Today, a look at rifling. The University of Houston's College of Engineering
presents this series about the machines that make our civilization
run, and the people whose ingenuity created them.
Football season is here again. This time, I grow more
curious about rifling. We've all watched that long-lens shot on TV: A football
comes toward both us and the downfield receiver. It spins like a top, even in
slow motion. Steady as a rock, it moves toward us at about fifty miles an hour.
So, exactly what do quarterbacks gain by rifling their passes like that?
For one thing, they temporarily minimize drag on the football by making it go
end-first -- by presenting the least cross-section to the air. Long narrow
projectiles will naturally tumble as they fly through air. Tumbling increases
drag and ruins accuracy.
That's no problem for a knife-thrower. His dense knife travels a short distance.
He intentionally makes it tumble end-over-end an exact number of times. He's
learned to make the knife rotate, say, twice before it reaches the target point first
When we study the physics of rotation, we find something surprising about footballs
and knives. If a rotating body is disturbed while it's in motion, it'll find a way
to rotate about the axis that gives the greatest moment of inertia. The knife starts
out that way. The football does not, but it'll eventually get there too. Its most
natural rotation would also be tumbling.
If a football could fly far enough, it would flip over and tumble, and it sometimes
does. When a quarterback gives the ball a strong spin, its motion is held gyroscopically
for a while. With luck and skill, it stays so until it reaches a receiver.
If you've ever used a bow and arrow, you may've noticed the arrow wobbling slightly
after its rotating feathers brush your fingers. But its strong rotation soon stabilizes
the wobble so the arrow can fly true. Same story for a bullet: It sometimes wobbles
as its stern clears the barrel, then it straightens out.
The person throwing a football lays fingers on the strings and pulls on them during
the throw. If the ball gets a quarter turn in a four-foot swing, it then keeps rotating
at around 300 rpm. Compare that with a target-rifle bullet: One turn per foot is typical
for gun barrel rifling. So a bullet leaving at 3000 feet-per-second rotates a lot
faster than a football -- maybe 180,000 rpm. (Bullets have been known to tear apart
under huge centrifugal forces before they hit anything. That's rare, but it happens.)
Bullets travel a lot further than footballs, and they have a longer, narrower, more
unstable shape. Now and then, after traveling, say, six hundred yards, bullets also
occasionally flip and rotate end-over-end. And they go far wide of the mark.
So, if I find myself watching football on TV this season, I'll look a lot more closely
at the passing game -- at that lovely arcing ball. I'll look for so many forgotten
details of its remarkable complex journey -- all the way from scrimmage into the distant
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
One may learn a great deal about the various projectiles discussed here by looking
at them individually on the web. I'm also grateful to Lewis Wheeler, UH Mech.
Engr. Dept., for his expertise on target shooting. The rifling question is also
addressed by Jearl Walker in his Flying Circus of Physics (New York: John
Wiley & Sons, Inc., 2007): pp.55-56.
For more on aspects of rotational momentum, see Episodes 1313
My thanks to the UH Sport Information Dept. for the photos used here. For more
photos, see: http://www.pbase.com/carboncopy/sports