Center of what?

Bonjour, mes amis! I have returned, refreshed and eager to instruct. The trophies of my triumphal efforts with hook and line are six bass, two catfish, and a number of tree limbs. All except the tree limbs have before now been consumed, so I have nothing but my happy memories as proof.

Now, where were we? But of course, we were talking about rocket fins. To begin, we will conduct a simple experiment. Hold a soda straw by one end, and being very careful not to hit anyone or anything with it, toss it into the air with a flick of the wrist, causing it to spin end over end.  If you do not have a straw, then an unsharpened pencil will do, but you must be even more careful with this, and especially do not use a sharpened one. We would not want to cause an eye injury. Lacking either of these, a short, straight stick or dowel will do, but no longer than about fifteen inches and no less than eight.

What did you observe? It should have spun around the middle of the straw or pencil. Exactly what you would expect, no? Yes. Now, tape a small weight onto one end of the straw, say, a small block of wood, a bottle cork, or something similar. Hold the other end and toss it into the air as before. Notice that the point about which the straw or pencil rotates is no longer in the middle, but is closer to the end with the weight. Why is this? It is because of something known as the Center of Gravity, or Center of Mass.

You may have heard the term mass before, but perhaps you have had difficulty with it. I will say very little about it here, because soon enough you will be accosted by plenty of science teachers brandishing that term and threatening you with Newton’s laws. What is the difference between mass and gravity? Let us deal with things that occur on this earth, as few of us will ever experience space flight and zero gravity, where the concept of mass is much more meaningful. We say that a kilogram is equal to two point two pounds, and indeed, if you compare upon a balance scale a kilogram weigh and two one pound weights plus two tenths of a pound, you will find the balance scale, if calibrated accurately, will be exactly level.  So this makes us believe kilograms and pounds are merely different measures of the same thing. Preposterous! Apples and artichokes. Nothing could be further from the truth. But recall the lesson of the ruler and exact measurements. We define things so as to make the definition work for us, and here on earth, for most purposes, and certainly for the purposes of this discussion, the two systems might as well be the same. For the record, mass and weight differ by an acceleration factor, but that is, for the moment, of no consequence; thus, we will use the term Center of Gravity.

Find a piece of string and tie a slip knot in one end. Place this around your straw or stick and move the straw toward the weighted end until you find the point at which it wants to balance. Then tighten the knot upon the straw, but not so tightly as to deform the straw.  Where you have tied the string is approximately the center of gravity of the straw and weight together.  It is not precisely so, but it works for our purposes.

Next time, you will need a few supplies. Cardboard from a cereal box will do nicely for our experimental fins, and you will need more tape and more string. If you have it, a wooden dowel would be helpful to create a more stable approximation of a rocket body, but if you can obtain three straws, they can be taped alongside each other to increase the strength and stiffness.

Au revoir!

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