5616 x 3744 px | 47,5 x 31,7 cm | 18,7 x 12,5 inches | 300dpi
Date de la prise de vue:
14 février 2010
Informations supplémentaires:
Newton's cradle, named after Sir Isaac Newton, is a device that demonstrates conservation of momentum and energy. If one ball is pulled away and is let to fall, it strikes the first ball in the series and comes to a dead stop. The ball on the opposite side of the series instantly acquires the speed of the first ball and swings in an arc that one would expect of the first ball.The first time observer may find this visually intriguing and counter-intuitive. If a person charged into one end of a row of adjacent people then one would intuitively expect the whole row of people to be moved, rather than the last person in the row to acquire the kinetic energy.The intermediate balls appear stationary. In fact, the cradle continues to work even if the intermediate balls are physically clamped still. This is also counter intuitive - transmitting motion without moving.What actually happens is that the first impact produces a shock wave that propagates through the intermediate balls. A hard material such as steel is very good at doing this, where common objects like people in a bus queue are not.The shock wave travels at the speed of sound in the medium. The speed of sound in steel is much faster (around 4699 m/s) than in air (around 343 m/s). The time to travel a few centimeters is too small for human perception, and so is the physical distortion of the balls as the shock wave passes through them.In the real world, none of these processes has perfect efficiency. Energy is lost in the suspending wires, friction of the surrounding air, and sound. The last is obvious as the clicking of the balls can be heard. Toward the end of oscillation, even the intermediate balls are jiggling a bit.Further intrigue is provided by starting more than one ball in motion. With two balls, exactly two balls on the opposite side swing out and back. While this is satisfyingly symmetrical, why could not one opposite ball swing out twice as fast, or four balls at quarter speed?
