Newton's cradle, named after Sir Isaac Newton, is a device that demonstrates conservation of momentum and energy via a series of swinging spheres. When one on the end is lifted and released, the resulting force travels through the line and pushes the last one upward. The device is also known as Newton's balls or "Executive Ball Clicker".
A typical Newton's cradle consists of a series of identically sized metal balls suspended in a metal frame so that they are just touching each other at rest. Each ball is attached to the frame by two wires of equal length angled away from each other. This restricts the pendulums' movements to the same plane.
If one ball is pulled away and is let to fall, it strikes the first ball in the series and comes to nearly a dead stop. The ball on the opposite side acquires most of the velocity and almost instantly swings in an arc almost as high as the release height of the Last ball. This shows that the final ball receives most of the energy and momentum that was in the first ball.
The impact produces a shock wave that propagates through the intermediate balls. Any efficiently elastic material such as steel will do this as long as the kinetic energy is temporarily stored as potential energy in the compression of the material rather than being lost as heat.
Intrigue is provided by starting more than one ball in motion. With two balls, exactly two balls on the opposite side swing out and back.
More than half the balls can be set in motion. For example, three out of five balls will result in the central ball swinging without any apparent interruption.
While the symmetry is satisfying, why does the initial ball (or balls) not bounce back instead of imparting nearly all the momentum and energy to the last ball (or balls)? The simple equations used for the conservation of kinetic energy and conservation of momentum can show this is a possible solution, but they cannot be used to predict the final velocities when there are three or more balls in a cradle, because they provide only two equations to find the three or more unknowns (velocities of the balls). They give an infinite number of possible solutions if the system of balls is not examined in more detail.
Christiaan Huygens used pendulums to study collisions. His work, De Motu Corporum ex Percussione (On the Motion of Bodies by Collision) published posthumously in 1703, contains a version of Newton's first law and discusses the collision of suspended bodies including two bodies of equal size with the motion of a moving body being transferred to one at rest.
Newton's cradle can be modeled with simple physics and minor errors if it is incorrectly assumed the balls always collide in pairs. If one ball strikes 4 stationary balls that are already touching, the simplification is unable to explain the resulting movements in all 5 balls, which are not due to friction losses. The simplification overestimates the kinetic energy in the 5th ball by 2.2%. All the animations in this article show idealized action (simple solution) that only occurs if the balls are not touching initially and only collide in pairs.
The conservation of momentum (mass x velocity) and kinetic energy (0.5 x mass x velocity^2) can be used to find the resulting velocities for two colliding elastic balls. For three or more balls, the velocities can be calculated in the same way if the collisions are a sequence of separate collisions between pairs. In Newton's cradle, all the balls weigh the same, so the solution for a colliding pair is that the "moving" ball stops relative to the "stationary" one, and the stationary one picks up all the other's velocity (and therefore all the momentum and energy). If both are moving, one is picked to be the "stationary" frame of reference. This effect from two identical elastic colliding spheres is the basis of the cradle and gives an approximate solution to all its action without needing to use math to solve the momentum and energy equations. For example, when two balls separated by a very small distance are dropped and strike three stationary balls, the action is as follows: The first ball to strike (the second ball in the cradle) transfers its velocity to the third ball and stops. The third ball then transfers the velocity to the fourth ball and stops, and then the fourth to the fifth ball. Right behind this sequence is the first ball transferring its velocity to the second ball that had just been stopped, and the sequence repeats immediately and imperceptibly behind the first sequence, ejecting the fourth ball right behind the fifth ball with the same microscopic separation that was between the two initial striking balls. If the two initial balls had been microscopically welded together, the initial strike would be the same as one ball having twice the weight and this results in only the last ball moving away much faster than the others in both theory and practice, so the initial separation is important.
When the simple solution applies, the balls more efficiently transfer the velocity from one ball to the next, maintaining the effect. So the effects are more noticeable when the balls are not touching and therefore more closely follow independent collisions.
When simple solution applies
In order for the simple solution to theoretically apply, no pair in the midst of colliding can touch a third ball. This is because applying the two conservation equations to three or more balls in a single collision results in many possible solutions.
"Touching" in this discussion means when a ball is still compressed on one side during a collision, it begins compression on the other side from the next collision. So "touching" may include small initial separations, which will need the complete Hertzian solution described below. If the separations are large enough to prevent simultaneous collisions, the Hertzian differential equations simplify to the case of independent collision pairs.
Small steel balls work well because they remain efficiently elastic (less heat loss) under strong strikes and hardly compress (up to about 30 µm in a small Newton's cradle). The small, stiff compressions mean they occur rapidly (less than 200 microseconds), so steel balls are more likely to complete a collision before touching a nearby 3rd ball. Steel increases the time during the cradle's operation that the simple solution applies. Softer elastic balls require a larger separation in order to maximize the effect from pair-wise collisions. For example, when two balls strike, there needs to be about 0.5 mm separation for rubber balls in order to get the fourth and fifth balls to eject with nearly the same velocity, but only half the width of a hair for steel balls.
The extra variables needed to determine the solution for three or more simultaneously colliding elastic balls are the relative compressibilities of the colliding surfaces. For example, five balls have four colliding points and scaling (dividing) three of the compressibilities by the fourth will give the three extra variables needed (in addition to the two conservation equations) to solve for all five post-collision velocities. The compressions of the surfaces are interacting in a way that makes a deterministic algebraic solution difficult to find. Numerical step-wise solutions to the differential equations have been used.
As air resistance and string friction slow the "ejected" ball(s) down, the other balls may come back together and collide and separate before the faster ball(s) return, thereby allowing the simple solution to apply on subsequent strikes even if the first strike did not.
More complete solution
Determining the velocities for the case of one ball striking four "touching" balls is found by modeling the balls as weights with non-traditional springs on their colliding surface. Steel is elastic and follows Hooke's force law for springs, , but because the area of contact for a sphere increases as the force increases, colliding elastic balls will follow Hertz's adjustment to Hooke's law, . This and Newton's law for motion () are applied to each ball, giving five simple but interdependent ("touching") differential equations that are solved numerically. When the fifth ball begins accelerating, it is receiving momentum and energy from the third and fourth balls through the spring action of their compressed surfaces. For identical elastic balls of any type, 40% to 50% of the kinetic energy of the initial ball is stored in the ball surfaces as potential energy for most of the collision process. 13% of the initial velocity is imparted to the fourth ball (which can be seen as a 3.3 degree movement if the fifth ball moves out 25 degrees) and there is a slight reverse velocity in the first three balls, −7% in the first ball. This separates the balls, but they will come back together just before the fifth ball returns making a determination of "touching" during subsequent collisions complex. Stationary steel balls weighing 100 grams (with a strike speed of 1 m/s) need to be separated by at least 10 µm if they are to be modeled as simple independent collisions. The differential equations with the initial separations are needed if there is less than 10 µm separation, a higher strike speed, or heavier balls.
The Hertzian differential equations predict that if two balls strike three, the fifth and fourth balls will leave with velocities of 1.14 and 0.80 times the initial velocity. This is 2.03 times more kinetic energy in the fifth ball than the fourth ball, which means the fifth ball should swing twice as high as the fourth ball. But in a real Newton's cradle the fourth ball swings out as far as the fifth ball. In order to explain the difference between theory and experiment, the two striking balls must have at least 20 µm separation (given steel, 100 g, and 1 m/s). This shows that in the common case of steel balls, unnoticed separations can be important and must be included in the Hertzian differential equations, or the simple solution may give a more accurate result.
Gravity and the pendulum action influence the middle balls to return near the center positions at nearly the same time in subsequent collisions. This and heat and friction losses are influences that can be included in the Hertzian equations to make them more general and for subsequent collisions.
Heat and friction losses
This discussion has assumed there are no heat losses from the balls' striking each other or friction losses from air resistance and the strings. However in the real world, these energy losses are the reason the balls eventually come to a stop. The higher weight of steel reduces the relative effect of air resistance. The size of the steel balls is limited because the collisions may exceed the elastic limit of the steel, deforming it and causing heat losses.
The principle demonstrated by the device, the law of impacts between bodies, was first demonstrated by the French physicist Abbé Mariotte in the 17th century.  Newton acknowledged Mariotte's work, among that of others, in his Principia.
Invention and design
There is much confusion over the origins of the modern Newton's cradle. Marius J. Morin has been credited as being the first to name and make this popular executive toy. However, in early 1967, an English actor, Simon Prebble, coined the name "Newton's cradle" (now used generically) for the wooden version manufactured by his company, Scientific Demonstrations Ltd. After some initial resistance from retailers, they were first sold by Harrods of London, thus creating the start of an enduring market for executive toys. Later a very successful chrome design for the Carnaby Street store Gear was created by the sculptor and future film director Richard Loncraine.
The largest cradle device in the world was designed by Mythbusters and consists of five one-ton concrete and steel rebar-filled buoys suspended from a steel truss. The buoys also had a steel plate inserted in between their two halves to act as a "contact point" for transferring the energy; this cradle device did not function well. A smaller scale version constructed by them consists of five 6" chrome steel ball bearings, each weighing 33 pounds, and is nearly as efficient as a desktop model.
The cradle device with the largest diameter collision balls on public display, was on display for more than a year in Milwaukee, Wisconsin at retail store American Science and Surplus. Each ball was an inflatable exercise ball 26" in diameter (enclosed in cage of steel rings), and was supported from the ceiling using extremely strong magnets. It was dismantled in early August of 2010 due to maintenance concerns.
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