# Efficiency of a hollow sphere of bouncy balls?

1. Jan 5, 2012

### Sirloin

I'm tasked with creating an egg protector that bounces. I'm thinking of gluing bouncy balls together side-by-side around a template sphere. The glue being used should be very strong, it's a super glue that works on rubber.

Would this be able to bounce effectively? Will I need to add mass within the bouncy ball sphere?

2. Jan 5, 2012

### Rayquesto

Ok, so, as I understand, you also don't want to break the egg if it is used as an egg protector. I would think that in order for the system of bouncy balls to bounce is possible with enough air pressure (more air) in the bouncy balls, yet there is a possibility that the egg could break if there is too much air in the bouncy balls, since the surface tension of the balls them-self would be much too small to obtain enough energy on impact to sustain the inertia of the eggs.

What exactly is your objective and can you draw out what this may look like?

3. Jan 5, 2012

### Sirloin

Well this is just the outside. Inside, I plan to incorporate a spring suspended, smaller container lined with cushionny material in which the egg will be placed.

I just need to know if the hollow bouncy ball sphere will efficiently bounce.

4. Jan 5, 2012

### Rayquesto

I believe it mostly has to do with how much weight the ball could use its surface tension without popping or breaking.

5. Jan 5, 2012

### Rayquesto

or rather how much weight each bouncy ball could use it's surface tension to suppress without breaking or popping.

6. Jan 5, 2012

### Sirloin

I mean solid rubber balls, I don't understand what you mean by popping them...

7. Jan 5, 2012

8. Jan 5, 2012

### Sirloin

Like these. Glued together side by side to form one big spherical shell.

9. Jan 5, 2012

### sophiecentaur

To protect the egg you need to avoid rapid changes in velocity (acceleration) superballs work as efficient bouncers because they do not distort much and their contact time with the ground is short. This means high acceleration (meaning high force) is involved and that acceleration would transfer to the egg. Something your egg doesn't need. So you have a conflict with this solution.
One way to achieve good energy storage with low acceleration could be to use a very rigid hollow sphere with an egg container of high mass. The container would be suspended at the centre of the sphere with radial steel springs (efficient energy storage). But the problem would be to get a light enough yet strong enough sphere that could contain the spring /mass system.
How 'bouncy' does this all need to be?

10. Jan 5, 2012

### jetwaterluffy

I really suggest bubble wrap for that. It's compact, and absorbs impacts really well.
Those balls are quite hard and can transfer a lot of energy (as this shows), so be careful.

11. Jan 5, 2012

### sophiecentaur

This is where there is a conflict of interest. If you use a soft, absorbent material to cushion your egg, then this will absorb energy, which will reduce the height of the bounce of the container. To get a good overall bounce, there must be a minimal amount of energy dissipating material in the system.
Tricky eh?

12. Jan 5, 2012

### Sirloin

I'm thinking the suspending springs should do a good job of dissipating the acceleration of the egg.

What do you mean by suspended with radial springs?

The more clear bounces, the better.

13. Jan 5, 2012

### Sirloin

This is a sort of cross section of what I'm thinking. Imagine that the bouncy balls continue into the page to form a hemisphere (not a full sphere, because it's a cross section).

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14. Jan 5, 2012

### sophiecentaur

If you want it to bounce a lot then you must avoid energy dissipation. The mountings must be resilient - which I suggested "radial springs" -i.e. like spokes on a wheel but radiating out from the centre to the surface of the sphere.
Also, it would be better for the sphere not to distort as you have shown. It would not bounce many times, if at all. Having the surface made of super balls wouldn't help in that respect as it would only be the balls in contact with the ground that would give you any benefit of their super-bounce quality.
I think your supporting sphere might be best made like a beach ball and inflated quite hard. But you'd have the problem of sealing the hole through which you put the egg and suspension. Alternatively, you could have an open sphere, made of steel hoops. These would be strong and resilient but otoh would be difficult to manufacture. Sod's Law rules

15. Jan 5, 2012

### Sirloin

The only energy I want to dissipate is the egg's energy, which is independant of the shell, no? And my diagram is very imperfect, imagine a perfect circle. The purpose of a sphere of superballs is to maximize the force of gravity on the entire structure as a whole, rather than on a component of say, a rectangular prism, as well as to have a round surface contacting the ground on every collision, no matter what the orientation.

I don't understand how radial springs would be used, I'm not familiar with this type of spring. Also, what do you mean by supporting sphere? The sphere I'm using as a template to make the bouncy ball shell? I intend to use just a foam ball I bought, glue together half the bouncy balls to be used, and then do the other half without gluing the two halfs. I'll have to find a method to attach them for trials, I may do an entire half, fashion the springs to the interior, then do the next half without gluing the top "cap" (think kippah (jewish headwear)), so when I'm done fashioning the springs to THAT half, I can glue the two halves, and have a removable cap.

16. Jan 5, 2012

### jetwaterluffy

What you might want to do is make the sphere of bouncyballs really big, so the energy lost on padding the egg is negligible in comparison. But I don't know whether in practice, increasing the mass of the bouncyballs increases the error and thus decreases the bounce height.

17. Jan 5, 2012

### Sirloin

I have a ridiculous amount of bouncy balls of perfect size ready to go, I don't think there's anything I can to do increase their mass, but I could add mass to the interior of the device.

18. Jan 5, 2012

### jetwaterluffy

You increase the mass by adding more of them. I don't know if my idea would work, but increasing the mass may mean the energy absorbed by the padding that sophiecentaur was talking about is reduced to negligible compared to the total energy of the system.

19. Jan 5, 2012

### sophiecentaur

But you don't need to 'dissipate' the energy of the egg. All you need to do is to limit its acceleration so that the forces aren't too high. There is an essential difference between the two ideas. It really wouldn't matter what actual speed the egg reaches during a bounce as long as it is not subjected to an accelerating force that could break it.

In your diagram you show a hollow shell that is a sphere when not under stress? But you now imply that the ball will be almost full of foam. I was assuming a rigid sphere (stronger than your pictured bungee design) which would have spokes going from the centre to the edges. The spokes (radials) would be long coil springs, supporting the egg carrier at the centre. These springs would allow the egg to move freely under moderate acceleration whilst the sphere is doing its violent bouncing.

Your main trouble will be in making a bouncy ball that is actually bouncy enough. In order to be bouncy it must not flex very much because flexing will involve energy loss due to hysteresis.
Have you seen the sort of sphere used in 'Zorbing'? That was the sort of structure I am envisaging - with the egg supported like the human is, only with springs instead of the straps.