Tennis could be a better game. Millions of players suffer from painful wrist, shoulder, and especially elbow injuries. Millions more try the game, but quit because it's too difficult to keep the ball in play or to get enough exercise per precious hour of recreation time. Pro tennis can be boring when points last only one or two shots, such as when a big server is playing on a fast surface. Could much of this be solved by making tennis balls lighter?
Most arm injuries in tennis result from strains caused by the ball knocking the racquet backward and making it twist in the player's hand. Stroke execution and racquet design can reduce stress on the arm, but even for the best players using the most protective racquets, thousands of ball impacts add up to considerable stress and strain. Each collision between racquet and ball involves a complex set of interactions between ball, strings, and racquet frame, but at the root of every collision are transfers of momentum and kinetic energy, both of which are directly proportional to mass. Momentum is the product of mass and velocity; kinetic energy is one-half the product of mass and the square of velocity. At a given speed of racquet and ball, reducing the mass of the ball by a certain percentage would reduce the momentum and kinetic energy with which it impacts the racquet by the same percentage.
A lighter ball should also arrive from the opposing player at a somewhat lower velocity. At a given racquet swing speed, a lighter ball will leave the opposing player's racquet at a higher speed, but it will slow down more due to increased air resistance, which is inversely proportional to mass, and arrive at the other end of the court at a lower speed than a standard ball would. Reducing the ball's incoming velocity will reduce its momentum in direct proportion, and it will reduce its kinetic energy substantially more, by the square of the reduction in velocity.
While their arms enjoy less stress from hitting a lighter, slower ball, tennis players will also benefit from having slightly more time to reach the ball, which will make the learning curve easier at the beginner level and lengthen rallies at every level, making tennis more fun and better exercise. Pro tennis will be more exciting to watch if rallies last longer and players need to create sharper angles or combinations of angles to win a point. To achieve these benefits, tennis has tried introducing larger tennis balls intended to fly somewhat slower because of increased air resistance, but the weight was not reduced, and the slight decrease in speed wasn't enough to make the balls significantly easier on the arm. The best known of the larger balls, the Wilson Rally, never caught on, as the beginners for whom it was primarily intended didn't find it noticeably easier to hit.