Last week’s puzzle about heated washers was an easy physics problem. Before I give you the solution, here’s another physics problem that is more challenging:

The figure on the right shows a hollow cylinder filled with water. There are two questions:

1. Where should a hole be punched in the cylinder wall so that the water goes as far from the cylinder as possible? A hole punched in the bottom accesses water with a lot of pressure on top. But the water doesn’t have far to fall. A hole punched near the top has a long way to fall, but there’s not as much pressure pushing it out. As shown in the figure, the water in the cylinder is continually refreshed from overhead. Thus, no matter where the hole is placed, water will come out.
2. For a given fixed hole, say in the middle, will the water spurt out less or more if you are on the Moon where there is less gravity?

This is not an easy problem. The interesting answers will be given next Monday.

Solution to Micro Softy 15: The Hot Washer     

Last week’s Micro Softy asked: When a metal washer is heated, does the hole in the middle get bigger or smaller?

You may be familiar with a trick for getting a tight lid off a jar. Running hot water over the lid expands the lid. That can make the fit looser. The same principle applies to a heated washer. When the washer is heated, the hot washer is a bigger version of the cold washer. So, as shown in the figure on the right, the hole gets bigger.

And here are the puzzles (and solutions) from the last four Mondays:

Micro Softy 12: Can You Connect the Dots? You may use no more than four perfectly straight lines and the lines must be connected. This classic puzzle in simple graph theory resulted in a commonly used phrase. Can you guess it?

Micro Softy 13: Garbage Trucks, String Theory… … and Stained-Glass Windows. What connects them? These puzzles, associated with the great mathematician Leonard Euler (1707‒1783), have a practical use, for example, in laying water pipes.

Micro Softy 14: How Did the Blind Ticket Seller Know? This puzzle doesn’t require math skills so much as advanced common sense reasoning. About last week’s solution, given here: If you code, the second part of the puzzle could also be offered to a computer.

Micro Softy 15: What Happens to the Hole in a Hot Washer? When a washer ring is heated, does the hole in the center get bigger or smaller? The answer to last week’s Micro Softy turns on the question of what form of currency Claire gave the blind ticket seller. Did you guess it right?

Note: At Monday Micro Softy 11: What Happened to That Other Dollar?, you will also find links to the first ten Micro Softies. Have fun!