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Question Anyone built a big Newton's Cradle or Pendulum Snake?

1 month 3 weeks ago #41809 by AndyG
I'm about to build a big pendulum snake (eg like this youtube ) and a massive newton's cradle. Anyone done this and got hints or tips? Before anyone asks, the reason is because lock has driven me nuts and I need to have something to concentrate on and produce something good rather than watching another painted wall dry ...!
The snake will use billiard balls and 0.09mm support cords (if I can see the cord to tie a knot!) and should be fairly easy to build and trim correctly.
The Newton's Cradle will use 50, 80 or possibly even 100mm hardened ball bearings. The weight will be large (between 0.5 and 4kg per ball) but the support cords will only be either 0.09 or 0.14mm thick so the balls should almost appear to float in the air and I'm looking at a long period well in excess of 1.5 seconds so it should look froopy (a technical physics term ... :hb ).

Has anyone built large versions of these and has any hints or tips? Should Newton's Cradle balls actually touch at rest or just not touch? What about fine adjustment methods for support cord lengths? Is there an ideal spacing between the two main support rails on a NC - is closer better? What happens if you change the number of balls?

I watched the demo of a "super dooper" commercial NC costing £2500 which was horribly made (I could knock one out as badly for less than £800 in a morning) and it died after 5 oscillations but I'm hoping that was due to the quality of manufacture rather than massive inherent damping in the system. I hope I can reduce damping to just impact friction as air resistance will be tiny as will bending friction on the support cords.
1 month 3 weeks ago #41810 by prep_room_boy
We once tried a Newtons cradle hung from the beam in the physics lab. It involved 4 old netballs, hairy string and Gaffa tape. The results were hilarious, and a disaster. Another bored lunchtime badly spent.

Good luck on yours and let us know how it works out.

I can feel a Wiki entry coming on here.
1 month 3 weeks ago #41811 by AndyG
Thanks, if it doesn't work I'll have to learn how to play Boules with the balls!
1 month 5 days ago - 1 month 5 days ago #41816 by AndyG
The primary phase of the Demo Pendulum Snake is complete and my creation is alive! (insert Frankenstein laugh in the background).

What have I learned so far?
String length is important (obviously as the wobbulation factor depends on it)
Nylon monofilament is horrible if you want two, matched support strings and the bob is not very heavy ...
A jig to set initial pendulum height makes life so much easier.
Excel is your friend for pendulum length lists.
Do not use knots as knotted knots are not easy to adjust accurately and monofilament does not knot well.

The secondary phase was testing.
More things I learned:
You look stupid when it basically works first time and you get over-excited in the prescence of the boss :laugh: :silly: :silly:
Boss headed to his office for his phone and meanwhile I tweaked only the two longest pendulums - which were quite obviously wrong - so always use the jig and if the length looks wrong it probably is!
When the Boss comes back with a phone to video it, you still look silly and overexcited on the second run ... but everyone was so impressed it didn't matter! B)

Now for the tertiary phase which will take longer ... and the line will be only 0.09mm thick and very slippery to handle! Hopefully adjustment will be built in this time ...

This is the Youtube link for version 0.5 beta ...
1 month 5 days ago #41817 by AndyG
For reference, I dished the 8mm diameter head of a little pop rivet body so the concave form roughly matched a 50mm steel ball bearing and Araldited it on ...
The idea is to drill a small hole through the body and use that as a cord fixing point for the big Newton's Cradle balls - it needs to support 2.5kg maximum load (a 2kg steel ball swinging) and for safety I want 5x that ... They glue planes together so it may work!

And so the test began.
The rivet suspended the 0.5kg ball which was a start.
I supported the ball in a retort clamp and loaded the string - I was hoping for 10kg but the retort assembly flexed too much as that was approached. Much jiggery pokery and more mass later, the string broke. :-(
I doubled the string, used the edges of two tables close together to support the ball and loaded some more mass on the string. At 19kg the double string broke!
This became a test of brawn over glue!
I replaced the string with 100lb test fishing line and loaded on some more mass. Eventually the fishing line ... ripped through the aluminium fixing! 29.5kg but the adhesive was still secure :-)
That exceeds my specification by a factor of 3 at least so I'm happy to run with that idea.
Waste not, want not ... I wanted to reuse the steel ball. Removal of the rivet was easy - heat the rivet very gently with a blowtorch or bunsen until the adhesive degrades, pop off the rivet then polish off any advesive remnants on the ball away with a polishing mop in a Dremel. Simples :-)
3 weeks 1 day ago #41818 by AndyG
And now a 5-ball Newton's Cradle exists. :)
Bit of a bodge but uses 2kg steel bearings on a tubular steel frame which looks fairly neat but the whole thing weighs a ton and the balls are not demountable.
Started with monofilament again, supported and strung the balls and as I let each go the line stretched by about 3/4 of an inch ... This makes it an absolute killer to adjust on the less-than-ideal fixiing points.
Ho hum.
Anyway, got it working after a fashion. My little unit with 20mm balls does about 4 or 5 oscillations before things start to get wobbly. Although mileage varies, and seems very sensitive to the way the first bob is released, at one point "Big Bob" did 50 good oscillations :-)
What's interesting is the appearance of very high frequency, small amplitude oscillations that occur in the "static" bobs during a simple, single bob start condition. The bobs touch when stationary but to the eye they appear to be 1-2mm apart whist the oscillations are in progress and at some points I can't see the oscillations so the frequency must be very high ... some calculation of the natural frequency of 80mm hardened steel balls is already in progress!
I still haven't worked out whether the balls should actually touch or not at rest. Making them "just touch" is more difficult than it sounds as there is an optical issue such that the eye thinks the balls touch before they actually make contact (this is also observed on a massive scale during some solar/planetary transits and called the "black drop effect") - try it using two large ball bearings looking towards a light source. A simple circuit with a meter/led/buzzer or whatever and a pair of wires taped to the balls will tell you when they really touch B)
As I said, the stretch of the monofilament is a real issue as the length of the suspension cord directly impacts the height of each ball and even fractions of a millimeter variation will change the angle of impact and the efficiency of the system. Equally the adjustment of the distance between each ball (via the suspension points) and the balance of the two suspension points across the frame (squareness, height, left/right) for individual bobs are obviously important. However I feel that matching the length of the suspension wires (thus the periods of each pendulum) may also be an important factor - a ball in the correct position but with shorter suspension wires because the suspension points are in a different position will have a different frequency. So all I have to do is be able to micro-adjust the position of the suspension point in three dimensions and the micro adjust the length of the cord ... more engineering bodgery coming up ...
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