In fact, there are a whole set of frequencies. At the right frequency, waves travelling down the string will interfere constructively with each other and with waves travelling in the opposite direction. The toothbrush can be connected to a small potentiometer to reduce the dc voltage and hence the frequency of the motor, allowing the tuning of the standing waves with a fixed tension. One can set up standing waves on a string which is fixed at both ends by vibrating it at just the right frequency. ![]() We have found up to five harmonics with a 150-cm cord. 2) may be tuned by changing the tension of the cord while keeping its length constant. As the toothbrush vibrates at a fixed frequency, standing waves (see Fig. ![]() The toothbrush unit is placed near one end of the cord, which is wrapped once around the rod. Elastic cords that come with some school notebooks work well for this experiment, but a longer cord allows the observation of a larger number of modes. The ends of the cord are attached to stands. 1, which shows our Braun Oral-B®) to which a brush is normally affixed. Most suitable for this experiment is the kind of unit that has a metal rod protruding from the front (see Fig. ![]() electric toothbrush to produce a regular sideways motion that can be easily transmitted to an elastic cord. We have come up with a very simple and cheap way of exciting stationary waves in a string, which anyone, particularly children, can try at home. There are a number of ways of exciting standing waves in ropes and springs using non-commercial vibrators such as loudspeakers,1 jigsaws,2 motors,3 or a simple tuning fork,4 including the rhythmical shaking of a handheld Slinky.
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