A homemade string shooter and slow moving waves in rope

This string shooter uses two wheels on motors to push a string forward while a tube guides the string back through the wheels, creating a constant loop that appears to defy gravity and demonstrates wave phenomena.

This is one of science teacher Bruce Yeany‘s home made demonstrations from years of teaching in his central Pennsylvania classroom. He notes: “I have found that designing and building my own equipment has taught me more than any course that I have ever taken.”

“The idea for this device came from a baseball pitching machine. The idea is that the two wheels will turn, and that will push the string forward. Since the string is a continuous loop, the two wheels throw the string out and it feeds back in on itself. There’s a little plastic tube that guides it back into place in between the wheels.

“First thing to notice that once the machine is put into motion, if you tap on the string, waves will move away from the device on both the upper string and the lower side of the string.”

Related oscillation: His Slow Moving Waves in Rope demo.

“The speed of waves through a stationary rope can vary due to the tension and the density of the material. What happens when the rope itself is put into motion and then a wave disturbance is added. This idea is the start of our investigation using a few different loops of ribbon and rope. The ropes are made into loops by melting and joining the two ends together and then hang limply until put into motion. The behavior of the rope as it hangs stationary and limp versus when it is put into motion is quite remarkable.

“The closest analogy that might help to understand what is going on would be to imagine throwing rock in a moving stream or river. The waves traveling upstream would slow down and if the stream would be fast enough, the waves wouldn’t be able to move in that direction at all. The waves headed downstream would move very fast since both speeds would be in the same direction.”

Related reading: What is a wave?