Impossible equilibrium

Balancing on knives

How can you build bridges with just knives – no nails or ropes?

Safe­ty pre­cau­tions

Ob­serve cau­tion when work­ing with knives.

Equip­ment

  • three but­ter knives;
  • three thin wood­en sticks;
  • three pa­per cups.

Step-by-step in­struc­tions

Ar­range the cups to form a tri­an­gle. Set the han­dle of each knife on the base of a sep­a­rate cup and in­ter­weave the blades so they over­lap with one an­oth­er. You can even bal­ance a heavy glass of wa­ter in the cen­ter, where all three blades are con­nect­ed – the con­struc­tion can sup­port it! Re­peat this ex­per­i­ment with thin wood­en sticks. The con­struc­tion still holds!

Process de­scrip­tion

The whole sys­tem re­mains bal­anced be­cause the piv­ot points of its var­i­ous com­po­nents com­bine to sup­port each oth­er. Each knife rests with one end on a cup, and the op­po­site end on an­oth­er blade. In oth­er words, for each knife there are two oth­ers; the force from the first is di­rect­ed up­wards, and the force from the sec­ond is di­rect­ed down­wards. There­fore, move­ment in any di­rec­tion from any of the knives sets the en­tire sys­tem in mo­tion. The forces are ar­ranged in such a way that if the pres­sure on one knife in­creas­es, the oth­ers tend to com­pen­sate. The same goes for the thin wood­en sticks.

Try it