Prop ideas sought for tabletop tension & compression science demo.

I’m doing a basic science demonstration about bridge building to some youngsters and their parents. I want to illustrate with props how various materials withstand tension and compression differently.

Demonstrating a material that can handle tension is a snap. I can use a piece of rope or cable. I’ll ask two kids to have a little tug-of-war and it won’t break. When I put the rope under compression, however, it will fail miserably, going limp under its own weight.

Compression is trickier. Sure, I can use a brick or a piece of wood. I can tell a kid to squeeze it as hard as he can or stand on it and it will remain whole and undistorted. The problem is that the brick or wood could just as easily pass the tension tug-of-war test (at least for my little tabletop demonstration). A smart-alecky kid could rightly ask, Why not just build the whole structure out of brick or wood?

So, I’m looking for a readily available material that can withstand compression but will pull apart under modest, human-strength tension. Any ideas?

(Oh, and I don’t want to cheat by using, say, a stack of checkers. The same smart-alecky kid could say, Well, the rope would fail too if you cut it into pieces! Duh!)

clay?

Brian

As a former civil engineering student (but no expert) I have my doubts such a material even exists. Perhaps a spring?

The trick is going to be in how you subject the material to tension. Taking a clay tile and putting weights on each end might work.

Technically, you’re not really looking at materials so much as the shape of the material. A rope or cable is made up of natural fibers, nylon or steel, all of which can handle compression just fine if shaped into a solid brick. Actually, outside of a liquid, or an air filled foam, just about any material can handle compression. You can fill a bucket with sand and stand on it, if you wanted to.

Anyway, you CAN build a bridge out of wood or brick, it’s been done for centuries, but cable is the right choice when you have to span long distances, it’s easy to make thousands of feet long and can hold a great deal of weight. Try to build the Brooklyn Bridge without cable, you’ll need 10x the amount of material, you’ll need 2 or 3 more towers (in even deeper water) then arches between the towers, it would be a horrible mess.

If you’re spanning 50 feet over a roadway, you buy some 50 foot long steel I beams and build your road on them. You’re spanning a creek, drop a couple of logs over top and you’re set. If you’re spanning the lower Hudson River, you better have some cable on order.

When I was in middle school I was on the Science Olympiad as the official “bridge builder” for the bridge building event (now, apparently, the tower building event.)

The point of the event was to build the lightest possible bridge (with size restrictions) that would hold the most weight, represented by a bucket hung from a block that rested on the bridge into which you slowly poured sand. When the bridge blew apart into a million pieces, the weight of the sand that made it into the bucket divided by the weight of your bridge was your score.

I had some bridges (made of balsa and bass wood, and wood glue) weigh just under 3 grams and hold over 7 kg of sand. Those bridges were so fragile that I also had to build special carrying boxes just to get them to the event, and they could quite easily fall right apart if you even looked at them wrong.

I always thought this was an amazing display of how, by using the right design techniques (triangles and arcs), you could devise a structure so so sound under one direction of force and yet so fragile in another.

How about an arch made of styrofoam or balsa wood? Or an egg?

You could use a piece of printer paper. If you cut it into strips and fold it accordion style, then place it on the table so that it’s lying on its side and curve it around on itself, then you can place quite a bit of weight on it without it collapsing. Try it with a brick or something similar. If you then try to use it as a rope, you can probably break it fairly easily. It would probably also work if you simply coiled the strip of paper and then placed a weight on the coil.

A second approach would be to use something that’s completely granular, like dirt or sand. But that may be too much like the checkers example. On the other hand, something like mortar might work well, if you can find (or make) a piece of it that you can set something on top of, then try to use as a “rope” to pull against.

A final approach is to use a thin wooden dowel. That will lift quite a bit of weight in tension, but try to use it in compression (in the same form) and it will buckle and collapse fairly easily. It will have to be long relative to its diameter for this to work. Of course that’s the opposite of what you were looking for, but it does illustrate the difference between the two rather well.

A stack of quarters?

Elmer’s glue? Any two items glued with it. You can press until the item itself breaks and the glue patch is still there looking happy. Then you repeat and tug the items apart. The glue should fail first (for most other reasonable materials, of course)

A bicycle pump in fully compressed position?

toothpicks or other small pieces of wood.
Glue a couple of them to a heavy weight (experiment to see just how heavy) and suspend the weight from the toothpicks.
Then put the same weight on top of a couple of toothpicks, and they will break.

how about making an arch with say floral foam.
on a board have 2 blocks screwed down to form the start of each side of the arc and cut wedges of foam to make an arch( exactly the same as a brick arch around a old tunnel)
when the wedges are in place- quite a load can be places on top. take the wedges out and they can be pulled apart quite easily.