The Pendulum Challenge

Think you could stand still with a bowling ball swinging towards your nose? It’s tough! This scary experiment is governed by the same principle that decides the dynamics of a car crash and guides trick shots on a pool table: a gigantically important physical law called conservation of energy. This law states that if a system is left alone, its total energy doesn’t change.


A system’s total energy is composed of two parts: kinetic energy (the system’s motion) and potential energy (stored energy determined by the system’s position). Let’s think about our bowling ball when it’s suspended at head height. It’s not moving, so its kinetic energy is zero. We know that if we stop holding it up, it will fall. This means it has potential energy! this case, the potential energy comes from Earth’s gravity pulling on the bowling ball.

When we release the bowling ball, it begins to move. In other words, its potential energy starts turning into kinetic energy. This happens bit by bit: when the bowling ball has fallen a few inches, it’s not going very fast yet, and it still has most of its original gravitational potential. As it falls farther, more of its gravitational energy is converted to kinetic energy, so the ball picks up speed. The tradeoff continues smoothly and proportionally until all potential energy has been converted to kinetic energy. The bowling ball moves fastest (has highest kinetic energy) when it’s closest to the ground (has lowest gravitational potential energy).


Credit: BBC


No physical system is perfectly efficient. Air resistance and stretch in the tether damp the swinging bowling ball’s momentum. A tiny bit of energy is dissipated with each turn of the pendulum until, finally, it comes to rest. Since each pass of the bowling ball carries less energy than the one before, the pendulum swings a little lower every time. As long as you drop the weight cleanly from your nose, it’s perfectly safe to stay put.


It’s one thing to know that the bowling ball can never swing back up to its original height and crash into your nose… it’s quite another to override your body’s instinct to flinch or step back!


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