The energy has left the ball! Friction robs the system of energy, slowly bringing the balls to a standstill. But due to the conservation of energy, the total amount of energy stays the same. Skip to content Users questions. March 29, Joe Ford. Orbit Micro Instt. Have a Question? Ask our expert. Speak your question. It explains Newton's laws of motion. DIY activity kit. Metal frame, steel balls, wooden base included.
Newton Ask Price. Newton Cradle Ask Price. Newton 3 electric lunch box Ask Price. Science Emporium Santomalan, Najibabad, Dist. You may know Newton's Third Law: For every action, there is an equal and opposite reaction. Pull back the polished steel ball on one end and let it fall. When it hits, it transfers its energy to the ball at the far end of the line, setting it in motion.
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Ball 1 has initial speed V, balls 2 and 3 are initially at rest, touching each other. Ball 2 has speed 6. This is the observed outcome, which we will call "case 1", summarized below. But why does case 2 not occur? Case 1, the observed outcome Quantity Initial 1 Initial 2 Initial 3 Final 1 Final 2 Final 3 velocity 6 0 0 0 0 6 momentum 6 0 0 0 0 6 kinetic energy 18 0 0 0 0 18 Case 2, two balls emerge at the same speed.
Quantity Initial 1 Initial 2 Initial 3 Final 1 Final 2 Final 3 Net velocity 6 0 0 -2 4 4 momentum 6 0 0 -2 4 4 6 kinetic energy 18 0 0 2 8 8 18 Case 2 clearly satisfies conservation of energy and momentum. Yet it is not observed to happen.
Suspicion focuses on the processes happening during the impact, when the three balls were in contact for a brief time interval. What's going on there? The balls deform elastically near the point of impact. Can you find any other hypothetical situations that would conserve energy and momentum but do not happen? If we can answer this question for the three-ball case we might gain insight into the N-ball general case.
In fact, if we look carefully at the two-ball case we might learn something about how the elastic properties of the balls store and release energy. Some textbooks and web sites tell you nothing other than a description of the behavior of the system and note that this behavior satisfies the conservation of energy and momentum.
Perhaps they are "playing it safe" by not attempting to answer the obvious questions that "inquiring minds want to know. Why is just one of many momentum and energy conserving outcomes selected by the laws of physics, to be the only outcome that happens?
Ball 1 has initial speed v o , balls 2 and 3 are in contact at rest. Case 3, all three balls emerge at different speeds. The successive impacts model. The simplest model to understand is one that invokes a "cheat". It assumes the N balls are initially not touching. The first ball is pulled back and strikes the second with speed. The first ball comes to rest and the second moves forward with speed V, hits the third ball, and so on down the line, till the last ball is ejected with speed V.
This is valid when the balls are actually separated. But then some folks assume that the explanation is also valid when the balls are touching. Well, the results are nearly the same in both cases, but the dynamics of the processes are certainly different. We will move on to look at the interesting case, where all balls are initially touching each other. The compression pulse model. This assumes that the balls are initially all touching.
A compression pulse begins in the metal balls at the point of first impact, traveling through the balls with the speed of sound. The speed of sound in the material of which the balls are made is much greater than the speeds of the balls.
So the pulse "does its work" before any of the stationary balls have moved. The pulse travels forward and backward, reflecting from the ends of the string of balls and meeting again simultaneously at one point.
Where is that point? Well, if the pulse originated between the first two balls, the pulse meets between the last two balls, where it gives up its momentum and energy, giving the last ball a kick, and slowing the others to a stop before they have moved much.
This sounds plausible at first, and it agrees with experiment. But there's a troublesome issue. This model requires that a pulse of energy and momentum from the first ball ends up at one localized point, the point where the last two balls touch. How does it do that without dispersion, for the compression pulse initially goes in all directions within the balls, forward, backward, up, down and all directions in between?
It is reflected from the ball surfaces the balls are spherical after all in very complex paths and most of these paths are not equal in length from start to finish. Though it sounds good, it fails to convince the skeptical student.
But the model does work remarkably well in predicting where the chain of balls will break first. One seldom discussed confirmation test is this. The model predicts that the initial break point of the ball chain is determined by the length of the compression pulse paths through the chain, and not on the mass of the balls. Therefore if mass were added to one or more of the balls, without changing its diameter, the initial break point should be the same.
This can be done with the real apparatus by attaching weight to the bottom of one or more balls. Experiment confirms the prediction. The balls-and-springs model. This model imagines a linear string of balls with small springs between them.
It treats the system as a lattice array. This, it is argued, is a result of the balls being spherical. A linear array of objects of different shape, say cylinders, would behave differently. While interesting, this model is not an exact simulation, for its predictions do not quite match the real behavior.
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