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Posts: 6693
12/31/12 04:05 PM
Grand Poobah
DM - 10:37am Oct 16, 1998 PST (#118000 of 118003) That trick never works! The Fhysics of Football Lesson 1, part 2, The Spiral Now, grasshoppers, you remember at our last meeting, we were discussing how the laws of thermodynamics make it preferable to throw a football "nose first" in order to increase the down field distance of the throw. I promised that in this lesson that I would explain why making the ball spin (i.e. to throw a "spiral") helps that to happen. It all comes down to Newtonian mechanics. To those of you old enough (like Shores) to have been hanging out under that old apple tree, you remember Isaac putting forth these three laws governing the motions of matter: A body remains at rest or in uniform motion unless acted upon by a force. A body acted upon by a force moves in such a manner that the time rate of change of momentum equals the force. If two bodies exert forces on each other, these forces are equal in magnitude and opposite in direction It is really Newton's first two laws that are the most relevant to explaining why a spiral keeps the nose of the ball pointed down field. Again, to put these laws into something approximating plain English, Law 1 has to do with momentum. An object moving in a particular direction wants to keep moving in this direction. This is a good thing. If it weren't for momentum, a football might, for example, decide to suddenly reverse direction mid-flight, hitting the QB in the forehead, creating an unusual situation where the QB himself would be called for roughing the passer! The second law, for our purposes, can be distilled to the statement that momentum proportional to force. For our purposes, the important thing to remember, is that it takes force to change the momentum of an object. (It is a curious sidelight that sports announcers generally consider an object or team that is "moving" to have momentum, while one that is sitting still does not. In fact, both have momentum. Some momentum is just better than others). So now comes the part that is easy to understand, but hard to explain without using any of those fhysicscal drawings with lots of arrows and angles. So, I'm sorry, but you are going to have to use your imaginations here. Don't hurt yourselves. So...imagine a football. Still with me? Now, imagine a line drawn through the football from pointy end to pointy end. Now, imagine the ball spinning around that line (i.e. spinning just as it would if it were thrown in a perfect spiral). That line will be the axis of rotation of a football thrown in a perfect spiral. OK, now, sit down for a minute and have a drink, it gets harder from here. Here is the gist of the whole thing. As long as it is spinning, the ball wants to keep spinning exactly around that line. It doesn't want to deviate at all from that. Now of course, the ball doesn't "want" anything. It is an inanimate object, made of pigskin or plastic or some similar substance. When I say "want", I am using the familiar device of assigning human motivations on the ball, a practice commonly known in scientific circles as "wishful thinking". What I really mean, is that there are forces acting on the ball that tend to keep it spinning exactly around that line. It is in the understanding of these forces that the answer to the original question (why does a spiral work?) lies. The answer, grasshoppers, has to do with angular momentum. "What is angular momentum?" I hear you asking. Well, grasshoppers, angular momentum was invented in 1884 by a sadistic physics teacher in Camden NJ, who suffering from low self-esteem, wanted something to make his students feel stupid just when they thought they were getting the hang of regular momentum. As you will see, it still serves its purpose very well. OK, so, now that you are imagining that football spinning around a line, now imagine one single point on the surface of the football. Imagine the motion of that point. Stop whinning and take a Dramamine and continue to imagine while I finish here! Of course, those of you with an imagination, will see that that point will be traveling in a perfect circle around our aforementioned axis of rotation. So what kind of momentum does a point traveling in a circle have (all together now!)? "Angular momentum, Dr. Dim!" The point on the surface of the ball is not traveling in a straight line like Newtonian mechanics says it will, but rather is traveling in a circle. But it wants to travel in a straight line (there is that "wishful thinking" again). At any given instant, it would like to fly off the ball and travel in a straight line away from the ball. The force of momentum created by the spinning ball is in the direction of having each particle of the football fly off into space in a direction (here is an important part) perpendicular to the axis of rotation. Of course, these elementary football particles can't fly off because they are attached to all the other football particles, and some of these are on the opposite side of the ball where they want to fly off in the opposite direction. In a fit of pique at not being able to do what they want, these particles exert a force on the football as a whole which (here is comes) is of a direction and magnitude to keep the football pointed straight along the angle of rotation(!) "Man! you lost me there, Dr. Dim!" OK, let me try to say it another way. Since the angular momentum on each part of the football makes the direction of travel (and the momentum) of each part perpendicular to the axis of rotation (the well-known "centrifugal force; coming from the Latin centra, meaning 'from the center' and fugal, meaning 'going out there somewhere'), it would take a force to move these particles off this intended line of flight. Since each piece of football is attached to all the other pieces, the only way that they could deviate from their intended line of is to change the axis of rotation. SO, you'd need a force to change the axis of rotation. The orientation of the ball requiring the least force is for it to maintain its current axis of rotation. SO, unless you apply an additional force (a from a linebacker's hand, for example), the ball will maintain it's axis of rotation, and will stay pointed "nose first" as it flies down field. Still don't get it? Consider the alternative: If the ball is NOT spinning, then there is no centrifugal force, and thus there is no particular momentum keeping the ball pointed straight down field. "So you are beginning to bore me here! Is that it?" you ask. Yes, that is it. OK, so now you with understand why making the ball spin keeps it pointed in the direction of travel, or, you've sworn off ever reading one of these things again. NEXT TIME: Why some QB's have to get more "air under the ball" than others.
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