Demonstrating InertiaDate: 10/10/95 at 22:3:51 From: Pat S Pattillo Subject: 5th Grade Physics To introduce myself, I am an instructional assistant at Leander High School in Leander, Texas. I have two children, Jack, an eighth grader, and Katie, a fifth grader. Katie's fifth grade class was given this hypothetical problem: A hole is bored through the exact center of the earth, from the North Pole to the South Pole, A mass is dropped into the hole in the exact center. WHAT HAPPENS NEXT? I supposed the mass would oscillate. I talked to physics teachers where I work who agreed with me. They, in turn, sent me to the calculus teacher who said this is a pretty common hypothetical problem but not especially for a class of fifth graders. The calculus teacher said there is a formula to figure this out. Do you know this formula? Can you e-mail it to me? Please.. Pat Pattillo Rt. 3 Box 166-A Leander, TX 78641 (512) 258-5585 Date: 10/11/95 at 13:6:13 From: Doctor Jonathan Subject: Re: 5th Grade Physics Mr. Pattillo: You are right: the object dropped will oscillate, and will eventually come to rest at the center of the earth (assuming air friction). The formula needed to figure this out is actually a differential equation, relating the forces due to gravity, inertia and air friction in terms of time and position. As such, this probably isn't a problem that fifth graders could solve analytically as opposed to just verbalizing an intuitive answer. I could go through the theory and solution of the differential equation mentioned above, or I could try to explain what would happen in intuitive terms at a level appropriate to a fifth grader. Either way or both is fine, and I'll be happy to provide you with as in depth an answer as I can, but I just wanted to know what exactly you were looking for first. -Doctor Jonathan, The Geometry Forum Date: 10/11/95 at 22:13:16 From: Pat S Pattillo Subject: Re: 5th Grade Physics "Dr. Math" Thank you very much for the prompt answer. Now to the meat of the problem. I'll leave to you the way you would like to explain the solution to this problem. Keeping in mind that these are 10 and 11 year old students. I just wanted some example to show to the kids to illustrate the complexity of the problem. It would be interesting to have a chart or equation to show them that gives a solution to the problem. I'm sure I couldn't solve a differential equation if you were sitting beside me and guiding my pencil. I'm indebted to you and anxiously awaiting your next message. Mr Pat Date: 10/17/95 at 16:7:48 From: Doctor Jonathan Subject: Re: 5th Grade Physics Mr. Pattillo: Sorry it has taken me so long to get back to you--we just had midterms. In order to understand this problem to any real degree, two concepts must be first understood: Force and Inertia. In this problem, the force is due to gravity, and the inertia is due to the mass of the object. The force due to gravity can be thought of as a rubber band stretching from the object to the center of the earth. Obviously, we're pretty far away from the center of the earth so the rubber band is really trying hard to pull the object toward the center of the earth. This feels like it's pushing "down" to us. So what would happen if we let go of the object over a hole that goes all the way through the center of the planet and out the other side? Well, the "rubber band" starts pulling the object "down" into the hole. As the rubber band pulls on it, the object races faster and faster toward the center. When it finally reaches the center, our rubber band is no longer stretched, and so it exerts no force on the object. However, the object doesn't just stop, and for the same reason that a car doesn't stop when you take your foot off the gas. This is called inertia. Inertia is the tendency of objects that are moving to want to keep moving. The only way to stop an object from moving is to push on it in the opposite direction. If someone is ice skating and he can't stop, you have to grab and pull him the opposite way so he doesn't crash into the wall. Because of inertia, our object doesn't just stop at the center of the planet, but instead keeps going "up" the other side of the earth. As it does so, it starts to stretch the rubber band the opposite way. Like the brakes on a car, the rubber band starts to slow the object down until it stops. At this point, the object is just about on the surface of the opposite side of the earth. Of course it then begins to "fall" back towards us, and the process repeats itself forever. If the kids are familiar with Sines and Cosines, the object's location will be a sinusoidal function of time. An experiment you can do to demonstrate this would be to suspect a heavy object between two vertically oriented springs. Stretch them pretty far so that real gravity doesn't really affect things and also so that they act linearly. (One spring will work, as the mass will find an equilibrium point where the spring and gravity cancel out, but it will be hard to explain this to 5th graders, and will also ruin the visual symmetry of the problem.) Where the object naturally rests can be thought of as the center of the earth. If you lift the object up, it will be pushed down by the two springs acting as gravity. Letting go, the mass will oscillate around the equilibrium point (center of the earth) as the above discussion predicted. To show that the motion is sinusoidal, you could attach a pen to the mass and move a piece of paper behind it at a constant speed. This might be awkward to do, but the resulting graph with be a pretty good sinusoid. Most likely the kids will figure out that eventually the mass will lose energy and come to rest back at the center. This is because we have neglected to mention the effects of air friction. Each oscillation, the object is slowed down by air friction. However, unlike the spring, which stored the objects energy and gave it back by sending the object speeding in the opposite direction, energy lost to air friction is lost forever and eventually takes away all of the object's energy, leaving it motionless at the center. Hopes this helps. If you feel any of this needs elucidation or would like me to come up with a differential equation, feel free to write. I can't really even give the equation for the solution of the differential equation, because it involves Sines and Exponentials, which I assume 5th graders haven't learned yet. -Doctor Jonathan, The Geometry Forum |
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