Brain Exercises – September 5

  1. An airplane flying at several thousand feet casts a shadow on the ground. Will it be larger, smaller, or the same size as the airplane?
  2. What happens when you barely touch the edge of a stream of water with a spoon?
  3. Wave a flexible corrugated tube around in a circle, and it will make a sound. Can you explain why?
  4. Three men sat by open windows on a steam train that passed through a tunnel. All three of their faces became covered with soot. When the three passengers saw this, they started laughing at one another. Then one of them suddenly stopped because he realized that his face was also soiled. What was his reasoning?
  5. Many years ago, a man married the sister of his widow. How did he do it?
  6. How can you build a house that has a window in all four walls, but every window faces south?
  7. What happens when you blow between two burning candles?
  1. Titanic sinks (1912)
  2. US women get the vote (1920)
  3. Insulin discovered (1921)
  4. Hindenburg Explodes (1937)
  5. First polio vaccine (1952)
  6. Martin Luther King, Jr. shot (1968)
  7. Man walks on the moon (1969)
  8. Nixon resigns (1974)
  9. Berlin Wall falls (1989)
  10. Nelson Mandela freed (1990)
  11. O.J. Simpson not guilty (1995)
  12. Hurricane Katrina (2005)
  1. Because the sun is so large, the shadow will be smaller, but the difference in size is imperceptible. But if the sun is at an angle to the shadow surface, such as an hour or less before sunset, the shadow can be much larger. Light rays from a distant object may appear parallel, but that is not necessarily the case. If the light source is larger than the object, the shadow (on a flat surface perpendicular to the light source) will be smaller. If the light source is smaller than the object, then the shadow will be larger. The difference in size, however, is barely perceptible if the difference between the two objects is great.
  2. The stream will follow the curve of the spoon. That’s called the Coanda effect. On a microscopic scale, a minuscule electrostatic force is generated when two molecules come close, a force that tends to draw the molecules together. This attraction, called the van der Waals force, is the reason why poured liquids often dribble down the side of a glass rather than exit clearly over the side.
  3. The air pressure at the moving end of the tube is lower than the pressure at the end being held. That pressure difference causes the air to flow through the tube and the air vibrates as it passes over the corrugated walls of the tube.
  4. The passenger realized that if his face were clean, one of the other passengers would have realized that his own face was blackened by soot. Since neither of them stopped laughing, he realized his face must be sooty as well.
  5. He married the sister first.
  6. The question is, rather, where can you build it? Only on the North Pole.
  7. The stream of air causes a low-pressure area, drawing the flames together.