1. 2010

    Waterslide Wipeout

    Like everyone else, I feel a need to analyze the giant water slide in the latest Mythbusters episode. But honestly, there isn’t much left to say. The original video has been around for a while and everybody else who does this kind of analysis has already had the chance to do it. Example 1; example 2 (okay, so I’m only linking to one blog, but there must be more).

    I guess I might as well do the obvious calculation, but I’ll use the Mythbusters’ parameters instead of those from the original video (which are unknown). The slide starts with a downward ramp \(\unit{165}{\foot}\) long at a \(\unit{24}{\degree}\) slope, which then curves upward to a \(\unit{30}{\degree}\) launch ramp that terminates \(\unit{12}{\foot}\) above the surface of the lake. They didn’t say how long the launch ramp is, but I can work without that information. I’ll be trying to calculate two quantities mentioned on the show: how far each Mythbuster flies from the end of the ramp, and his maximum speed.

    (here’s a full-size version)

    There are two parts to this problem:

    1. The slide
    2. The flight

    The first part …

  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 …

  3. 2009

    How the Mythbusters skipped a car

    On the last episode before breaking for Christmas, the Mythbusters build team undertook the slightly ambitious project of skipping a car across a pond, as shown in the movie Cannonball Run. At first this probably seems like a ridiculous thing to try — of course, on Mythbusters, what isn’t? But this one actually worked. Here’s a look at the rather interesting physics behind it.

    As Jesse explained on the show, there are basically two physical principles that allow you to skip a stone (or a car) across water: the spin, and the reaction force of the water. This isn’t buoyant force, like they’ve dealt with on previous shows; if buoyancy alone were the only thing pushing up on the stone, it’d float. Stones don’t float. (Neither do cars.) The force that keeps a stone skipping across the water is related to its speed. Spin and speed, that’s the magic formula.

    First, the spin. Any spinning or rotating object has angular momentum, which is like a rotational equivalent of linear momentum: roughly speaking, it measures how difficult it is to change the object’s motion. Objects with a lot of momentum are either very massive …

  4. 2009

    Buoyancy, part 2

    Following up on my calculation of the lifting power of helium balloons, it’s time to see how the same argument applies to ping-pong balls being used to raise a sunken ship.

    Raising a ship with ping-pong balls is, in fact, nearly the same situation as raising a child with helium balloons. All you have to do is replace the air with water, the helium with air, the rubber balloons with plastic balls, and the child and harness with a boat (though preferably not in that order). The physical principle at work (Archimedes’ Principle) is exactly the same, and so the same equation I used last time is equally applicable here: the buoyant force on an object (ping-pong ball) immersed in a fluid (water) is equal to the weight of the water displaced by the fluid,

    $$F = \rho g V$$

    Let’s see what this says about how many ping-pong balls it would take to raise the Mythtanic II, which weighs about \(\unit{3500}{\pound}\) according to the show. We can start by figuring out how much mass it takes to balance out the buoyant force on a single ping-pong ball, using \(-m_\text{load} - m_\text{ball} + \rho V …