Newton's Cradle...Or Is It?
Newton’s cradle! A scientific demonstration of one of the most famous principles in classical mechanics, with two startling facts: 1) Newton had nothing to do with it, and 2) Though the principles on which the Cradle are based were discovered in 1662 (and Newton lived from 1643 – 1727), it didn’t get its name until…1967. Over four hundred years later. And it wasn’t even a scientist who named the device, it was an actor.
At it’s most basic, the Cradle demonstrates two scientific concepts: the conservation of momentum, and the conservation of energy. Conservation of momentum indicates that when two objects in a system collide, the total momentum before and after would be the same; in this case, the momentum is transferred from the first ball, through the second, third, and forth, until it reached the fifth ball, which swings out with almost the same momentum as the first ball had when it hit. Energy is also conserved, as the potential energy of the first ball is converted into kinetic energy that is then transferred down the line…along with a tiny amount of heat energy, and of course the “clacking” sound you hear.
So, back to Newton. Though the conservation of momentum is widely known through Newton’s Third Law, the principles demonstrated by the Cradle had actually been known decades before Isaac even started working, first published in papers presented to the Royal Society in England by John Wallis, Christopher Wren, and Christiaan Huygens. Wallis was a clergyman who helped develop infinitesimal calculus, Wren an astronomer and famous church architect, and Huygens a Dutch mathematician who invented the pendulum clock, proposed the wave theory of light, and discovered the Saturnian moon Titan. So a bunch of heavy hitters. It was Huygens who noted that an explanation of the motion of a Cradle-like system required both conservation of momentum and kinetic energy; though he actually didn’t use the expression “kinetic energy”–to him it was “a quantity proportional to mass and velocity squared.” The first recorded experiments on the collision of pendulums with balls were conducted by Edme Mariotte in 1670; Newton actually did write about them, which may be as close as he got to the mechanism that now bears his name.
And how about the actor? That’d be Simon Prebble. He was in a bunch of British TV series in the 60s, the 1983 Tom Selleck vehicle “High Road to China,” and if you recognize Prebble at all, it may be for his voice work on “Courage the Cowardly Dog” as the family’s sentient home computer. In the mid-60s, after reading a book on Newton’s laws, he got to wondering about how to illustrate them with a physical mechanism. He experimented with different materials, alignments, and arrangements of the balls; he called it “Newton’s Cradle” because it reminded him of a cat’s cradle. Prebble had a difficult time moving them as merchandise, though–people at the time had little concept of fun toys for desks at the office. A buyer at Harrod’s department store picked up on the Cradle, and within a few years, Prebble had sold 20,000 of them. Apparently he thought the whole thing was actually sort of strange, he was a young actor holding cool alternative parties on a houseboat in Chelsea, but much more famous for inventing what he described as a toy.
And believe it or not, for a 50 year-old toy that’s illustrating a 400 year-old concept in classical mechanics, there is still a surprising amount of research being done on Newton’s Cradle. Some of these studies involve making esoteric versions of the Cradle: coated in viscous liquid, or made out of Rydberg atoms (highly excited atoms that, among other things, take up 1000 times of the space of their regular counterparts), or a Bose gas (essentially, a gas made of bosons, of course). But some of them are investigating the same Cradle you have in your hands right now–investigating nonlinear waves in a Cradle that yield something called a “discrete p-Schrödinger equation,” or the viscoelastic dissipation in the impacts of the balls. Even something as simple as how fast the balls are going after the collisions is still up for investigation. Two researchers recently noted, just trying to develop a way to predict the post impact velocities of the balls “has baffled investigators in the field of impact research for many years." What you hold is not a toy, or a relic–it’s a piece of scientific equipment that still yields up valuable knowledge today. See what you can discover yourself!