Simple Investigations for High School Students

Attach one end of each rod to a table so that the same amount overhangs. Add incremental amounts of weight to the furthest point of each rod plotting the deflections! weight against deflection!

If you roll up a tube of card so that it has the same dimensions, repeat the experiment! (May need a bit of giggery pockery on that one!)

Have fun teaching those kids South African guest

Mr B's suggestion prompts me to say it sounds the ideal moment to seperate out the concepts of 'strength' and 'stiffness'. Not the least bit the same. You could expand it to deal with hardness and suchlike (how easy is it to make an impression in the surface of the wood and the metal with an implement).

Talk to Vermin re relative "strength" and "stiffness".

Assuming you have fairly small diameter rods...

Clamp the rod horizontal in a vice (or with a lab clamp or a "C" clamp). Use something to clamp the free end a distance of, for example, 5 cm out from the clamping point. If you have it available, use a pair of locking pliers (we call them vise-grips but that's a trade name). have the clamp or pliers also horizontal, but perpendicular to the rod. Have the students hang weights off the end until the rod breaks or deforms. If you don't have weights, have them pull with their hands and make estimates. Or, you can use a bucket and fill it slowly with water (or sand).

Place the rods between supports perhaps 10 cm apart. Hang a bucket in the middle. Slowly add water to the bucket till the rod breaks or deforms. Make sure the bucket is near enough the floor that it will not spill. Compare the amounts of water.

If the 'strength' of wood and iron need to be explained, the suggested experiments are good enough.

But when you say, 'compressive strength', it is very specific. Compressive, tensile, bending, shear, twisting are different in the direction of load application.

The easier experiment for 'compressive strength' could be, stack up (one touching the other's cylindrical face) a short wooden cylinder (or any shape) and an iron cylinder of same shape/dimension between the jaws of a vice. Then tight them up until one of them give way...

I'm very sorry. I did not read the question well, it is said as, 'comparative strength' and not 'compressive strength'.

All experiments suggested are good, including mine...

What size rods will you be using?

the west countries' high school studnts have more time to play than chinese students. I was told.

And that answers the question how?

so they are happy and luck than ours. they havnt got pressure.

Why dont you try giving them thin rods of metal that are just about bendable. A peice of wood would break easily while the metal wouldnt. It will also explain the difference in the nature of the two samples.

Make them try to break and beat them in different ways. Let them explore it too. Maybe not allowing destructive natures to grow though.

Floating it on water etc will also explain additional differences in density etc.

I think a few simple experiments would be as follows: (1) Extend the two "rods" over the edge of a desk, about half the length of the rod. Put a heavy weight on each to hold them in place, then hang increasing Kg weights until one rod breaks or bends. (2) Use a saw blade (hacksaw) being pulled backward and forward to show cut resistance; (3) butane torch flame applied to each; chemical resistance (where the metal could be attacked by acids for example, while the wood would resist a lot longer), (4) using a ring stand and clamp, clamp each rod vertically and then add weights to the top of the rod (the weights should be loosely secured from above so them don't fall when a rod breaks) until a rod bends or breaks, (5) suspend a rod between 2 chairs or 2 desks and stand on each carefully until it bends or breaks, (6) long term saline, acid or base solution exposure to see how each material is affected, also (7) leave the rods in an oven at 80 - 100 C over night (or over a weekend) and then test to see what effect the heat has had on each rod; (8) Drill holes near the ends of each rod and string nylon or similar cord thru the holes, and let students pull in opposite directions to show tensile strength, and so on.