If you look up how to do a spiral pass on the Internet —
which is something I had to do for this video
because I really suck at throwing footballs —
the main thing you'll see is that you need to position your fingers
in a way that'll make the ball spin as it flies through the air.
I was breaking my arm trying to do this
so I could see a physics problem up close.
And the problem is right... here.
When you throw a good spiral pass, the ball starts pointed up
tips over mid-air and ends pointed down.
We're used to seeing a football do this.
But according to physics, that's highly unusual.
When we throw objects, they are usually
going to either retain the same angle as when they originally thrown
like this loaf of bread.
Or flip over themselves several times until they land.
Like this box of girl scout cookies.
So what is it about a football that makes it want to tip over?
I first got interested in this problem, I'm guessing 2000.
We published the paper in 2020, so it really was 20 years.
My name is Timothy Gay.
I'm an atomic physicist by day
and my secret identity is football physicist at night.
The best way to understand why the ball turns over
is to make an analogy with a spinning top.
Spinning tops are like catnip for a physicist.
On its own, a top falls over.
Give it top a spin and it stands up.
It balances itself around a line
a center point that physicists call the reference axis.
This axis is determined by the main force acting on the object
which in this case is gravity.
Gravity is pulling straight down
and the top is fighting this by spinning around
wobbling around this vertical line, this reference axis.
A football is a top.
You can spin it in the exact same way
and this is in part because of its shape.
Both a football and a top are cylindrically symmetrical.
A cylindrically symmetric object is one which if I rotate
you can't tell that I'm turning it.
Objects which have this symmetry have a big impact
on how the object behaves when I apply forces to it.
So when you spin them...
they both wobble around a reference axis.
Bending makes all the difference.
But once you spin football in the air
it's reference axis gets a little more complicated.
When you throw a nice spiral pass
gravity is only acting as a secondary force on the ball.
Gravity is trying to pull the ball down
usually making its path to the ground look like this.
But the main force acting against the ball
is the air molecules that are pushing against it
as it flies through the air.
The ball pushes against the air and the air pushes back against it.
But because gravity makes the ball's trajectory a parabola
the air pushing against the ball changes directions as it flies.
When the ball is moving up
the molecules are acting as downward pressure.
When it's coming down, the molecules are pushing up.
And just like the spinning top
pointing against the downward force of gravity to stay vertical
the spinning football wants
to push against the main force acting against it.
The air molecules like the tap on the table,
the ball is going to want to wobble around
its center line, its reference axis.
And in this case, that ends up being the ball's trajectory —
its path through the air.
What the football wants to do
is exactly what the top wants to do on the table.
It wants to circle its axis, wants to circle around
that force direction.
But in the case of the top
the force of gravity never changes.
It's always vertical.
Whereas for the football
the axis of the force is changing.
You can see all these forces at play in this slow-mo 3D recreation.
The ball takes off
pointing towards the air pushing against it
wobbling around its reference axis.
It follows the trajectory gravity laid out for it
eventually reaching a point where it starts coming back down
at which point the air forces are pushing up against it
and the ball points against them.
But in regular motion, it just looks like the ball is tipping over.
If you throw a football without it spinning
it would follow a similar trajectory, but it wouldn't tip over.
And if you try to throw a spiral loaf of bread or some other object
it wouldn't tip over either.
It only works on objects that are cylindrically symmetrical
like a football or a rugby ball.
A very smart person once said to me
"Everything is simple once explained."
It's a hard problem because, first of all
there are a lot of potential explanations
which seem enticing at the beginning but turn out to be wrong.
And unfortunately, you can't really solve a problem
without using some relatively ophisticated mathematics.
But the basic ideas are simple enough for anybody
that's interested and is paying attention
to understand.