Subway Station Anxiety

Last Problem:

A steel washer (used in conjunction with a bolt in construction work) with a hole in the center is heated until the metal expands by five percent (5%). Will the hole get larger, smaller or remain unchanged?

Answer:

Every dimension of the washer will expand, so the hole will get larger too as the washer is heated.

Today’s Problem:

Is it really dangerous to stand too near the edge of a platform where a subway passes through a subway station in New York City at high speed? Or, is that a myth perpetuated by overly anxious mothers and fathers?

The Expanding Washer

Yesterday’s Problem:

Have you ever seen a smooth golf ball? The answer is no because they do not make smooth golf balls. So how come? Why does a golf ball always have a dimpled surface?

Answer:

Golf balls always fly with backspin. The dimples trap a layer of air that spins the ball. The top layer of trapped air moves faster than the bottom layer, giving the ball greater lift. This is called Bernoulli’s principle, which is also the basis of airplane flight.

A smooth golf ball would travel about half the distance that a dimpled golf ball can cover.

Today’s Problem:

A steel washer (used in conjunction with a bolt in construction work) with a hole in the center is heated until the metal expands by five percent (5%). Will the hole get larger, smaller or remain unchanged?

The Magical Shape of Golf Balls

Last Problem:

Is it possible to lift a marble off of a table using only a wine glass and nothing else? If so, how?

Answer:

Yes, is is possible and doable. Here is how it can be done.

Place the wine glass over the marble. Begin to move the wine glass in a circular fashion until the marble begins to spin around the inside of the glass. Start rotating the glass faster and faster. Once the marble is rotating fast enough it will rise off the table.

When the marble is spinning fast enough you will be able to lift the glass off the table. The marble will not drop immediately even if you stop spinning the wine glass. It will continue to spin around the inside of the wine glass at its own momentum. And presto – the marble has been removed from the table using only a wine glass.

Today’s Problem:

Have you ever seen a smooth golf ball? The answer is no because they do not make smooth golf balls. So how come? Why does a golf ball always have a dimpled surface?