Newton's cradle

[Matty F]

OK I've decided to use billiard balls for my Newton's cradle. But I would rather not drill into the balls or try to glue things on them. How can I hold the balls in position? This very large version has some rings, which is what I am considering:

people are likely to pull the balls out if they can, and I would rather have a couple of large rings, one on each side, centered on the point of impact. I'll have to tie them together somehow. Cable ties? I'll probably use heavy monofilament nylon to hold the balls in line. Unless someone thinks of something better. I have to make this in the next few days!

[Weatherlawyer]

You are far too fat headed for a successful operation. Not even brain surgery will help you. Have you considered mummification?

[Matty F]

Having a bad day are you? I've managed to make many things so far. Often people in here have ideas that I had not thought of. The Newton's cradle is for a science show that thousands of people will be looking at.

[Tim Watts]

Weatherlawyer wibbled on Saturday 03 July 2010 05:45

Are you austistic or just hard of thinking?

[Peter Watson]

How about making a net of six rings (top, bottoms and four sides)? The rings could be made out of fishing line and joined together by the same?

[Matty F]

Yes, they would end up being roughly squares. On the other hand I'm coming around to the idea of using epoxy glue to glue an attachment to the top of the balls so the fishing line can be attached. Most cradles have a gap betwen the attachments but why can't both lines be at the very top?

[Mike Barnes]

Matty F :

Jubilee clips.

[Jón Fairbairn]

Ideally you want the balls only to swing about the axis through the top of the cradle. If you mount them at the very top, they can wobble about that point and twist around it, so it's better to support them from two points lower down.

You could machine rings with a groove around the inside of each and cut them through (making an almost-closed C shape) so that it will spring open far enough for the ball to pop in.

[Robin]

Doesn't the gap serve to stop the balls rotating about the vertical axis? There's no way my addled brain can model such things these days but I think such spin would be a "bad thing" - unless you are also trying to demonstrate snooker shots :)

[Matty F]

Yes spin would be a bad thing. So I need a gap.

[Matty F]

At first I laughed. But they would be a good idea if they looked OK. Perhaps I can make some elegant brass clips.

[dennis]

A couple of large rings as you say, then just wire them together and twist the wires. Four wires spaced around the rings should be OK. The supporting nylon can be attached to the top two wires and I am sure you can twist a loop in the wires to attach them.

Why not get your angle grinder out and just mill a couple of notches in the top, drill the bit that now sticks up and attach to that?

[dennis]

The gap makes them swing straight, if you just have one the balls will just bounce to one side and not impart all their energy to the next ball.

[Duncan Di Saudelli]

plane disk whose diameter matches the flat. A metal disk would enable you to solder or braze or machine two pickup points at the edges of the disk, diametrically opposite one another.

DDS

[Matty F]

I can probably make the rings out of wire.

If the cradle doesn't work that well or management decides that they are a danger to small kids, I'd like to sell the balls as billiard balls again!

[John Rumm]

The spacing prevents the balls twisting...

I would have thought a couple of rings - a bit less than the ball diameter, which were then lashed or tied together with the ball sandwiched between them.

[Gib Bogle]

The motion wants to be: (a) In-plane, and (b) Irrotational.

Even with separated attachment points, flexible lines will permit a degree of out-of-plane movement, which tends to grow. Probably the most satisfactory compromise between in-plane constraint and low wind resistance would be a solid strip (or two, to obviate out-of-plane deflection) of a rigid material, aspect ratio say 5:1, with the long dimension in the plane of motion. If this was rigidly attached to the ball, and hinged at the top, the various requirements would be met quite well. A bearing or two (one each side of the strip) would be better than a hinge, but might be overkill.

[Gib Bogle]

The suspender could be made from a strip of alloy, bent twice to make a thin square U shape. The bottom of the U could be glued to the flat on the ball. At the other end, a ball bearing glued to each outer face of the strip, or if that's too complicated, a simple low-friction hinge. Alignment of the hinge axis perpendicular to the plane of motion could be a trick.

[Gib Bogle]

With fishing line, and the sort of separation envisaged, the spacing will reduce but not prevent twisting, IMO.

[Roger Dewhurst]

Forget the billiard balls and go looking for the largest ball bearing available. They might come from some very heavy machinery, not necessarily with wheels! Steel is much more elastic than billiard ball material. I mean elastic in the physical sense not like rubber.

R