A really fun to play with gyroscopic toy, with amusingly translated (from Chinese, I would imagine) instructions. Anyone can get them to spin up really fast, but people with weak arms tend to get out of control and freak out! It is very amusing to watch....

Addendum, 2001-09-07: I now have three PowerBalls, my original blue one, with no fancy features, and two new orange ones, which have LEDs that light up when you're spinning the ball. The original blue one, at just over two years old, it's still going strong, although slightly worse for wear, seeing as it's been dropped on concrete several times now. When I got the orange ones, I also got a pair of speedometers, so now I know I can say with relative confidence that these things actually can spin at over 12,000 RPM - my own personal best is around 12,900. Anyway, they're great toys to play with when you have people over - head to http://www.bodytrends.com/ to pick up some glowey-LED ones, or to a local sports store for a plain ol' blue one. (It'll probably be called a "Dynabee" there, but you know the truth - they just licensed the design...)

From www.nsd.com.tw:

Item: POWER BALL

Volume: Diameter: 7.1cm, a little smaller than a baseball

Weight: About 0.26 kg

Theory: Like gyroscopes, centrifugal force and inertia are applied.

Dynamic: Non-battery. Non-electricity. It is powered by rotating the wrist.The faster we rotate the ball, the stronger the dynamic and torsion is produced. The speed of rotation can reach over 12,000 RPM, and that can bring about the torsion over 10kgs.

Functions: It is a very effective tool to increase the strength of fingers, wrist, and arm. Besides, it can make the circulation of the blood better in a short time, strengthen the ligament of the wrist, and reduce the injuries caused by sports.

Effects: (1) It can benefit those who are fond of sports, like badminton, golf, bowling, baseball, basketball, tennis, table tennis, and so on. It can make them more powerful while playing sports.
(2) It can avoid the typists' and piano players' ligament injuries.
(3) It can make the hurt hand return to original condition by degrees and prevent the hand from becoming stark.
(4) People who often use their wrists to work need to play it more often to strengthen their wrists.

Merits: (1) It is a small ball, which can make people achieve sporting effects in a very short time.
(2) Almost everyone can play it, no matter the old, adults, or teenagers.
(3) No time or location limits. It can be used at any time and in any place while chatting, watching TV, going outing and mountain climbing, and taking a bus.

Features: (1) The balance of the rotor has been corrected by computer, so the ball has very excellent stability.
(2) There is a circle of silicone rubber on the surface of the ball. That can prevent the ball from sliding down to the ground, and the feeling of grasping is also better.
(3) The plastic material is especially durable.
(4) The axle is made of steel, not iron. It is not easy to be broken

You give me one minute, and I will give you a powerful hand.
NANO-SECOND TECHNOLOGY CO., LTD.

The Dyna-Flex PowerBall

Dyna-Flex International (http://dynaflex-intl.com) is the manufacturer of the PowerBall listed above. For specs on the different styles of PowerBall available, please refer to that site. For generic information, check out gyroscope, gyroscopic motion or gyroscopic stability. For specific physics of the PowerBall, keep reading!

Physics

Angular momentum is the quantity in question when dealing with gyroscopes. I don't remember the exact calculations, but every object has an 'angular inertia', usually labeled I; then the angular momentum p=I*a, where a is the angular velocity. This is the groundwork.

Next up is torque, or 'angular force'. This quantity is equivalent to the angular acceleration times the angular inertia. Torque can be achieved in several ways, but the most interesting (related to the PowerBall) is the following: when an object has angular momentum, and a torque t1 is applied in a direction perpendicular to the object's angular momentum, a second torque t2 is generated (by the cross product of the angular momentum with t1). This second torque is perpendicular to both the angular momentum and the direction of t1.

The PowerBall is a gyroscope with a ring around its middle (perpendicular to its motion). The ring is relatively free to move within the PowerBall's structure, though there is a little friction. So the gyroscope has two directions of angular freedom. Now suppose that the gyroscope has an angular momentum a, so it is spinning (in the gyroscope direction, call the direction g). Suppose that a torque is applied in the non-free direction (the direction perpendicular to the gyroscope and its ring, call the direction nf). Then a torque is generated in the ring direction (call it r). But there is friction in the r direction, so the cross product of the actual torques t1 x t2 is something other than the angular momentum a. The point of the PowerBall is to time the forced torques (in the nf direction) just right so that t1 x t2 is greater than the previous angular momentum a, which will make the gyroscope portion spin faster.

The Workout

At its top speed, the 'heaviest' PowerBall (the one with the greatest angular inertia) will generate about 38 lbs. of torque (this is at about 9000 rpm, see the website mentioned above for more details). I recommend this 'workout toy' for anyone and everyone; it's a lot of fun!

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