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The Secret of Oscillations
by Thomas Oestereich
Have you ever seen a pendulum swing? The pendulum of a grandfather's clock, maybe? Or of a cuckoo clock?
Keep your eyes at the weight. No, don't get hypnotized. It swings left-and-up, it stalls, halts, reverses, goes down and speeds up, with maximum speed it then passes the lowest point, goes right-and-up, stalls, ... and so on.
What is happening from the perspective of energy?
At its lowest point, the pendulum has the highest speed. A mass moving at any velocity has some movement energy, called "kinetic energy". Again an ancient Greek word. But where is the energy going when the pendulum gets slower, finally halts?
Yes, you are right. Because it has risen in position, the pendulum has acquired the possibility to fall down again. It has acquired energy of position, oftentimes called "potential energy". This happened because it moved against the attractive force of the earth, its momentum lifted it against its weight. The weight slowed it down and finally accelerated it in the other direction.
The interesting point here is that the energy changes form without any incentive, so to speak. Normally energy only flows in or out or anywhere when and if something called "entropy" rises along the way. But here the energy goes from kinetic energy to energy of position and back to kinetic energy and... and so on. Permanently? Well, not really, because the joint of the pendulum is not completely free of friction, and because, at least in the case of an oldfashioned clock, the pendulum swings in air which as well causes "dissipation" of energy. It's a damped oscillation. That's why the clock has to push the pendulum at every swing a little bit to make up for the damping.
But if you have seen a really great pendulum like let's say the one in the Griffith Observatory near LA, a pendulum a couple of storys high with a really heavy weight at its end, then you know that friction can be practically non-existent. Once moving, that pendulum will swing for days without any push.
So there really is a special process here, energy swings back and forth between two states, and the loss of energy out of the system is not the reason for this constant transformation (because it can be made arbitrarily small without affecting the main process). So it is justified to regard the entire pendulum as just ONE system containing energy, the more so as you cannot only reduce the kinetic energy or only the potential energy.
Why is this so interesting? Because this same principle is the basis for so many beauties of nature of which we are part. Just think of waves in a pond into which you have thrown a stone. Water is going up and down. There is an additional element though: The water waves move ahead - what the clock (hopefully) doesn't do. Another example: Light, light is an electromagnetic "pendulum", the energy swings from a magnetic field into an electric field and back. And light waves have the same property, they swing while propagating. And the entire swinging system is just one entity which keeps a certain amount of energy.
So the secret of oscillations and waves is that energy swings in them between different states, kinetic and potential, magnetic field and electric field, back and forth.
Why is this property of oscillations so special that it is worth mentioning at all? Well - wouldn't it be great if energy in the state of fuel, which we transform into the state of movement of our car, at the end of the day came back as fuel again? But nay, here energy is transforming due to an "incentive", namely the production of entropy. So you wouldn't expect the fuel to come back into the tank by itself, would you?
Oscillations ARE special.
For more thoughts and ressources have a look here:
http://physics.global-momentum.net
Regards Thomas Oestereich
After 20 years of being a physicist, Dr. Thomas Oestereich turned from scientific research to a life as an author and editor of his home school curriculum. He now seeks to help his readers gain access to the insights of physics. Bringing the achievements of science to a larger public, he hopes to finally contribute to a better knowledge of the options and constraints of decision making in our democratic society.
http://www.physics.global-momentum.net
Contact the author, Thomas Oestereich
, at speed-up@gmx.net
.