of the Week about them.

• Question of the Week Archive
• A Flame In Free Fall? - Question of the Week 5/11 - 5-/15
• A Puzzle Of Combustion: Candles Low and High - Question of the Week 5/4 - 5/8
• A Look At The Trajectory Of A Falling Ball - Question of the Week 4/27 - 5/1
• How Do These Rods Cross? Question of the Week 4/20 -4/24

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

Here's a followup to last week's candle puzzle.

A candle mounted on the lid of a gallon jug is lit, and the lid quickly affixed to the jug. In this configuration the candle will remain lit for over one minute before the oxygen in the jug is sufficiently used up by the combustion process and the flame is extinguished.

Now suppose that the candle flame is lit and the lid again quickly affixed to the jug. However, the bottle is now dropped about six feet starting from the orientation shown in the photograph below.

What will happen? In particular, by the time the jug falls six feet the candle flame will:

• (a) burn more brightly.
• (b) remain at about the same brightness.
• (c) burn less brightly.
• (d) go out.

Two candles, shown in the photograph at the left below, are lit and then covered by a plastic tube (sealed at the top), as seen in the photograph at the right below. In the apparatus as pictured the tube is about 50 cm tall and 10 cm in diameter, and the upper candle is about 30 cm from the bottom.

After some period of time, as the oxygen in the tube is consumed by the candle flames, the candles will both cease to burn. The question involves the order in which the candles will go out.

Which of the following statements is true?

• (a) The top candle will go out first, then the bottom candle.
• (b) The bottom candle will go out first, then the top candle.
• (c) Both candles will go out at the same time.

An experimenter walks briskly forwardwith a ball in one hand, and attempts to release the ball so that it will fall into a bucket, as shown in the photo below:

The question is where one must release the ball in order for it to fall into the bucket. In the photograph the ball has been released and is falling, but whether it will fall into the bucket is not certain. As you walk along, must you release the ball before or after the position shown here? Must you wait until your hand is directly above the bucket, or slightly beyond that point, due to the effect of conservation of momentum between your hand and the ball at the time when it is released?

In order for the ball to fall into the bucket, you must release it:

• (a) somewhere to the left of where the scientist is in the picture.
• (b) somewhere to the right of where the scientist is in the picture, but not as far as the bucket.
• (c) when your hand is directly above the bucket.
• (d) when your hand is just past the center of the bucket.

Shown in the photographs below are the front and the top view of two straight wooden rods, one red and one blue. They are mounted between two plastic plates, not shown.

 Front Top

The question this week involves what we can say about this geometry. In particular, are the two rods might be touching each other at any point?

The two rods:

• (a) are definitely touching at some point.
• (b) are definitely not touching at any point.
• (c) may or may not be touching; it is impossible to say.