Sidespin

Sidespin refers to the rotation of a ball or similar object about a largely vertical axis, so the ball spins left and right as it travels forward. When a ball is given sidespin, the air flow around it becomes asymmetric, and a sideways force develops—often described by the Magnus effect—causing the ball’s path to curve toward the direction of the spin. Sidespin is distinct from topspin (spin about a horizontal axis perpendicular to the flight path) and backspin (spin about a horizontal axis parallel to the flight path).

The physics behind sidespin involve the interaction of surface roughness, air density, speed, and spin rate. The resulting sideways force increases with higher spin rate and slower forward speed, and it can also affect the ball’s bounce after contact with a surface. In many sports, sidespin is used intentionally to deceive opponents, create advantageous angles, or control the ball’s position after impact.

In practical sports applications, players generate sidespin by striking or hitting the ball with a snug, glancing contact that impart a sideways component to the rotation. Examples include slice and sidespin serves in tennis, side-spin shots in table tennis, curling and English on a billiard ball, and curved kicks in soccer. In golf, similar effects produce hooks or slices when the clubface and path impart sideways rotation.

Measurement and analysis of sidespin rely on motion capture, radar, or high-speed imaging to infer spin axis and rate from trajectory and bounce data.