How Physics Applies to Disc Golf - Intro
Written by Team Disc Store Member Joseph Johnson
Physics… When I tell people what I teach, I usually get one of two responses. “Oh wow! Cool! I love physics!” or “Oh wow... I hated physics” with very few reactions in between. The visceral reaction of some, I believe, is due in large part to how it is presented in school as just a bunch of applied math and equations to memorize. Physics, in actuality, is the study of EVERYTHING. It is our way, as human beings, to make sense of the universe that we live in and codify the rules by which things work. So, everything that exists in the universe is related to physics in some way or another. You might be asking, “What does that have to do with disc golf?” Well as it happens, disc golf generally takes place in this universe. While understanding physics isn’t a prerequisite for playing disc golf, digging into the physics can help us understand the game in new, deeper, and better ways. My intention in this blog series is to present the physics of disc golf in an understandable (and hopefully interesting) way, so that it is accessible to everyone. Physics can be intimidating with heavy math and difficult concepts to wrap your head around, but my goal is to help you to think differently about your disc golf game using a physics mindset.
Topics I intend to cover in the series include:
- Kinematics (displacement, velocity, acceleration) - For throwing motion and disc flight
- Work and Energy - Transferring energy to the disc in the throwing motion
- Vectors - Push putt vs. spin putt
- Torque - Grip and rotating the disc
- Torque - Biomechanics of a throw and torque in the joints
- Wind speed and relative velocity
- Conservation of momentum - Whipping motion and the power pocket
- Impulse/Momentum for the throw – Reach back and follow through
- Impulse/Momentum for the basket/chains
- Newton’s second law - Disc weights and acceleration
- Newton’s Third Law - Air Bounce and disc angle
- Moment of Inertia - Disc shape and design
- Rotational kinematics - spin rate and rotational kinetic energy
- Angular Momentum and disc skip
- Angular Momentum and Gyroscopic precession
For this introductory post, I want to introduce some of the basic language we use in kinematics (descriptions of motion). This can be tricky in that many of these words are used in our everyday vocabulary, but not with the physics definition. As a result, we interchange words that don’t really mean what we think they mean [insert Princess Bride reference here].
First up is time. Time means essentially the same thing in physics as it does in regular language. It describes how long it takes for something to happen. The study of why time works the way it does and how it can change based on your state of motion through space is some deep and heavy physics. It is extremely interesting but beyond the scope of this series. Feel free to read up on special relativity or the book Now: The Physics of Time for more info. For our purposes, we will look at time in terms of rates of change, i.e. how fast does a disc’s position change or how fast does its velocity change.
Changes in position fall into two categories: distance, which is how far the object moved on its path, and displacement, which is the straight-line distance between the starting point and ending point. For example, a backhand hyzer drive might travel a long distance in a wide arcing path, but not have a large displacement as the disc doesn’t end very far from the tee.
The rate of change of distance (distance divided by time) is the object’s speed. Velocity is speed with direction, making it a vector quantity. Both speed and velocity are measured relative to some reference point i.e., how fast is my disc traveling relative to the ground or how fast is it going relative to the air around it? This latter reference, the air speed of the disc, is what the “speed” flight number is referring to. This is particularly important when thinking about how our discs fly in the wind. If the wind is moving in the same direction as the disc (a tailwind), it is moving slower relative to the wind (see picture below) and acts as if it was thrown slower.
We can take this one step further and talk about the rate of change of the disc’s speed or velocity. This is its acceleration. Our goal in a distance throw is to accelerate our disc from rest (velocity = 0 m/s) to a maximum possible final velocity as it leaves our hand. The instant it leaves the hand, the disc will immediately begin to negatively accelerate (slow down). In general, positive accelerations will increase the speed of the disc and negative accelerations will slow it down.
Accelerations are always the result of forces, pushes or pulls, on an object. More force will yield large accelerations; however, larger mass (more grams in the “disc weight”) will yield smaller acceleration (Fnet = ma). This equation, Newton’s 2nd Law, can be rewritten with a little algebra to give a new term, momentum. Momentum is a disc’s mass multiplied by its velocity. The equation becomes: change in momentum (Δp) equals force multiplied by time (Δp = Ft). Increase the force or the time you apply the force, and you increase the change in momentum and thus the final velocity of the disc. This explains the importance of the reach back and follow through in your throw. All these terms have rotational counterparts (force and torque, mass and moment of inertia, momentum and angular momentum to name a few), which will be covered in a later post.
These are a LOT of topics to digest at once, but I am excited to explore them all in more detail. We will be using these ideas to break down various aspects of throwing mechanics, disc design, and disc flight to better understand the sport and hopefully to improve our game!
Written by Team Disc Store Member Joseph Johnson