Saturday Science: Falling Footballs
Throwing a football is HAAAARD. I mean, I can’t do it, and I can do plenty of other things, and that must mean that it’s like super hard.
Or maybe not. Maybe it’s just not my thing. It’s entirely possible that you, dear Saturday Scientist, are fully able to throw a perfect spiral with hardly any effort. Good for you! It’s by far the best way to throw a pass in football and, believe it or not, there’s a scientific reason for that. Let me pose a question: why is it better to throw a spiral than to toss the football without a spin?
There’s a decent chance you said that the spin on the spiral makes it go further than it would if it was moving without a spin. There’s a pretty simple experiment we can do to test out this hypothesis. I call it the Tic Tac Test.
- Tic Tacs
- A nice smooth table
- Eyeballs. Not, like, disembodied ones. Just the ones in your head so you can watch the test.
- Open up your Tic Tacs. Have a couple. I can smell your breath from here. Gross.
- Take out a single Tic Tac and hold it in your hand above the table a couple of feet up.
- Drop that Tic Tac! Watch it bounce.
- Do you notice anything weird? Any time when it bounces higher or lower?
- Pull out a second Tic Tac and try again, just to make sure it’s consistent. Still doing the same thing?
- Why do you think that some bounces were higher than others? Can anything else you observed help explain that?
- If you have a football handy, try the same thing with it on some nice smooth ground. After all, we used the Tic Tacs because they have an oblong shape just like a football.
Unless your Tic Tacs were all sorts of messed up, you probably noticed that some bounces were higher than others. If you watch football, you may have seen this very thing happen on the field. Maybe there’s a fumble, and the ball starts moving unpredictably across the field, bouncing erratically, some high and some low, some short and some long.
Now, this isn’t like a round object, like a super ball or a basketball. When you drop something that’s more or less spherical, each bounce is lower than the one before it. The key here is in the shapes of each object. When you drop a basketball, it’s not spinning, and it’s got a smooth, regular shape that will generally maintain that lack of spin as it bounces up and down. When you drop an oblong, like a Tic Tac or a football, even a slight change in the angle it hits the ground can send it spinning, and a different change in the next bounce can cancel out that spin.
And now your observation: the highest bounces were the non-spinning bounces. When your Tic Tacs hit the right way to start spinning, they didn’t bounce as high. Why not? Well, it’s all about energy.
See, when you dropped the Tic Tac it had a finite amount of energy, and any motion that it goes through can only use that much energy. If it hits at the right angle to bounce without spinning, most of the energy of the bounce is used in getting it up. If it hits at the right angle to start spinning, then some of that bounce energy has to go into the spin and whatever’s left over goes into the bounce, making a spinning bounce a lower bounce.
Here’s the technical breakdown: a Tic Tac held up in the air has gravitational potential energy. When you drop it, that potential energy converts to kinetic energy. A non-spin bounce contains almost pure linear kinetic energy going back up, while a spinning bounce has to split the same amount of initial potential energy between linear kinetic energy (going up) and rotational kinetic energy (spinning).
What this means is that our initial hypothesis up there is wrong! Throwing a spiral will actually use some of the energy of the throw to make the ball spin, which means it will travel a smaller distance than a ball thrown without a spin! So why the spiral? Well, it makes for a smoother flight. The spiral spin gives the ball something called angular momentum, and when the football has that it will basically keep the same position throughout its flight. If it’s thrown with its nose facing a bit up into the sky, as long as its spinning, it’ll pretty much stay that way until it hits the ground (or is caught by a receiver).
That means the flight is more predictable, which means that it’s easier both for the quarterback to aim a spiraled pass and it’s easier for a receiver to catch it. See, the spiral is important…But for a really, really weird reason.