20 Student Chair Trick

A Science Lesson in Static Equilibrium. Static equilibrium is the balance of forces in systems at rest. The students start in a circle sitting in chairs. They are in static equilibrium because their weight pushing down is exactly balanced by the chair pushing up, in accordance to Newton’ Third Law. When they lay back onto one another’s laps, their weight is precisely balanced by the force on their feet, the chair, and the lap of the person behind them pushing up. By shifting the weight from the chair to the person behind them, the students support themselves on their own feet and the feet (and lower legs) of the people behind them. Once this equilibrium is achieved, the chairs are no longer supporting the students and can be removed. Mutual Trust This is an exercise in team-building and mutual trust. To be successful, each student must trust the student behind them to support and protect them. Each student knows that, if the student behind them fails, they will fall to the floor back-first. This is a very uncomfortable and vulnerable position. If anyone tries to sit up, or get out of the circle, the equilibrium is destroyed for the group and they will all fall down. The circle depends on 100% buy in and trust. Beyond the Physics, the circle is built on the confidence that their team mates literally “have their back“.