Every golfer, from the weekend amateur to the seasoned professional, knows the uniquely frustrating moment when a seemingly perfect putt reaches the edge of the hole, circles the rim, and then spins away. This maddening event, known as a lip out, can feel like a cruel twist of fate, but new research reveals it is governed by predictable, albeit complex, principles of physics. The study, published in Royal Society Open Science, deconstructs this golfing heartbreak, offering a scientific explanation for what happens in those few critical seconds on the green.
A team of researchers led by scientists at the University of Bristol has analytically detailed the mechanics behind why a golf ball fails to drop into the cup. They identified two distinct ways a putt can lip out, based on whether the ball’s center of mass dips below the level of the green. The findings move beyond golfers’ anecdotal wisdom, providing a mathematical framework that accounts for the ball’s velocity, angle of approach, and its interaction with the hole’s rim. At the heart of the phenomenon is a precarious balance of forces, where the slightest variation can mean the difference between a successful putt and a missed opportunity.
Two Distinct Categories of Failure
The research fundamentally classifies lip outs into two types: a “rim lip out” and a “hole lip out.” The rim lip out is the more common and intuitive of the two. In this scenario, the golf ball’s center of mass never drops below the putting surface. It rolls to the edge of the cup and tracks around the upper rim before being ejected back onto the green. This type of rejection often happens when the ball is traveling with slightly too much speed or strikes the edge at a glancing angle.
The second, and much rarer, type is the “hole lip out.” This occurs under a very specific set of conditions where the ball actually falls into the hole, only to emerge again. For this to happen, the ball must enter the cup in such a way that it begins a pendulum-like motion, rolling along the inner wall of the hole. If it maintains enough momentum without touching the bottom of the cup, it can climb back to the rim and roll out, a sequence that often leaves players in disbelief.
The Physics of a Precarious Balance
At the core of both types of lip outs is a concept the researchers describe as a “degenerate saddle equilibrium.” This term from mechanics refers to the delicate tipping point where the ball is teetering on the very edge of the hole. It is in a state of unstable balance, where it is neither securely on the green nor definitively falling into the cup. It is from this exquisitely sensitive position that the ball’s fate is decided.
For the common rim lip out, tiny external factors, or perturbations, can be decisive. A slight puff of wind, an errant grain of sand, or the smallest amount of side spin on the ball can provide the necessary force to push it one way or the other. When a ball arrives at this equilibrium point, these minuscule influences can be enough to nudge it away from the hole instead of into it, causing it to rotate around the rim and back onto the green.
The ‘Wall of Death’ Effect
The researchers found the dynamics of the “hole lip out” to be particularly fascinating, dubbing these events “golf balls of death.” The name is a direct nod to the circus stunt known as the “wall of death,” where motorcyclists ride horizontally around the inside of a cylindrical arena, seeming to defy gravity. A similar principle applies to the golf ball. As it drops into the hole, its potential energy is converted into kinetic energy in the form of spin.
This spinning motion, combined with a pendulum-like swing along the cup’s inner wall, can generate enough force for the ball to find a state of zero downward pitch and climb back up to the rim. A standard golf cup is approximately 10 centimeters deep, and as long as the ball does not make contact with the bottom, this escape is physically possible. The ball uses the spin it gained during its brief journey into the hole to propel itself back out, completing the most improbable and frustrating of all putting misses.
Mapping the Zones of Success and Failure
The study analyzed how the outcome of a putt depends on its speed and its position as it approaches the hole. The researchers effectively mapped out different regions around the cup that lead to different results. A putt that is not moving with too much pace and enters the hole closer to its center is almost certain to drop. This is the safe zone that all golfers aim for, where momentum is low enough for gravity to easily take over.
However, as the ball’s entry point moves closer to the edge and its velocity increases, it enters a region where a “rim lip out” becomes the expected outcome. There is also a specific, narrow set of conditions involving higher speeds and precise angles where the rare “pendulum-like” motion of a hole lip out can be triggered. The analysis confirms that while spin can be a factor, the primary determinants for most lip outs are simply the ball’s speed and its line into the hole.
Applying the Science on the Green
While the physics may be complex, the practical advice derived from the research is straightforward and confirms what many experienced golfers already know. To avoid a lip out, players should focus on two key elements: accuracy and pace. Professor Hogan noted that the research confirms the wisdom passed down by his golfing friends: hitting the ball too hard is a primary cause of lip outs.
The study’s scientific conclusion is that a golfer’s best strategy is to aim as close to the center of the hole as possible and to deliver the ball with the minimum speed required for it to reach the rim. This ensures that the ball arrives with just enough momentum to tip over the edge and fall into the hole, but not so much that it has the energy to be carried around the rim and away. Although this advice may not seem revolutionary, it is now supported by a rigorous mathematical foundation, giving golfers a clearer understanding of the forces at play in their moments of putting triumph or despair.
