Demo Details: Glasses & Stick
How Do We Do It?
The sticks we use are cut on a table saw from a cedar board to a half inch square four feet in length. We tried other wood and I’m sure there may be more good choices, but the sticks need to break clean and we found cedar broke in such manner almost every time. Pine often splinters sideways almost always upsetting and/or breaking the glasses. It is hard to find a clear cedar board so when I’m at HomeDepot or other such stores I check their eight foot cedar boards for a near clear and straight specimen. After cutting sort through all the sticks and throw out any that have a flaw (most often a knot) unless it happens to be at the very center. If you have a weak spot not at the center the stick is likely to break off center (or in two places) which will most likely cause the demonstration to fail.
The wine glasses can be quite fragile, but should not have a flared lip nor should the lip be further out from the center than the outer edge of the foot. In the video the shock wave did reach the glass on the left just as the stick was lifting (if it had not been lifting the glass would have been broken) and the absence of a flared lip and proper foot gave us a little tolerance. However, if done properly there is absolutely no stress on the glasses other then the dead weight of the stick. Jack and I have toyed with the idea of having people from the audience come up and put a finger under each stick and then do the demonstration. However, it would be tough for someone to not jump back as you swing the striker as hard as you can.
Our striker is a hardwood handle thirty inches long and one and an eighth inches in diameter. We drilled a hole through one end for a cord tied in a loop that goes around our wrist as a safety precaution. Timidity in swinging the striker is of no use the faster the better. We did some guess work on where the striker needs to be in order for the stick to be broken so rotation could begin and we thought maybe a quarter of the way through. If so, the striker would need to be going around thirty-five miles per hour. Data on pro baseball players show that the end of their bats often exceed sixty miles per hour so it’s not to hard to believe that the end of our thirty inch striker can be going thirty-five miles per hour or faster. At the moment of impact the striker needs to be level to prevent a forward or backward vector to the force applied to the stick. The stick won’t fly off, but it will often tip over one or both glasses. Not impressive!! A little practice is needed