Plank resonance

What can you do here
  1. Walk over the plank a few times, every time putting your feet in a different rhythm. What are you noticing?
  2. Stand in the middle of the plank and gently bob up and down, changing the pace of your jumps every now and then. What are you noticing?
  3. Stand in the middle of the plank with someone else. Gently jump in a synchronised way, changing the pace once in a while. Try to jump simultaneously. What is happening
  4. Repeat the above but try not to make synchronised jumps. What are you noticing now? 

How does it work
Hopping on a plank causes it to oscillate. If the frequency of jumps matches the natural frequency of the plank, the amplitude is large and we jump quite high. The jumping frequency resulting in the largest amplitude is called the resonance frequency. If there is more than one person on the plank, then the amplitude depends on all jumpers. The jumps will be highest if they are synchronised and match the natural frequency of the plank.

Why is this happening
Bridge engineers need to bear resonance in mind. On 7 November 1940 in Tacoma, Washington the 840-metre-long main span of a suspension bridge collapsed just four months after its opening. Gusty winds of about 60 km per hour (40 miles per hour) made it vibrate and destroyed it. Curiously, the event was captured on film, which can be easily found on the Internet.