Resonating Rods

Here is a simple physics demonstration that will cause an uproar on what the principle behind this demonstration is. Well here is the explanation that can help you understand and videos that will give you a better outlook!!

Materials

-Aluminum ROD

-Violin Rosin

PROCEDURE

1. Hold the rod in the middle with your first and second fingers on top and the thumb supporting the rod from below. The key is to make as little contact with the rod as possible so as not to dampen the vibrations.

2. Apply some rosin to the thumb and fingers of your other hand and lightly coat the aluminum rod with rosin.

3. Use your rosin coated fingers to pinch the metal while sliding your fingers from the middle to the end of the rod. Don’t stop. As soon as your fingers reach the end, repeat this pinch-n-slide process...and again...and again...maybe upwards of 20 times before the metal bar begins to resonate. Don’t give up.

4. Because of the sticky nature of rosin, your fingers should stick and slide across the rod causing it to resonate.

5. The sound of the vibrations will be soft at first but will strengthen with each successive stroke. The resulting high pitch sound will be ear-piercing! Each stroke reinforces the vibrations of the last. Pinch and slide, pinch and slide, pinch and slide...don’t give up!

6. Try holding the bar in the middle and tapping on the sides. Compare this sound to the sound produced by hitting the bar directly on the end. How does this sound compare to the sound made by stroking the bar?

WHAT IS THE IDEA BEHIND IT?

In terms of making the bar vibrate, the rosin is responsible for making your fingers stick and slide as they move across the bar. In turn, this repeated stick and slide action sets up vibrations in the bar. This is caused due to ~natural frequency~. since the rosin and the rod contain the same natural frequency, the vibrations starts to create a high pitched noise.

You probably noticed that holding the bar in the middle and tapping it on the sides produced a lower pitch sound and striking the bar on the very end created a higher pitch sound. The same high pitch sound is also made by stroking the bar with your fingers. In either case, the high pitch sound resulted from the formation of compression waves or longitudinal waves throughout the bar. Each successive stroke of the bar reinforces the strength of the previously established longitudinal wave, resulting in a louder sound.

Here are pictures of the Materials....

aND HERE IS A DEMONSTRATION ON HOW IT IS DONE....