How Physics Applies to Disc Golf

Off the Chains: Impulse, Momentum, and Why Chains Catch Discs

Written by Team Disc Store Member Joseph Johnson

For avid disc golfers, there is no better sound than a putt hitting the center of the chains and dropping into the basket, but why have those chains at all? Is it just for that Pavlovian response every time you hear the satisfying "clang"? No, obviously not. There is some fascinating physics at work when a disc hits the chains. One of the key principles is impulse, which equals the change of an object’s momentum and explains how the chains effectively stop the disc and guide it into the basket (most of the time).

What Is Impulse? 

Since impulse also equals change in momentum, we need to define that term first. Momentum (p) is one of those terms that can be confusing because we use it differently in physics from in our everyday language. In physics, it is a way to quantify the motion of any object considering both the size and velocity of the object. A fast-moving baseball and a slow-moving bowling ball might actually have the same impact when they hit something. We can look at their momentum to figure it out. We calculate momentum simply by multiplying an object’s mass and velocity. We can find the momentum of a 172 g disc (0.172 kg) moving at 60 mph (26.8 m/s) just by multiplying those two quantities together.

p = m x v = 0.172 kg x 26.8 m/s = 4.6 kg m/s

Impulse describes the change in momentum of an object when a force is applied over a period of time. It’s mathematically expressed as:

Impulse = Force × Time

For example, I measured the force of one of my throws to be approximately 12.5 N (around 2.8 pounds of force). The throwing motion lasted only 0.3 seconds. This means I imparted an impulse of 3.75 kg m/s on the disc.

Impulse = Force × Time = 12.5 N × 0.3 seconds = 3.75 kg m/s

It is worth noting that this is one of the reasons it is important to reach back and follow through with your throws. By doing so, you increase the amount of time you are applying your force to the disc and therefore increase the resulting changing in momentum. Larger change in momentum means it is leaving your hand at a fast velocity and hopefully flying further down the fairway. You can see that in the equation showing impulse as the mechanism by which an object’s momentum changes:

Impulse = Δp (change in momentum) = mass × Δv (change in velocity)

Putting all of this together, we get:

F × t = m × Δv

What does this have to do with baskets and chains? In simple terms, when your disc hits the chains, the impulse provided by the chains changes the disc’s velocity, slowing it down and stopping it from continuing its flight. If the disc hits the band or cage of the basket, this change in momentum happens very quickly. The chains extend the time over which the change happens.

Looking at the equation, increasing the time will decrease the impact force the disc feels and thus reduce the likelihood that the disc spits out of the basket.

The chains in a disc golf basket serve a dual purpose:

1. Absorbing the Disc’s Momentum: As described above, when the disc makes contact, the chains provide a force over a longer period, reducing the disc’s velocity while reducing the force required.
2. Redirecting the Disc: The angle of the chains also guides the slowed disc downward into the basket, hopefully preventing it from flying out or bouncing off. This is obviously not foolproof as chain outs do happen, but in general, that is how they are designed to work.

The ability of the chains to catch and hold the disc effectively depends on how they apply impulse. When a disc hits the chains, its momentum is high because of its velocity. The chains apply a force over an extended time, creating an impulse that reduces the disc’s forward momentum and dissipates its kinetic energy. It is very similar to the physics behind your car’s airbags.

The key here is the force distribution over time: 

• If the chains are too stiff or heavy, they deliver a large force over a very short time. This might stop the disc quickly but could cause it to rebound away.
• If the chains are too loose, they may not provide enough force to stop the disc completely.

Properly designed chains strike a balance, applying enough force over a brief time to capture the disc without rebounding out.

Material Matters

The materials and arrangement of the chains also play a role. This is why different basket designs from different companies catch putts differently (Check out the Disc Store Grow the Sport Basket for an awesome championship-level basket at the best price on the market). In general, metal chains are excellent for providing a consistent force because they don’t stretch or deform significantly. Their flexibility allows them to absorb and redistribute the disc's energy efficiently, while their weight prevents excessive swinging that could cause a disc to escape. Once the chains absorb the disc’s momentum, gravity takes over. The chains redirect the disc downward, and its reduced velocity ensures it lands safely in the basket rather than bouncing out. This process showcases a perfect demonstration of momentum transfer, impulse, and gravitational force.

I did a quick comparative analysis of a disc being received by a championship-level basket, a light practice basket, and a disc hitting the band of the basket. The championship-level basket took 1/3rd of a second to bring the disc to a stop and dropped it nicely into the basket, while the much lighter chains on the practice basket took much longer, nearly a full second, to bring the disc to rest, nearly allowing the disc to pass all the way through, catching on the back edge of the basket. At the other extreme, hitting the bend brought the disc to a stop in less than 1/30th. This imparts so much force due to the rapid momentum change that the disc actually rebounds back in the opposite direction. You can learn more about this by studying elastic and inelastic collisions.