## A little physics with your football

The gyroscopically-stabilized lifting body, spiraling through the air, is stopped by high-friction material on the inside of a glove. A massive body begins propelling itself down the field at high speed until a second body, traveling on another vector, collides with an impact force of more than 700 newtons.

The point of impact is below the centre of mass of the first body, causing it to rotate about that point, changing its orientation from vertical to horizontal. The momentum of the combined masses carries the two bodies on a ballistic curve towards the ground, where friction with artificial turf changes their kinetic energy into heat, bringing the entire system to rest. First down.

Since my home team is not in the Grey Cup this weekend, I can still be entertained by watching all the principles of physics playing out on the field. From passes and kicks to runs and tackles, Newton himself would enjoy his famous laws of motion manifesting themselves in such an entertaining way.

When the quarterback throws the ball, his fingers grip the laces to impart a spin for that perfect spiral toss. Any spinning object becomes a gyroscope, which has the tendency to keep itself pointed in the same direction, which, in the case of the football, is pointy end forward.

The shape of the ball cuts through the air more effectively than a sphere, reducing drag from the air, but it also develops a bit of lift, like the wing of an airplane. If the ball is thrown with the point slightly up, called the angle of attack, the air striking the underside of the ball will be forced downward, producing an opposite reaction, lifting the ball up.

At the same time, the air passing over the topside follows the curve of the ball and, thanks to the Bernoulli principle, speeds up, creating low pressure. The combination of higher pressure on the underside and lower pressure above produces the same kind of lift that keeps jumbo jets from falling out of the sky.

The lift produced by a football is far less than an airplane wing, but it does help the ball hang in the air a little longer.

While a football can be thrown farther than a soccer ball, because of its elongated shape, it won’t go as far as a round ball when it’s kicked. Watch a field goal and you will see the football tumbling end over end, which creates a lot of aerodynamic drag. Soccer kicks, on the other hand, seem to fly from one end of the field to the other. Hmm, I guess that’s why round balls are more appropriate for a game played with the feet rather than hands.

Tackling is a great demonstration of a transfer of momentum between two moving bodies. Every object has a point called the centre of mass, sometimes called the centre of gravity. Watch a tumbling gymnast, high diver or acrobat on a trampoline and you will see that their body always rotates around a point just below the rib cage. That point follows the laws of motion, while the rest of the system orbits around it. If you want to bring down a runner, hit above or below that centre of mass and the body will rotate around like a propeller around the hub. So the most successful tackles are either high or low on the body. Hit a runner in the middle and you might change their direction, but they could remain upright and get away.

Finally, there is the equipment designed to reduce the force of impact when large men run into each other. The amount of force produced by a collision depends on three elements: mass, speed and time. The shorter the time of an impact, or the less time it takes to come to a stop, the greater the force.

If your head stops too quickly, your brain continues like a crash test dummy inside your head and hits the inside of your skull. The result is a concussion. Helmets and shoulder pads provide cushioning, which extends that collision time just enough to reduce the force of the impact.

Of course, European and Australian rugby players who butt bare heads together laugh at the padded equipment used in North American football, but frankly, I’d prefer to have an intact brain.

Then there is the piece of equipment that has more to do with biology than physics, the tight spandex pants. They’re supposed to make it more difficult for an opponent to grab onto, but they’re really meant to show off all those tight butts.

Enjoy the game, whatever you see.

— Bob McDonald