Physics in Action: The Giant Nose-Basher

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I'm standing next to this 13-pound bowling ball, hanging from an 80-foot cable here at the Elliot Hall of Music at Purdue University, and I'm going to use this bowling ball to demonstrate what is arguably the most fundamental principle in all of physics, which is energy conservation. It took physicists much of the 19^th Century to discover this principle and this principle is the most fundamental principle guiding all processes that occur physics.
Now, this is a real bowling ball. If I pick it up and bowl it, you're can see it's going to knock down the pins. Now, at the bottom here, when the ball is at rest, it has no kinetic energy, because it's not moving. It also has no potential energy, because it is at the lowest point that it can possibly be. But what I'm going to do is pickup this ball, carry it all the way over to the other side of the stage and, in the process, lift it off the ground and give it potential energy. As the ball is released, it's going to require some combination of potential and kinetic energy, and the principle of energy conservation is that the sum of the potential energy and the kinetic energy is always a constant. In particular, the ball will not acquire any more energy as it sweeps across the stage and back, so, if I release it, it's going to come back to exactly where it was. And if I release it from my nose, it'll come back to my nose, but not smash my nose. Let's see what happens when we really do this.
At this point over here, the ball has acquired a very large amount, for this 13-pound ball, of potential energy, because it's high off the ground. It has no kinetic energy. When I release the ball, it's going to sweep along the floor and have some kind of combination of potential and kinetic energy, but always the sum of the two will be the same. When the ball passes the midpoint, it will have no potential energy, only kinetic energy, and at either end, at the far end and when it comes back over here, it will have only potential energy. The point of energy conservation is the total energy is always the same. Now, let's go through this demonstration. If energy conservation holds, the ball will be released, come back to this point and I will be okay. So, here we go.
The ball touches my nose. On the mark, go! I've released the ball. Now, if energy conservation's okay, my nose will be okay, too.
There it was. The ball came back almost exactly to the point where I released it. That's a demonstration of energy conservation. It's the classic demonstration of energy conservation, the most fundamental principle in physics.
Well, is this really what happened? Let's take a closer, behind the scenes look at what really happened in this demonstration. Okay, it has not come back, because of air resistance and we know that. So, we're going to try to pretend that it actually did come back by cheating. While the camera is not looking at me, while we really do it, I'm going to step forward to the blue line. Let's see what happens. I'm stepping forward, where I hope the ball will - back up a little bit, I'm told. I'm going to catch it. Not quite, so let's do again.
Let me now show you what you would have really seen, had we done this demonstration without the benefit of camera tricks. I'm going to move back to my green mark and carryout the demonstration exactly as before. I hold the ball to my nose, I release it, I don't move, I let it go. The ball is coming back. Note the ball stopped. I had to reach out to get the ball. It did not come anywhere near my nose. Does that mean that energy conservation really doesn't work? Not at all. In the motion of the ball across the stage and back, there are at least two sources of friction: one is the friction between the ball and the air, sometimes called drag, another is a source of friction between the top of the cable and whatever holds the cable to the Hall of Music. If we do not recognize those sources of energy loss, then it looks like energy conservation does not hold, but it really does. And we can demonstrate this by redoing this demonstration in an environment where the ball swings over a very short distance, where air friction does not play a major role, and we can show you that energy conservation holds exactly, without the benefit of any camera tricks.
Now, we're going to do this demonstration for real, no cheating. What you're going to see is exactly what's supposed to happen. For one thing, the bowling ball is going to move over a much shorter distance, so air resistance won't be as much of a problem, neither will friction at the top of the cable. But now, because it is a real demonstration, we have to be very, very much more careful. And I'm warning all those of you who are watching this demo, listen carefully to what I have to say. For one thing, you want to make sure that you head doesn't accidentally lean forward. My head is pressing against something, keeping my head in place. For another, and this is very important, you have to release the ball without giving it an accidental push forward. So I'm going to hold this with just my fingers to prevent my thumbs from pushing it forward. If we do everything right, this will be a real demonstration and we hope it will work. And also, we hope it won't hurt me. Here goes. On my mark, set, go!
Voile! It really does work.
Preliminaries
Conservation of Energy
Physics in Action: The Giant Nose-Basher Page [1 of 2]