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Indian rope trick

This trick saw a long piece of rope stant on end in the night. A small boy could climb theis rope, the his master. Dr Karls explains the physics behind lesser versions of this trick.

The Indian Rope Trick is very famous. A performance would begin just before sunset. The Indian Fakir (a Hindu miracle-worker) would take a length of hemp rope from a wicker basket and throw it into the air. It would fly up, in the growing darkness, until the top was no longer visible and then miraculously stay there. A slim young boy would climb the rope and be seen to vanish into thin air. He would refuse his master's call to come back down. The Fakir would draw a lethal-looking knife, clench it between his teeth, and clamber up after the boy - and also vanish from sight. Then there would be a series of blood-curdling yells, and various dismembered limbs of the young boy would fall to the ground, followed by his head. The Fakir would then shimmy down the rope, which would then collapse after him, throw a cloth over the scattered body parts, clap his hands, say "Hey presto", and the young boy would leap up - miraculously reassembled.

The earliest written account of the Indian Rope Trick comes from the Arab traveller Ibne Battuta, who said that he saw it in 1346, while travelling in Hangzhou in China.

How would they do this?

The trick would be performed in imminent darkness, with nearby hills or trees. The magician would rely on a thin strong black cord slung between two high points. The rope that he would throw up would have a heavy black ball on the end which would hook over this cord, and which would be strong enough to support the weight of the slim youth. Once the boy had reached the top he would hook the vertical rope onto the horizontal black cord, which could then take the weight of the magician.

And the boy's dismembered body parts? They were bits of a shaved monkey that were previously dressed in clothes similar to the boy's, with a bit of red sauce splattered around.

As conjuring tricks go, it's a classic.

But if magic isn't your bag, David Acheson from Jesus College at Oxford, and Tom Mullin, a physicist at the University of Manchester, have worked out the physics of how to get your Indian Rope Trick off the ground.

Imagine that you have a stick resting vertically on the palm of your hand - and you don't want it to fall over. Back in 1908, Andrew Stephenson, a mathematician from the University of Manchester, proved that you could keep this stick upright simply by moving the palm of your hand rapidly up-and-down, instead of side-to-side. This is one show that you can do at home, folks!

Now suppose you get two sticks and link them together with a little metal loop. Once again, you can keep the two sticks standing vertically by moving your palm up-and-down "fast enough", while moving through a distance that is "short enough". Of course the tricky words are "fast enough" and "short enough".

Luckily, a Swiss mathematician, Daniel Bernoulli, worked out exactly what "short enough" and "fast enough" were, way back in 1733.

So David Acheson and Tom Mullin linked three rods together, each about 19 cm long. They used a DC motor to vibrate the bottom support up and down 40 times per second, through a total distance of around 2 cm. The set of three linked rods stood up perfectly straight. The scientists even pushed the three rods over 45o from the vertical - and they just wobbled right back up to the vertical again.

Now why is it so, that this up-and-down motion works better than a side-to-side motion? When the motor is pulling the assembly downwards, it's actually pulling it down faster than it would fall by gravity alone. Think about one of the linked rods that would fall, thanks to gravity, off to the side. Pulling it down rapidly makes it straighten out, and become vertical.

The relationship of this experiment to the Indian Rope Trick is that rope is made of many short stiff physical fibres. Each individual fibre, if it is short enough and stiff enough, will stand up by itself. So you can look upon a rope as being a linkage of many short fibres. Unfortunately, the scientists couldn't get their motor to vibrate rapidly enough to keep a real rope standing vertically, let alone support the weight of a person.

Despite what the scientists have discovered, the secret to the full Indian Rope Trick remains the domain of Indian Fakirs, magicians and conjurers.


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Published 26 July 2000

© 2014 Karl S. Kruszelnicki Pty Ltd