Resonance in a swimming pool
Jearl Walker www.flyingcircusofphysics.com
April 2016 In a small, backyard pool, you can splash all you want but the waves are only small. You can get a bigger wave with a belly-flop into the water, but that is only momentary. How can you can produce a sustained large amplitude wave? Here is how one father did it:
The father built up the wave by pumping on the water surface with the board, just as he could build up the swinging of a playground swing by pushing. In both cases, the pushing must be in step (in phase) with the oscillations, that is, in resonance. Although only a small amount of energy is added to the oscillations with each push, those small amounts add up until the oscillations are significant (and thus fun).