Resonant Swings

Let two persons sit on the swings. Set one of the swings into motion. After a while, the second swing, at rest, begins to oscillate picking up speed during each oscillation. At the same time the first swing slows down and comes to rest. Then, it again begins to oscillate while the second swing gradually loses speed and comes to a halt. The cycle continues.

The phenomenon is due to resonance.

In this exhibit we have two swings suspended from a common rod. This is not unusual since most amusement parks have two or more swing suspended from a common support. But , what  is unusual in the exhibit here is that, if you set one of the swings oscillating, the other swing also oscillates – without anyone pushing it. This does not happen in the usual amusement park swing. Why is it so ?

This is due to the manner in which the rod supporting the swings is held. The supporting rod in the amusement parks is fixed and rigid. However, in the exhibit here, the rod itself is suspended and hence, is free to move back and forth when one of the swings oscillates. The back and forth motion of the rod transfers the energy of motion of one swing to the other at rest. This sets the second swing into motion. As the second swing gains energy, its oscillations cover greater distances from its rest position. Meanwhile, the first swing gradually comes to rest. Now, the transfer of energy from the second swing to the first begins. This goes on until the second swing comes to rest. And, the cycle of transfer of energy between the two swings continues.

The scientific phenomenon behind the transfer of energy is the one known as Resonance. It is one of the most common phenomena occurring in nature. There is whole lot of diverse effects that resonance brings about. The colour of an object, sound produced by musical instruments; tall structures collapsing during earthquakes, sensing sound and light are some of them.

If a swing is pulled to one extreme and released without pushing, it makes a certain number of oscillations in a unit time, say one minute. This number is a constant for a given swing and is known as its natural frequency or Resonance frequency. The natural frequency of a swing depends on its length. Longer the swing, lower the natural frequency. Suppose we have more than one object with same resonance frequency. If we have a medium connecting those object, the motion of one of them brings about the motion of the others. This is resonance. The transfer of  energy then is complete and maximum. On the other hand, if we have objects at different natural frequencies, then the transfer of energy is less than maximum. That is the reason why a cavalry marching on a suspension bridge is asked to break steps. If all of them marched in unison and if the frequency of the steps matches with the natural frequency of the bridge, a calamity is not too far. With every step, the bridge executes an up-down motion with increasing amplitude that could lead to the collapse of the bridge itself. Sometimes resonance can be troublesome.