Physics of Tennis

Leave it to an engineer that plays tennis to find enjoyment in understanding the technology for hitting and controlling a tennis ball… I’ve been reading this fascinating book called the “Physics and Technology of Tennis”, which attempts to use scientific methods to objectively answer the questions:

• Is a stiff frame good or bad?
• What difference does string tension make?
• Should I use a lighter or heavier racquet?
• How can I hit with more power?

Since the book is filled with obtuse scientific concepts such as Vertical Angular Acceptance, Coefficient of Restitution, etc., I’ve attempted to summarize the finer points. A few interesting insights:

1. A standard Type 2 professional tennis ball should rebound 55% when dropped from a height of 100 inches onto a concrete surface.

2. The ITF has recently allowed the use of a new Type 3 ball, which is slightly lighter and softer but 6% larger in diameter.

3. Even though you can’t see it with the naked eye, during impact, the ball squashes by approximately 50% and the frame bends out of shape. 

4. The number one determinant of serve speed is the velocity of the racquet at the impact point on the ball.

5. Professional players can serve almost as fast with an old wood racquet as with a modern graphite racquet – the difference is rarely more than 5%.

6. Tennis has conflicting adages in “keep the ball on the loose stings longer for better control” and “tight strings give better control”. Lower tension will provide a little more power, and an open string pattern will also produce more power.

7. String tension is largely a matter of preference.  Lower string tension will give you more power but only 1% more.

8. Stiff strings normally increase jarring since they cause the handle to vibrate more and to slam into the hand.

9. A common view is that a racquet with a stiff frame will generate more shock at impact.  In reality, stiff frames vibrate less and generate less shock.

10. A racquet also needs stability when impacting the ball, thus at a certain point (e.g. below 8 oz), a lighter racquet can never compete with a heavier one in terms of stability.

11. The number one way of improving service percentage is by hitting the ball at a higher elevation.  An increase of 6 inches in height increases the acceptance window by almost 30%.

12. By the time a 110 mph service gets to the other baseline, it has slowed down to 55 mph due to air resistance and friction from the bounce.

13. Moving 5 feet behind the baseline gives the receiver 10% more time to react to the service.

14. Unlike the bounce of balls in most sports, the angle of reflection does not equal the angle of incidence for tennis balls rebounding from a racquet.

15. Higher string tension actually increases string life because at higher tension there is less string movement, hence less “sawing” and “notching”.

16. A ball hit with topspin arrives at the receiver spinning between 50 to 100 revolutions per second.