Talk:Newton's cradle

Newton's cradle
What I don't understand is why it's always the same number of balls at the same acceleration from each side, and not 1/2 the number of balls at twice the acceleration [Hackwrench]

I'm going over http://www.lhup.edu/~dsimanek/scenario/cradle.htm, but it's making a mistake of saying speed instead of acceleration/deceleration.


 * A purely elastic collision preserves not just momentum, but also kinetic energy in the system, where $$KE = \frac{1}{2} mv^2 $$. The scenario you describe would end up with twice the mass in motion, but $$v^2$$ would be a quarter of what it was before, so only half the kinetic energy would remain (the rest of the energy having been converted into other forms, such as sound and heat). This is impossible is a purely elastic scenario, and you will find that the only case preserving both momentum and KE is the one where "the same number of balls" have the same speed before and after the collision.


 * Glad I can help answer year-old questions. :/ Twinxor t 17:04, 30 October 2006 (UTC)


 * every collision preserves momentum; an ideal perfectly elastic collision preserves kinetic energy. 2603:8001:D3F0:87E0:0:0:0:1DF6 (talk) 00:10, 18 December 2023 (UTC)
 * What is momentum 2409:4056:201D:783E:0:0:1C2:10A0 (talk) 14:26, 21 February 2024 (UTC)
 * Momentum is a measure of how much force an object could apply in collision.
 * (F=dp/dt)-‌‌‌‌‌‌‌‌‍‌Newton's Second Law
 * [N‌‌ote:F=ma assumes that the mass is constant which might not be true in all cases.] 27.147.202.233 (talk) 12:41, 5 March 2024 (UTC)
 * Suppose,you lift three balls and release them.The third ball will transfer all of its momentum to the fourth ball which will transfer its to the fifth ball.The firs two balls will do the same exact thing and what we are left with is that three balls moving from the right side.
 * [Note:Assuming all the ball have the same mass] 27.147.202.233 (talk) 12:36, 5 March 2024 (UTC)

Explanation of the Physics?
This article could really do with an explanation of the physics behind this. The first sentence mentions stuff like conservation of momentum and energy, but that doesn't really help to explain how this device works to the layman. I'd write it myself, but I don't know how it works. =P – Lantoka  (talk) 03:54, 12 February 2007 (UTC)

I think the best explanation would be that all the explanations are wrong! Amoung other things, an explanation/theory/model must be predictave. While the two conservation laws mentioned do fine on the one ball case; they fail miserably when you swing 3 or 4 balls to start it. As a retired rocket scientist (physicist) I delight in showing this as an example of "A simple mechanisim" needing a very complex analysis. 71.163.146.15 (talk) 11:10, 27 December 2008 (UTC)Ron

Elastic collision
Am I the only one who finds the elastic collision animation so distracting it's almost impossible to read the article? That and many other highly distracting gifs are already on the elastic collision page. Do we really need it here? Kafziel Talk 16:17, 28 February 2007 (UTC)


 * In particular, the bright green backgrounds are distracting and unnecessary. White or grey would be less intrusive, as shown by the 3d animations. 173.172.70.38 (talk) 19:28, 17 March 2014 (UTC)

Hmm, I think a picture says more than a thousand words, and an animation even more so when describing the motion of something. I think it should stay. I feel one fault of the Newton's Cradle animation is that it does not show the gradual decay of oscillation, which may have lead to some people thinking it keeps going forever. Maybe someone could produce a non-repeating animation, or more animations with more balls.

Besides, the animation soon scrolls off screen as you read through the article. SpareHeadOne 02:34, 30 April 2009 (UTC)

Does not need to be touched to stop
I am bothered by the fact that it says you need to touch it to make it stop, while that is obviously not true. This is not a "perpetuum mobile", it's just a device that provides a good example of the conservation of KE and impulse, and impulse transfer in solids. You don't need to touch it to stop it, it will stop on its own as some of the energy will be transformed into heat on each collision (making the collision partly plastic and not 100% elastic, which is an ideal theoretical case). Adrian

Other Names; History; How it works; Variations
Other names for this are: Collision Apparatus http://physics.kenyon.edu/EarlyApparatus/Mechanics/Collision_Apparatus/Collision_Apparatus.html

"Balance Balls", "Newton's Pendulum" and "Newton's Balls" http://www.officeplayground.com/newtonscradle3.html In particular, the first link shows pictures of this device dating to around 1890.

The "Invention and design" section says "in early 1967 ... coined the name 'Newton's Cradle'" but does not mention earlier dates.

I have not researched this myself, but I suspect that "Collision Ball Apparatus" was the earlier name used in physics demos, and that a search under that name might bring up better/alternative explanations of how this device actually works.

A variation of this physics demo consistes of balls on a track (straight or curved, grooved or parallel rods):

"version of Newton's Cradle" http://www.hope.edu/csi/Activity_Kits/Newtons_Balls_Background.pdf "Rolling Collision Balls" https://www1.fishersci.com/wps/portal/PRODUCTDETAIL?productId=703758&catalogId=29101&pos=23&catCode=SE_SC&fromCat=yes&keepSessionSearchOutPut=true&brCategoryId=56821&hlpi=y&fromSearch=

"COLLISION BALL APPARATUS" http://www.ginsbergscientific.com/0cat_details01.asp?cid=3676 I hope these observations are helpful. Quisicaluser (talk) 16:22, 29 June 2009 (UTC)

German tradition
Every German physics textbook of the 19th century shows a picture of such a device, it is most often called "Stossmaschine" (= impulse machine). One of my books names it "Stossmaschine Mariotte", thus one should look for Mariotte as the inventor. Every German Grammar school had/has such a gadget to demonstrate conservation of impulse. I own a 1972 catalog from Griffin & George (English company making and trading laboratory and school equipment) but there is no such device at all! For strange reasons the Mariotte impact machine was not popular in English schools/universities, maybe the same is true for US? http://de.wikipedia.org/wiki/Kugelsto%C3%9Fpendel names Mariotte


 * A drawing of Newton's cradle (not named) appears on p. 223 of The Book of Popular Science, Vol. 5 (1958 ed.) by The Grolier Society. It's in an article entitled Experiments With Sound which also contains older devices such as the Siren which is mentioned by 19th century authors such as Rayleigh and Helmholtz. The article could explain the popularity of the cradle and it appearance as a novelty in Canada and the US in the sixties. --Jbergquist (talk) 22:21, 17 January 2012 (UTC)

Do we really need a pop culture section?
I don't think the pop culture section contributes at all to this article. The pop culture references are far too obscure, especially when you consider that a newton's cradle is a common prop for an executive character's desk. It looks to me like some random users noticed a newton's cradle in their favorite pop media, looked it up on Wikipedia, and thought it would be cool to add cruft to the article. I was about to simply remove the entire section, but as I am Wikipedia (editing) noob I'm going to ask in advance. What do you guys think? AuntieNeo (talk) 04:40, 6 November 2009 (UTC)

What if the middle balls are not hang but fixed?
Will the effect be observed in the case only deformations of metall are involved and the middle balls are well fixed?--MathFacts (talk) 10:38, 12 February 2010 (UTC)

Newtons Balls?
Does anyone have proof that it has been called 'Newtons Balls,' as it does sound a bit, well, you know, DIRTY. He&#39;s just A Common Man! (talk) 07:53, 20 February 2010 (UTC)

Yes in... http://www.fas.harvard.edu/~scidemos/NewtonianMechanics/NewtonsCradle/NewtonsCradle.html http://hendrix2.uoregon.edu/~demo/Demo/Mechanics/ILM/Balls.html http://claymore.engineer.gvsu.edu/~flanderb/files/Newton.htm http://demo.pa.msu.edu/showdemo.asp?DID=DID26 Cantwealljustgetalong (talk) 15:26, 20 February 2010 (UTC)
 * Oh, so it was not only me who bursted out laughing at that name. ROTFL. -andy 77.190.12.208 (talk) 17:33, 25 August 2011 (UTC)

Explanation of the Physics?
Another thing I've noticed is that nowhere is there any reference to a text/source to support "the wave propagates through the balls at the speed of sound etc". I can categorically tell you that that is not the case. From some of my recent experimentation it is clear that the wave propagation time is on a scale of milliseconds not microseconds. Can anyone else shed any light on this?? —Preceding unsigned comment added by 136.206.1.20 (talk) 15:57, 3 March 2011 (UTC)

I agree with user 136.206.1.20 above in that shock waves are probably what is involved although sound waves would also occur at the same time. I'd like to know how he measured the propagation time and through what. Contrary to what Lensman003 says there would be no light speed propagation.--JCatania1 (talk) 23:05, 1 July 2011 (UTC)

The velocity of sound in steel is around 6000 m/s, thus the time for a wave to travel through all the balls is around 15 µs.Perarve (talk) 11:02, 26 October 2014 (UTC)


 * The effect of pressure waves was the original wiki explanation, and their presence is interesting so I just included a detailed description of how they are involved. Ywaz (talk) 13:39, 15 January 2017 (UTC)

What?
This article states: "The first time observer may find this visually intriguing and counter-intuitive. If a person charged into one end of a row of adjacent people then one would intuitively expect the whole row of people to be moved, rather than the last person in the row to acquire the kinetic energy." It is not well established that one person running into a row of people would cause them all to move or indeed any of them to move - there are just too many variables here for this analogy to be meaningful. — Preceding unsigned comment added by 192.158.61.139 (talk) 13:44, 24 June 2011 (UTC)
 * There's only one way to find out what would happen in such a scenario. 173.180.202.22 (talk) 07:40, 6 April 2012 (UTC)

Justification for detailed physics, and a spreadsheet
After spending a lot of time working on the physics explanation, I wanted to justify what may seem like a lengthy section. Other than finding out what it is, I think the main reason anyone would visit the article is to find out how it works. There are a lot of inaccurate solutions on the internet, and the previous explanation in Wikipedia was wrong (based on bouncing shockwaves which the authors of the reference subsequently renounced because the time it took was much slower as predicted by Hertzian compression). Rather than simply giving the 5 interdependent 2nd order differential equations and asking the reader to work out the details, or otherwise giving complex math, I gave a more wordy treatment in the expectation that the average reader would be at a high school physics level. Most presentations of the device and all of the iPad apps and Java programs I've seen use the simple solution, which is not completely accurate, given that it is supposed to be a physics demonstration. Even the best apps are not doing a real cradle justice because they do not include the errors away from the ideal. They have included friction and heat loss, but it is not enough to give the feel of a real device. Ywaz (talk) 16:04, 9 July 2011 (UTC)

"Drag the balls to create different motions"
This is totally misleading for unaware users! These are animated GIFs and would not respond to any mouse interaction whatsoever!! I make a bet many users already tried to take influence on the motions by using their mouse. But alas, this is not Flash, it's just a (beefed-up) image file. -andy 77.190.12.208 (talk) 17:38, 25 August 2011 (UTC)


 * I fixed them. Didn't know how to upload on top of the old ones on Commons, so if someone wants to fix that, please go ahead so the file history is preserved.  Lurlock (talk) 02:21, 6 October 2011 (UTC)

Who Invented It?
Joe Smitherman? Shouldn't the invention of this by Joe Smitherman in 1965 be mentioned? http://www.angelfire.com/home/sesquiq/joe.html -- 76.104.59.252 (talk) 16:54, 30 November 2011 (UTC)

Lee Trippett? A December 2007 article in the Eugene Weekly credits him: http://www.eugeneweekly.com/2007/12/13/news2.html

Energy losses
One of the other factors for losses must be the sound generated by each impact, not just heat from deformation and air resistance. In fact I think the sound is probably the major source of energy loss in an otherwise balanced system, but I have no evidence of this. Can somebody with the numbers add this ideas to the Losses section? Nutster (talk) 23:52, 25 April 2012 (UTC)
 * Also, if the balls are big enough that they deform on impact, some of the kinetic energy is lost to breaking metallic bonds and changing energy at the grain boundaries, not just to heat.--Wikimedes (talk) 19:00, 8 July 2012 (UTC)

Earthquake
I'm no expert, so I'll risk asking an ignorant question. Could this be considered as exemplifying how shock waves travel through the ground from an earthquake? If so, would it be worth mentioning?Campolongo (talk) 08:14, 26 April 2015 (UTC)


 * I don't think so. The energy released in an earthquake travels through the earth's crust by a wave motion that involves measurable movement of the soil and rock, up and down, or back and forth. In Newton's cradle with three or more spheres, there is no measurable movement of the middle sphere, or the interior spheres if there are more than three in total.
 * An earthquake doesn't generate shock waves; they are something different. A shock wave is generated by a jet aircraft or rocket or artillery shell or bullet that travels through air (or other gas) at a speed faster than the speed of sound. Dolphin  ( t ) 12:53, 26 April 2015 (UTC)

OK, thanks for that. I'll reflect on it and try to grasp the difference.Campolongo (talk) 18:41, 26 April 2015 (UTC)


 * Earthquakes generally shift down more than sideways, so the crust acts like surface waves in water, not sideways wavefronts, although the sideway movement might be like this effect in a certain way that the article mentions. The connection is too lose to mention it. I wanted to include pressure waves in water like when you see the surface explosions from depth charges, but someone objected. Ywaz (talk) 00:17, 9 April 2016 (UTC)

more complex arrangements of the five balled cradle
initial conditioning:


 * 1) one ball raised and released at once (from one side)
 * 2) one ball from each side raised and released at once synchronously
 * 3) two balls raised and released at once (from one side)
 * 4) two balls from each side raised and released at once synchronously
 * 5) three balls raised and released at once (from one side)
 * 6) four balls raised and released at once (from one side)
 * 7) all balls raised and released at once (from one side)
 * 8) separate the five balls in groups of 2 - 2 - 1 balls and synchronously release them
 * 9) all balls raised and released one by one at short intervals (by holding them carefully with one hand) (from one side)
 * 10) stabilize the central ball, release synchronously two surrounding balls - one from each side - and at the half of the extreme divergence position release the other two flank balls
 * 11) make the central ball to oscillate, then repeat the upper step, this looks like step 9 but less energetic

then:
 * 1) describe all the above mathematically
 * 2) replace one ball with one magnetic ball from one flank
 * 3) replace the two outer balls from both flanks with magnetic ones
 * 4) use magnets of various magnetic magnitudes
 * 5) describe all the above mathematically
 * 6) find new examples

— Preceding unsigned comment added by 2A02:587:4110:2B00:F0D5:204:77E2:3637 (talk) 09:53, 24 October 2016 (UTC)

Crappy grammar from template
five 6 in chrome steel ball bearings yields
 * five 6 in chrome steel ball bearings

which is not good grammar. Either amend the 'convert' template, change the template being used, or don't use a template. —DIV (120.18.150.196 (talk) 07:20, 20 November 2016 (UTC))

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Typo? Time to complete collision ____ (increases or decreases) in softer materials.
Hi, I noticed that the sentence reading "the time to complete a collision decreases in softer materials" seems to have it backward... isn't rubber a slower collision than steel?

Thanks. 208.76.28.70 (talk) 21:34, 22 February 2018 (UTC)


 * I made the correction. Ywaz (talk) 20:51, 6 March 2018 (UTC)

Sentence needs proof reading and correction.
In the section "Effects of Different Types of Balls", this sentence needs to be fixed. "If the solid balls are too large, is be lost as heat, because the elastic limit increases as radius 1.5 but the energy had to be absorbed and released increases as radius3" — Preceding unsigned comment added by Jamezism (talk • contribs) 16:20, 15 December 2018 (UTC)