Each of the two hanging wood dowel rods has some additional weight on the bottom to give the rod more momentum. The weights are iron washers but they are not magnets as some people suspect. Think of a child sitting in a playground swing. The child wants to be pushed so that he or she is swinging in a big arc from front to back to front. It is difficult to give just one push that is strong enough so that the child immediately is swinging in this big arc. Usually the child is given a small push and swings out and back a short distance; and the top of the back swing the child is given another small push. The child swings out and back but the arc is slightly longer. Each time the child returns to the top of the back swing he or she is given another small push. Over time the cumulative effect of these small pushes is a child swinging in a large arc.
The exhibit consists of the two dowel rods hanging from a horizontal rod. This horizontal rod is actually one of three horizontal rods linked together by the two triangles into a fairly rigid frame. Draw back one of the hanging dowel rods and let it go. It starts swinging forward and backward. As it swings forward it is pulling forward on the horizontal rod; as it swings backward it is pulling backward on the horizontal rod. In other words it is making the horizontal rod swing. When the horizontal rod swings forward and backward it in turn pulls forward and backward the other hanging dowel rod.
The dowel rod you start swinging has energy. With each swing forward and backward it gives up a little bit of that energy to pull on the horizontal rod. But the horizontal rod in turn passes that energy on to the second dowel rod. The second rod begins swinging, but is always slightly behind the first. This process continues until all of the energy of the first rod is passed on to the second rod. At that point the process starts over again, with the second dowel rod giving its energy back to the first.
Consider the situation when the first rod has given up almost all of its energy. It is now swinging in a small arc. The second rod is swinging in a much larger arc but the swing of the first rod is still ahead of the swing of the second. Therefore the first rod continues to give up its energy to the second until it has none left.
There are several sources of energy loss in the exhibit. The swinging dowel rods are moving through the air and thus continually lose some energy to air resistance. There is also friction loss as the dowel rods rub on the horizontal rod, air resistance as the horizontal rods move, and friction loss as the triangles swing.
The coupled pendulums always swing back and forth at the same number of times per minute. The number of times the pendulum swings per minute is called its frequency. Start one of the pendulums swinging and count its frequency per minute. (If the pendulum does not swing for a full minute, count the number of swings in thirty seconds and multiply by two.) Now pull both pendulums back together and let them go at the same time. Count this different frequency. Finally, start the pendulums in equal but opposite directions and count this third frequency.
(CAN SOMEONE EXPLAIN SIGNIFICANCE OF THESE THREE DIFFERENT FREQUENCIES?)
Every object has a natural frequency. If small pushes are applied to it at the rate of its natural frequency then it will begin swinging. On 7 November 1940 the Tacoma Narrows Bridge collapsed. Only ten months old, the bridge was broken apart by extreme oscillations in the bridge deck. These oscillations were caused by wind that vibrated the bridge deck at its natural frequency. (As of 19 November 1998 a web site on the Tacoma Narrows Bridge disaster could be found at http://www.nctrc.plattsburgh.edu/secondaryed/second/Kent/Kent.html.
To avoid the chance of vibrating bridges at their natural frequencies, when the army and other military marching units reach a bridge they intentionally stop marching and walk as individuals across the bridge.
During earthquakes, the ground moves from side to side. This ground motion in turn makes buildings move back and forth. If the frequency of the earth movements matches the natural frequency of the buildings then the buildings will gradually increase their sway from side to side until they collapse.
Tall buildings also have natural frequencies. Architects try to design these buildings to have natural frequencies unlikely to be matched by gusts of wind.