Same thickness, so why a different direction?
Separating the moment right after the collision from the travel on the cloth afterwards connects stun, follow and draw shots with the separation angle.
One thing to remember today
Thickness alone does not decide the path afterwards. Look also at the speed and spin at the moment the ball reaches the object ball, and at the cloth friction after the collision.
Written and edited by TTBoys · How we write and check sources · Report an error
Reading the motion in diagrams
Topspin → follows
- Dashed: cue ball start
- Red: object ball travel
- Yellow: cue ball after impact
No top/back spin → stops
- Dashed: cue ball start
- Red: object ball travel
Backspin → comes back
- Dashed: cue ball start
- Red: object ball travel
- Yellow: cue ball after impact
A concept diagram made by TTBoys for learning. It is not a measured graph or simulation result from a paper; arrow lengths and angles are there to explain the principle.
90 degrees is a reference with conditions
For balls of equal mass, one of them at rest, in an off-center elastic collision that ignores ball-to-ball friction and energy loss, the two directions of motion right after the collision are at a right angle. In a full-ball hit where the cue ball stops, its direction of travel cannot be defined, so treat that case separately.
The 90-degree stun rule used in play relies on this reference. But if forward or backward spin remains, cloth friction after the collision changes the cue ball’s direction. Do not confuse an angle measured on screen after the ball has traveled a while with the angle at the moment of collision.
The first collision changes the speed; the remaining spin acts on the path afterwards
In a full-ball hit, much of the cue ball’s forward motion passes to the object ball. If forward spin remains, the cue ball follows; if backspin remains, it can come back. In a cut shot some sideways motion also remains, so the path is not the straight line of this full-ball picture.
What matters is the state when the cue ball reaches the object ball, not when the cue touches it. The same bottom hit can give a completely different result over a long distance between the balls, because the backspin wears off.
Real collisions also have a little friction and loss
A 2023 collision study separates the ball-to-ball collision from the stage afterwards in which the ball changes to natural roll on the surface. You can use it as support for looking at both stages together to understand the path after a collision.
A 2024 Korean study also deals with inelasticity, ball-to-ball friction and table friction. This is why we do not present a single approximate rule as the exact answer for every thickness and spin.
An observation drill suggested by TTBoys
Watch the cue ball after a collision in the same full-ball layout
- Place only two balls, the white cue ball and a red object ball, on the same straight line. Start without sidespin.
- Repeat full-ball hits with a tip position near the center, then in the next set raise only the tip position slightly and compare. Do not count missed hits as successes under the same conditions.
- Watch the cue ball and the object ball separately at four moments: the start, just before the collision, just after it, and after a little travel.
What to watch: Look at whether the cue ball stops, moves forward or comes back, and whether the spin before the collision was really the same.
When interpreting: Even with a center hit, the ball can start rolling on its way to the object ball. Do not decide the collision was a stun just because the tip position was at the center.
Connect to Basics, Systems and Practice
Evidence from papers and where it applies
Sources reviewed: 2026-10-10 · The explanations and drills are TTBoys' editing.
Experimental and theoretical paper · Antonio Doménech-Carbó
Independent friction-restitution description of billiard ball collisions ↗
Mechanics Research Communications 131, 104149 (2023) · DOI 10.1016/j.mechrescom.2023.104149
What this source explains: Separates ball-to-ball friction and restitution from the process of reaching pure rolling after the collision to explain speed, spin and separation direction.
Limits: The experiments with several spheres, billiard balls included, were done on a PVC surface. The measured values cannot be used as-is as corrections for billiard cloth.
Theoretical paper · 김형찬 (Hyeong-Chan Kim)
Collision of Two Spinning Billiard Balls and the Role of Table ↗
New Physics: Sae Mulli 74, 621–633 (2024) · DOI 10.3938/NPSM.74.621
What this source explains: Analyzes inelasticity, ball-to-ball friction and table friction when a spinning cue ball hits a stationary object ball of the same mass.
Limits: The link is the author’s public version (arXiv). It may differ from the journal version; this article relies on the scope of the study as stated in the abstract.
Experimental and theoretical paper · S. Mathavan · M. R. Jackson · R. M. Parkin
Application of high-speed imaging to determine the dynamics of billiards ↗
American Journal of Physics 77, 788–794 (2009) · DOI 10.1119/1.3157159
What this source explains: Tracks changes in position and velocity in high-speed video to study sliding and rolling, cushion impacts and separation angles.
Limits: The experiments used a snooker table and calibrated camera equipment. This does not mean a smartphone gives the same accuracy, and tracking spin needs separate observation.