Stop-Go Magic Ball Trick

Printing

Print the ball out of PLA, and feel free to use jazzy colours, multiple colours etc. You may scale it to suit the cord size, but ensure that it weighs at least 28g by increasing infill if necessary. You will probably need a wide brim.

Preparation

You'll need a piece of 3.5-4mm diameter smooth cord for this, about as long as the distance between your fingertips and nose. I used 3.7mm paracord. Thread the cord through the central hole of the ball and put a knot in each end so no clever person can pull it out.

Use

Hold the ball in one hand while stretching the cord fairly tightly between your two hands. As long as you maintain tension, the ball will not slide on the cord. Hold the ball hand directly above the empty hand, and gradually reduce the tension on the cord.

The ball will start to creep downwards. Restoring tension will stop it immediately. With a little practice, the ball can be made to slide slowly, rapidly, or swiftly come to a complete stop.

Do not try sliding the ball at an angle as the audience will see the ball sagging when the cord tension is released.

Suggested Tricks

In a basic trick, the ball can be made to appear to obey start-stop commands.

As part of a more complex routine the magician can declare that "I'll start this this trick with the ball at the top of the string..." While magician looks at the audience, the ball is sent into a controlled descent, stops, and the magician looks at it. Magician returns the ball to the top of the string says once again "I'll start this trick...", looks back to the audience and the ball again starts to descend. The magician then looks at the ball, which stops. Magician looks back to audience, ball starts moving again, etc. back and forth. Magician sighs "Oh, I give up!" and moves on to a different rope trick.

The OpenSCAD file is provided so you can change the shape of the ball.