1. 2011

    Blow your own sail

    The latest episode of Mythbusters tested a slightly controversial and very physics-related myth: that you can propel a boat forward by putting a fan on the boat and pointing it forward, into the sail. What’s going on here?

    First of all, why wouldn’t you expect this to work? Actually, first of all, why would you expect this to work? Think about the naive explanation for why a sailboat moves: the wind pushes forward on the sail, and the sail pushes forward on the boat. So someone who had never heard of physics might think that putting a fan on the boat and pointing it into the sail just gives you a convenient, portable source of wind. Presto, instant speedboat!

    But, as explained on the show, that reasoning doesn’t work, because of Newton’s laws of motion. There are actually a couple of different ways to apply Newton’s laws to this scenario:

    • Newton’s first and second laws (which are kind of the same thing) say that an object maintains its state of motion unless subject to an external force. The key word there is “external”: if you want your boat to move, you need something outside …
  2. 2010

    Shockwave reflection

    The latest episode of Mythbusters features a myth with a deep physical explanation… no pun intended! Well, maybe. Anyway, the myth is that by diving under the water, you can escape injury from an explosion occurring above the surface. Adam and Jamie tried to solve this puzzle by experiment (what else), and their results seemed to show that the myth might actually be true, but I want to look at it from the theoretical standpoint: why might being underwater protect you from an explosion?

    There is actually a not-too-obscure answer to this puzzle, and it has to do with refraction and reflection. These are phenomena that occur when a wave (of any sort — light, sound, or whatever) crosses a boundary between two media in which it has different speeds. Part of the wave bounces back (that’s reflection) and part of it continues through, but in a different direction (that’s refraction). Exactly how much of the wave’s power is reflected and how much is transmitted through, as well as the new direction of the transmitted part, depends on the angle of the incoming wave with respect to the surface, and also on the relative speed of the wave …