Magic Pendulum Catch - Sick Science!
Have you ever seen a comedy bit from a black and white silent film? One of their favorite gags was someone hoisting a piano to a 3rd or 4th story window when someone cuts the string... CRASH! The piano comes crashing down as ivory keys and wood splinters go everywhere. The Magic Hex Nut Pendulum involves the same sort of thing, only on a much smaller scale. What will happen to your hex nuts? The result may surprise you.
- 15 hex nuts
- Shoe string or string/yarn of similar length
- Safety glasses
- To get started, thread your string through 14 of the 15 hex nuts.
- Take the end of the string you just threaded through the hex nuts and tie it back onto the string right above the stack of hex nuts. Basically, you are making a loop of hex nuts.
- Thread the string through the middle of the remaining hex nut and tie the string so that you have a string with 14 hex nuts at one end and one hex nut at the other.
- With the string-hex nut apparatus you have constructed, grab the single hex nut end with one hand and hang the string over your opposite hand's index finger.
- Pull the single hex nut end of the string so that the 14 hex nuts are touching your index finger. Make sure that the string is parallel or close to parallel with the ground.
- From this position, let go of the string. Be sure to keep your index finger as still as possible. Oh no! This is going to be really loud! Wait... the hex nuts didn't hit the ground. What happened? You better try it again to make sure that it wasn't a fluke.
How Does It Work?
The apparatus that you've constructed out of some string and hex nuts is a pendulum. A pendulum is a weight suspended from a pivot (or fixed point) so that it can swing freely, back and forth. Common examples of pendulums can be found in time pieces such as grandfather clocks.
Pendulums like the one you constructed operate using acceleration from gravity. When you release the hex nut, gravity accelerates it towards the ground, giving it velocity. In a normal pendulum, the velocity decreases as the pendulum swings. The amplitude (how high the pendulum swings) also decreases the more the pendulum swings. This happens because of friction.
In our pendulum, the distance between the pivot (your finger) and the bob (the single hex nut) is decreased very rapidly when you release the string. As the distance between the bob and pivot decreases, the velocity of the pendulum increases. With the velocity increasing so rapidly, its amplitude is increased to a point that it makes a number of full swings, wrapping the string around your finger. The scientific explanation as to why the Magic Hex Nut Pendulum works is this: as the length of the pendulum decreases, the velocity increases, thus increasing the amplitude.
Want to spend some more time experimenting with Magic Hex Nut Pendulums? Here are a couple of ideas:
- Try dropping the pendulum from different heights. Is there a point, either too high or too low, that the magic of your pendulum no longer works? Is there a height that works better than in our original experiment?
- How many hex nuts can you add to the "bob" side of the pendulum and still have it work? How few can you have on the heavier side?
- I like this version Review by A. Bottacin
I've done this trick with various elementary school classes using a wine glass in place of the bunch of hex nuts and a key in place of the single hex nut. At first glance,they think the glass will fall and crash into "smithereens" but this does not happen (a discrepant event). I ask for predictions and after the trick, observations/explanations. It always amazes.
(Posted on December 16, 2010)
- 3 reasons the pendulum stops falling... Review by Jason Giddings
Acceleration due to gravity
Conservation of angular momentum
The smaller weight is forced to fallow a continuously smaller arc. As the distance to the center is reduced the mass must accelerate much like a spinning skater who pulls in her arms to spin faster.
Coefficient of dynamic friction
As the string begins to wrap the finger the surface area of the string that is in contact with the finger increases along with the resistance due to friction. The resistance due to friction increases until it reaches equilibrium with the acceleration due to gravity.
(Posted on December 16, 2010)