Archive for December 2011

Why the Mythbusters Newton’s Cradle failed

10 December 2011

What with the Mythbusters having a bit of a mishap this week (a vast understatement by the way, and thank goodness no one was injured), folks have likely forgotten about their most elaborate and ambitious project of the current season: the supersized Newton’s Cradle. The thing was enormous, consisting of giant orbs hung from steel girders suspended over an empty drydock. It was an awesome concept.

But it was also a dismal failure.

Why? Well, there’s the inherent difficulty of precisely aligning such a massive structure such that the balls are in a perfectly straight line and a minimum of energy is lost to sideways motion. This was what much of Adam’s and Jamie’s fine-tuning addressed—but try as they might, they couldn’t get the giant clack-clacking effect they’d hoped for.

An in-depth analysis on Wired goes into the physics of a Newton’s Cradle, and what might have gone wrong, but ultimately punts a definitive conclusion by stating that “the camera angle wasn’t the best for analysis.” Now, I am not a professional physicist, but I think a hint at the real problem may be summed up in one comment in that Wired article: “It seems that these balls are not elastic.”

Right. See, the balls they used were not the solid steel balls of an ordinary Newton’s Cradle, scaled up, which apparently would have been prohibitively expensive to acquire. Instead they were homemade: spherical steel casings, each with a thick steel disk at the equator and both hemispheres filled with concrete.

As I said, I am not a physicist, so what follows might be off-base. But my impression of the impact event in a normal Newton’s Cradle goes like this:

  • When one ball strikes another, the first ball’s momentum is transferred as a force acting on a single point (ideally, that is) on the surface of the second ball.
  • That force of impact radiates in all directions through the second ball. The energy can’t escape from the ball (except for the bits that become heat, and that clacking noise), so as it crosses through the interior of the ball, the energy that reaches the surface is reflected (or refracted?) back into the interior.
  • Ultimately all that energy converges back at a single point on the surface of the ball, exactly antipodal to the impact point.
  • That energy convergence causes the ball to react and move—and if there’s another ball touching that convergence point, the energy is transferred into that next ball, and the Newton’s Cradle does its thing.

So far, so good. Here’s the problem: as I said, the interior of the Mythbusters balls were mostly concrete, not steel. Therefore most of the energy entering each impacted ball was muddled, diffused, slowed as it moved through that medium. Only the energy passing through the steel equatorial disk—a small fraction of the whole—was transferred efficiently into the next ball. The result was as seen on TV: powerful action, anemic reaction.

I believe that, had the Mythbusters used enormous, solid, hardened steel balls for their giant Newton’s Cradle, they might have come up with the amazing visual they—and we—were all hoping to see.