1. 2010
May
11

## The car crash

The blogosphere has been abuzz with analysis of last week’s episode of Mythbusters, in which they tested Jamie’s assertion (from a previous episode) that two cars crashing into each other at 50 miles per hour is the same as one hitting a wall at 100 miles per hour. OK, well, that turned out to be wrong.

I didn’t have the time to write about it just after the episode and there’s no point in me repeating what’s been said about it elsewhere, but I do want to point something out: the reason it was wrong is that in one case, a car crashes into another car, and in the other case, it crashes into a wall. The car and wall react differently in the collision — the car compresses, the wall doesn’t, so when there are two cars involved, the energy of the collision gets split between them. There isn’t any fundamental difference between two objects crashing together at 50 miles an hour and one object at 100 miles per hour crashing into a stationary one.

I would have liked to see the Mythbusters test one car at 100 miles per hour crashing into …

2. 2009
Dec
28

## 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 …

3. 2009
Oct
22

## Dirty vs. Clean Car

Hot on the heels of their Bullet Fired vs. Bullet Dropped episode, the Mythbusters have another result that’s poised to shake up the world of science… well, maybe not. But this week’s main myth, Dirty vs. Clean Car, is the kind of neat idea that most of us would never think to test and yet turns out to be surprisingly close to practicality. The myth that Adam and Jamie are testing is that dirt on a car has the same kind of effect as golf ball dimples, increasing the fuel efficiency of the car. To sum up the results (SPOILER ALERT ;-), it doesn’t work, at least not with dirt — but putting an actual dimpled coating on a car does increase the fuel efficiency by 11%. (Only on Mythbusters would they dimple a car…)

As with a lot of recent myths, this one deals with fluid dynamics — but not just the simple stuff like drag force, as in the bullet myths. The golf ball effect is based on turbulence, specifically the idea that the rough surface of the ball induces turbulence which disrupts the wake (pocket of still air) that trails behind the ball. That pocket of still …