Build Notes: A Wave You Can Watch
1Every Tuesday evening, the Riverside Commons Makerspace opens its workbenches to teens. This month, eighth graders Anselm Brandvold and Noemi Achike are building a wave machine for the town's children's museum, with help from their mentor, Mr. Thaddeus Lorne. The machine is simple: a strip of wide tape 2.0 m long is stretched tight between two clamps, and 100 wooden skewers are pressed across it, one every 2 cm. When the first skewer is twisted, the next skewer is pulled along a moment later, then the next, so a twisting wave travels down the line. Each skewer only swings up and down around the tape; what travels from one end to the other is the wave and its energy.
2A small motor with a crank arm twists the first skewer back and forth. A knob sets the frequency, the number of back-and-forth swings each second, in hertz (Hz). The length of the crank arm sets the amplitude, how far the tip of the first skewer swings from its resting position, in centimeters (cm). The team built two machines that are the same except for one thing: on machine B, a small ball of clay is pressed onto both ends of every skewer.
3To measure wavelength, Noemi filmed each machine in front of a wall marked in centimeters and measured the distance from one crest to the next. Anselm also timed single waves on machine A. A small swing of 2 cm and a large swing of 6 cm each took 1.25 s to travel the 2.0 m from the motor end to the far end.
4To measure energy, Mr. Lorne set up machine A so that its last skewer pushes a paddle on a toy motor that runs backward as a tiny generator. A meter adds up the energy that reaches the far end over 10 seconds, in millijoules (mJ). The team ran the motor at 2 Hz for every energy test and changed only the crank arm, and so the amplitude (bar graph).
5For the museum, the far end of machine A will tap a small bell. The bell rings clearly only when 30 mJ or more reaches the far end every 10 seconds.