Physics of Tennis: Calculating Racquet Swing Weight at Home

Suspend the racquet at its top cross string using two rounded sticks stabilized at the edge of a table. Then measure the distance from the hanging point to the balance point (R). Displace the racquet approximately 2 inches, then release it. Use a stopwatch to measure the time of 10 full oscillations. One oscillation is defined as a complete back-and-forth movement. After recording the time of 10 full oscillations, calculate the period of one oscillation (T).

The moment of inertia at the hang point: m*g*R*T²/4𝜋²

To find the “pure inertia of the racquet we must subtract the extra inertia caused by the distance between the hang point and the balance point: m*R².

RA is the reference axis located at 10 cm from the butt cap. It roughly aligns with where the index finger knuckle of a player’s dominant hand sits on the handle. The industry defines “Swing Weight” as the inertia at 10 cm from the butt cap. 

Therefore, we must add back the inertia from the distance between BP and RA: m* (BP-10)²

The final equation: SW= m (g*R*T²/4𝜋²-R²+(BP-10)²)