### Plank resonance

What you can do here
1. Walk over the plank a few times, putting your feet down in a different rhythm each time. What can you see?
2. Stand in the middle of the plank and gently bounce up and down changing your pace every now and then. What can you see?
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 happens?
4. Repeat the above but try not to make synchronised jumps. What can you see now?

How it works
Jumping on a plank makes it oscillate. If the frequency of jumps matches the natural frequency of the plank, the amplitude is large and you jump quite high. The jump 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 the jumpers. The jumps will be highest if they are synchronised and match the natural frequency of the plank.

Interesting facts
Bridge engineers need to bear resonance in mind. On 7 November 1940 in Tacoma (USA), the 850-metre-long (2,800-feet-long) the main span of a suspension bridge collapsed just four months after it first opened. The strong wind of 64 km per hour (40 miles per hour) made it vibrate and destroyed it. The event was filmed and it may be easily found on the Internet.