The Physics Behind A Design

[mike_302]

I am given the task to make an egg holder for a 20 foot drop. I have been looking at designs all night and I have come across a website that tells me about the best results from their school, of all time. There is one particular design that the teacher praises where they used a tylenol box, JUST barely big enough for the egg, and they packed it TIGHT with foam. IT says the tighter the better. Now, The site says this design has never cracked an egg, even from a 50 foot drop. Can someone explain the physics behind this?

Then it also says that the most proven way to make ANY egg holder design is the more rigid styrofoam board... Like the stuff that has the tiny styrofoam balls. Supposedly this stuff is purely amazing. Again, I don't understand the physics behind this.

I'll explain what hinders me.

I think that you need to let the egg move. I think of the dropper as something that doesn't stop the egg DEAD, but makes it land over a longer period. The tiny tylenol box with foam sounds to me like it would absorb ALLL the energy, and the egg really gets a big dose of energy from that... My brain tells me that it would crack instantly... Yet it's supposedly the very best contraption ever built for this project.

Again, could someone explain this physics to me?

 

[Dr.D]

I have not given the a lot of thought, but the basic consideration is this: The egg is very, very strong, provided the load is distributed properly over the surface. The trick then is to achieve the necessary distribution of the load. Most obviously, you do not want anything that looks remotely like a point load. I have no idea what the most ideal loading is, but the successful packaging you have read about must approximate this.

 

[nickjer]

Supposedly the egg has a shape where if you apply equal pressure all over it evenly it will not crack (due to its arch shape). But if you apply uneven pressure, like poking it with your finger at one spot, then it will crack. But this is something I heard a long time ago and may be a myth.

As for the egg dropping experiment, I did the same in high school a loooong time ago. I packed mine nice and tight, but it cracked :(

The winning design was trapping the egg in pantyhose and suspending it in a box. That way when the box hit the ground the pantyhose acted like a bungee cord.

 

[mike_302]

See, that was my first thought, but my teacher has restricted weight to a measly 75 grams. Which is why I think I need to go with the tightly packed box. Mind you, I still can't wait to try it out a few times to see if it works. Bought a dozen eggs :P

 

[Paul E.]

When we had to do this for our Physics class a few years ago there were a few designs that did well:
1. Allowed no bounce to occur, thus no vibrations/resonance in the egg from bouncing off of the ground, this was done with a peanut butter jar and some designs allowed it to splatter out (made for a fun clean-up).
2. Fill up a huge (5 gallon) or so sized container with some material that is fairly dense (jello was used and was also suspended in the middle of it with rubber bands and tape).
3. Someone got this to work and I still don't know how, they made a frame out of chopsticks that would not break from the fall and then suspended the egg in the middle with rubber bands, that was amazing to actually see work.
I'm assuming that your goal is to have the lightest system that would work without letting the egg break. Some basic principles of Physics that you can use to your advantage would be:
1. Not letting the egg resonate or bounce.
2. Increase the impulse or amount of time there is a force exerted on the egg.
3. Using increased surface area to lower the amount of pressure on the egg.

My final suggestion to you is to experiment yourself with different ideas! I spent many hours constructing something that would work and I had many a broken eggs on my driveway. Keep in mind that you're system may flip while in mid-air, or in my teachers case intentionally send it on a flip-trip. So you may want to add something to stabilize it in mid-air or make your make your center of gravity below the egg.

I hope this helps! Don't try to take ideas, but use your own intuition and think through how different designs would behave in air, and on impact.
Good Luck!

 

[mike_302]

Well, I made a working design last night. IT was effective in a 20 foot drop, but then I got to school today and found out the FULL extent of the project: for full marks: max 15 cm*15cm*15cm ... So Size AND weight restriction. Also, for full marks, can't simply be a box with styrofoam. That's poor restricting, but my design was a box with styrofoam, although I added something else: I wrapped it in excess bubble wrap, and twisted and tied the bottom together to make a sort of bonbon package... Like the little twist you get in the plastic wrapper of after-dinner candies, or those new years exploding things.

I think that twist is what softens the impact, cause it tends to stay down, land on that, and then fall softly.. It's about 30 cm though I think... I could probably shorten it, but that would be no fun if it just worked!

So I'm trying a new design today: Cone of paper (like a party hat) and a rubber glove, filled with air. I'll stuff it with tissue paper and blow it up with air. The cone shape should drop down first, and I'm thinking It will be a crumple barrier. To attache the glove, then I tie it closed, I'll pinch it in a hole at the bottom of the cone.

I'll post how it worked after :P I'm excited

 

[minger]

Eggs are not "designed" for evenly distributed pressures, they are designed for axial forces. If you hold an egg in your hand such that the "point" is at the bottom of your palm, and your fingers wrap over the wide side and squeeze, you will not be able to break the egg! (not sure how strong you are, so do it over a sink, just in case)

Having said that, if you start putting pressure on the sides, it will break quite easily. I've done egg drops many many times. The most important part is making sure the egg falls the way you want it to.

I've found that putting an egg inside of a toilet paper roll with a little cotton underneath it can be plenty. The egg fits just inside, and then you add a stabilizer to ensure it falls straight, and you're done.

 

[mike_302]

Well, progress out of tonight is this:

took the box that worked, kept the sponges, and added a layer of bubble wrap in order to let some energy disperse through yet another material before reaching the foam, then the egg. Okay, so now I have a box with a bunch of foam again...

Next thing I did was to take a plastic container lid (diameter=15cm) and cut an opening to squeeze the box through in the center, effectively making a skirt. Now the thing has some air resistance, and I haev tested it... It works, but needs more to not be considered a simple "box with foam".

So The last thing I did was to cut straws, 15 cm, and poke a hole at each of the corners of the base of the box. I put he straws up through these holes, and they can slide. Purpose of this is, at some small extension (say 5 cm) They will be hte first to land, and should hopefully retract, and/or break at the base of the fall.

That final design worked twice, then the box became soggy from the wet sponges, the eggs cracking, and all that jazz, so I am looking for some new box material.

Here's an issue though: How do I absolutly guarantee that the egg stays upright? I mean, I used to have one of those egg shaped toys that had a heavy sandbag in the bottom and it would always pop back upright. Should I remove the majority of the padding from the TOP of the egg (not all, but I had been putting a large amount in) and moving it to the bottom to create more wait there?

I noticed that the "skirt" helped level things a lot, but the windy conditions from the roof were not helping my observations unfortunately.

Can anyone give me some theory so that I could try it for myself, given that I don't have the practical means of determining an answer on my own (darn wind!)

 

[Paul E.]

The key to staying upright is keeping the center of mass of the system below the egg, other ways would be to use a parachute (I know your teacher told you you couldn't use that, but the skirt is maybe allowed?), or you could use fins of some kind. To put more weight below the egg is always tough because if you put too much your system will be too heavy, I might suggest putting a clump of playdough on the bottom or some kind of mashed "squishy" substance (there's a 'Physics' term for ya) to put more weight near the contact surface.

 

[mike_302]

Thanks. I'll try that. So it's about center of gravity I am guessing. Will still need a new box, but I'll try and get that this weekend and run a few trials.

 

[minger]

Yes, you need zero padding above the egg. In most the competitions I've had, several designs made the final drops, and the tie breaker is always weight, so keep it light.

If your current parachute isn't working, try just some streamers, or even a fan type design (can be pretty cool).

 

[mike_302]

the fan idea has been intriguing me for a while, and I've kept it in mind, but I made a brand new design that has, so far, been kicking butt. I will need to lose some weight on it after I determine whether it is the godsend I think it is, but as of right now, I have a sturdy laminate gift box that is JUST big enough for the egg, plus one sponge under, and one on each of two sides of the egg.

That holds it... Now the cool part: I found some light plastic "gutter filter" I guess is what it is called? It's like a plastic netting, 6 inches wide, in a roll... Buck store stuff. Anyways, I cut it into strips long enough for the diagonals of the box, and about 4-6 inches each, then rolled each one. The overlapping side is then taped to the box in a manner that the four SIDES have a diagonal strip, like a short, fat hot dog almost, taped across. The rolls do a good job protecting the corners, and then the bottoms have a wider piece, taped straight across the bottom. What these rolls have done for me is provided a bouncy cushion almost... It absorbs the energy from the fall, no matter the side it falls on, and rolls it off to one side or another where another cushion/roll takes out more energy. It got really windy so I didn't go to the roof, but I threw the thing up in the air and let it fall... The egg never cracked in 3 trys so... I'm guessing that's good.

 

[mike_302]

Here's one I reallly need help with guys.

Lets say I wanted to make a model that rotated like a maple key... You know what I mean? A blade or two that turned all that gravitational into kinetic. How would I go about building this model in under 75 grams? I'll bet I can brainstorm materials if I just knew the physics behind doing that. I could try a bunch of things, but I need places to start. Any idea if this is a common project and where I could find instructions on how to make a turning fan/blade like that?

Thanks