These Prague street performers have pulled off one of the finest executions of the levitating man illusion you will ever see. For those of you who hate fun and wonder, we've got the secret — and the physics — behind this ancient trick.
For centuries, Indian yogis have claimed to have mastered the powers of levitation. In reality, they've simply managed to dupe unsuspecting bystanders by tapping into fundamental design principles and basic physics.
Simply put, the floating man illusion is accomplished through the strategic placement of steel rods and plates. And though the performers appear to be defying the laws of physics, the structure is remarkably stable. Here's an image from 22 Words that shows how the one-person variation of the trick works:
The special wrap-around configuration, along with a solid base, provides the stability and strength required to pull it off. The three components include a solid base (3), the staff (2), and seat (3).
The two-man version is slightly different, but it borrow from the same solution. Writing in the James Randi Educational Foundation, Kyle Hill explains:
To fit underneath the carpet, the plate must be rather small. And from the looks of the picture [below], the sitting man isn't that elevated therefore the plate is not too thick. A thin plate can be heavy, so you may be thinking that the weight alone is balancing the whole apparatus, but a ~180 pound man on a steel rod can produce quite a bit of force. There is more physics to it than this, though the additional weight certainly helps. It could also be the case that the sitting man is helping to balance the levitating one, but considering that this trick can also be done with one person, we'll assume that the sitter isn't contributing to the stabilization. [image credit: Michael Shermer]
I'll do some engineering calculations in a process called statics to sort this out. I'll assume a 180 lb man whose center of gravity is directly above the bar connecting to the plate.
Now, the most efficient way to do this trick would be to sit directly above where the steel rod connects to the plate. This way, there is no torque in the plate-rod connection created by the levitator. This looks to be how the two-person levitation trick is set up.
The calculations are easy. To be in equilibrium, all the forces and moments (rotational forces) must cancel out. And what we are really concerned about, if we were the tricksters, is whether or not the apparatus will tip over. We can find this out by calculating how much rotational force is acting at the edge of the plate assuming nothing is moving. In the set-up above, the moment at point A is calculated by multiplying the downward force by the perpendicular distance (I guessed again here) to the point. This results in 180 ft-lbs that push the plate edge down into the ground (a clockwise force).
The trick, says Hill, is that the further the edge of the plate is away from the center of gravity of the levitator, the more stable the apparatus becomes. There must be a clever balance between rotational forces and deception. As Hill says, "The closer the weight of the levitator comes to any edge of the plate, the smaller the moment becomes, until it becomes zero when the levitator is directly over an edge. This is the near-tipping condition."
Here's a variation of the trick by performance artist Johan Lorbeer. I'll leave it to you to figure this one out.
Top image: George Dvorsky