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Old 01-19-2013, 08:41 AM   #192
Join Date: Jul 2008
Posts: 5,138

Originally Posted by toly View Post
No, it is not.
There is citation from Rod Cross article, PHYSICS OF TENNIS,

“4. The dead spot of a racquet

Clamp the end of the handle on a table, using your hand to press on the handle, so the rest of the racquet hangs over the edge of the table. Then drop a ball onto the strings at various points. The ball will bounce best near the throat. There is a spot near the tip where the ball doesn't bounce at all. That's the dead spot. At the dead spot, all of the energy of the ball is given to the racquet, and the racquet does not give any energy back to the ball.

The reason is that the effective mass of the racquet at that point is equal to the mass of the ball. The effective mass is the ratio of the force at that point to the acceleration at that point (F = ma so m = F/a). If a ball of mass m collides head-on with another ball of mass m at rest, then the incident ball stops dead and gives all its energy to the other ball.“

This racquet’s property is driving me crazy.
Here is a tool that shows the effective mass (also called "hittingweight") at various impact locations for most racquets produced in the past five years: You can open up several windows to compare racquets.

Hittingweight is pretty much proportional to swingweight for impacts along the longitudinal axis, with balance point and static mass playing only a small role. Twistweight (or polar moment of inertia) becomes a factor for impacts to either side of the long axis. The hittingweight at the balance point equals the static mass of the racquet and decreases the further away from the balance point you go. So it's tiny at the tip and butt of the racquet.

And here is a tool at USRSA that lets you play with swingweight, balance and static mass and see the effective mass along the longitudinal axis:

And this USRSA tool adds twistweight, allowing calculation of impacts at locations not on the longitudinal axis:

Also, if you spend much time reading old articles by Cross, Brody and other physicists studying racquets you'll run into lots of estimations of apparent coefficient of restitution (ACOR). These articles were all written prior to the creation of Tennis Warehouse University, where nearly every racquet produced in the past five years has had its ACOR measured experimentally, rather than estimated mathematically. This is probably the most useful tool to access the ACOR ("Power Potential") database, as it allows a comparison between two racquets with the specs of those racquets clearly listed: ACOR is pretty much proportional to hitting weight, with some minor role played by twistweight on off-center impacts, stiffness on impacts near the tip of the racquet. String pattern has a general, but small, effect on ACOR.

Last edited by corners : 01-19-2013 at 08:50 AM.
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