Demonstration apparatus for reflection after elastic shocks

Branch of study: mechanics
Inventor: Abbé Jean-Antoine Nollet, around 1745

The french cleric Jean-Antoine Nollet, later known as educator of the children of the french king, wrote a compendium of six volumes in the years 1743 – 1748 on experiments in the realm of mechanics called Leçons de physique expérimentale.

The apparatus is a wooden construct with three wooden screws in the base plate to level the machine parallel to the earth's surface. Mounted to the wooden stand is a box, whose entrance hole is variable in height by shifting the box upwards and downwards, fixing the height with another wooden screw by countering it against the vertical wooden plate. On top of the construction there is a wooden arm with a hole drilled through it. The hole can be closed by swinging a brass lid underneath it. This is the starting position for a wooden ball or a glass bead, which can be used in the demonstration. At the bottom of the apparatus you can see a wooden frame with a dark marble plate. The frame and plate can be swung at the experimenter's discretion to variable angles. The opening angle of the plate can be fixed by countering a small brass screw against the brass inlay of a wooden arc.

To conduct the demonstration the brass lid on top of the machine is opened and the ball falls downwards following the laws of gravity. Upon hitting the marble plate it is reflected towards the wooden box and if that in turn is positioned in a decent height, the ball will enter the box. From the relations of the angle of the marble plate and the height of the entrance hole you can learn, that the angles of inflection and reflection are the same. This law, today rather known from the realm of optics, can hence be transferred to the mechanics of elastic reflection.