The law-abiding disc
More disc golf physics
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May more forces be with your disc
It's been too long since we talked about physics here (I left you with only one force — gravity — in the last episode), and you can't do much with a disc and gravity except watch it fall to the ground. We need more forces! And we also need to talk more about spin and how it interacts with the other forces. But first things first. We'll talk about the forces now, and spin later.
Newton's first law
Newton's laws of motion describe the motion of discs (and other less-important things). In this episode, we'll consider the first law:
A body remains at rest, or in motion at a constant speed in a straight line, unless it is acted upon by a force.
Newton's first law is also known as the law of inertia. That is, it takes some effort (a force) to change the state of an object, whether that object is a frisbee on the tee or you lying on the couch.
Leaving aside the couch problem, the first important force in disc golf is you!—the force you apply when throwing the disc. When you throw the disc you apply a force: the strength (or magnitude) and direction of that force determine what will happen to the disc once you release it.
There are many factors that affect the flight of the disc while you are throwing it, but the magnitude and direction of the throwing force are two of the most important. Once you release the disc, it will travel in the direction of the throwing force you applied at a velocity that depends on the strength and direction of that force. (In other words, if you yank it, it will fly into the woods.)
(Some of the factors, which we'll discuss in another post, are the release angle, the launch angle, the nose angle, and the spin rate. In addition to the release variables that you control, there are the speed and direction of the wind. And there may be others that I'm forgetting)
After the throw
If there were no other forces than the initial throwing force you provided, your disc would just keep flying in a straight line forever. But once the disc leaves your hand, three other forces take over, and your disc will certainly not continue in a straight line:
Gravity: gravity pulls your disc back to earth. If you just drop the disc, it will fall straight down. But if you throw it, you'll get a much more interesting flight.
Lift: because the disc is shaped like a wing, when it's moving through the air it creates lift that keeps it from dropping to the ground like a lead ball of the same mass would.
Drag: an object (like a disc) moving through air encounters air resistance, which tends to slow it down. We call this force drag.
Each of these forces has a strength (or magnitude) and direction.
Drag always opposes the direction of motion (the velocity of the disc). The faster the disc travels, the higher the drag force on it. Drag also depends on the profile of the disc — a narrow-profile disc like a driver will have less drag at the same velocity compared to a putter or a midrange disc.
The force of gravity pulls your disc on a straight line between the disc and the center of the earth. The force of gravity is equal to the weight of the disc and is constant during the flight of the disc. The force is higher on a 180-gram disc than on a 150-gram disc.
Lift is more complicated because the direction depends on the design of the disc as well as the orientation and nose angle of the disc. While drag and gravity act at the center of the disc, the center of lift can move forward or backward from the center of the disc — that's what makes the flight of the disc so interesting!
For a simple flat launch, the forces look something like this a moment after you release the disc:
If you throw the disc hard enough, the drag force will be relatively small compared to the lift, though it will eventually slow your disc down and cause it to stop flying. Also, if you release the disc nearly flat with enough initial velocity, the lift force will be greater than the force of gravity (weight) and the disc will rise during its flight (that's why they call it lift!).
Spin
In addition to the forces of lift, drag, and gravity acting on it, a flying disc has spin. Like velocity, spin is not a force, but a characteristic of the disc's motion. For a right-hand backhand throw, the spin is clockwise when viewed from the top:
Next time we talk about physics, we'll get into the nitty-gritty of spin and how it interacts with lift to affect the flight of the disc. Things are about to get more interesting and more complicated!