Mechanics - Paradoxes
Paradoxal disc
Fenomeno paradosso
Fenomeno paradosso
Paradoxal disc
1818Nº //
1838Nº //
P.A.Nº //
Working D
MATERIALS: lead, wood
Dating: April 1881
In the A. M. Traversi Museum - Window D

There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy.
(Hamlet, Atc I, Scene V)

Immagine, Malfi, © D 2008

This tool has an apparently paradoxical behavior (hence the name). It was created in the first half of the eighteenth century (it's depicted in the Treaty of s'Gravesande) and then has been reproduced in several specimens more or less aesthetically different.

If it is positioned on a ramp (inclined plane), the wooden disc is in fact able to go up hill spontaneously for a short distance (corresponding to about half a turn) and then it stops. This behavior is contrary to the obvious motion of descending the slope: under the solid certainties dictated by experience the round bodies descend along the inclined planes.

But no law of physics is violated. The wooden disc inside has a small "trick" that can be discovered by reasoning. The important thing is to go beyond appearances!

One of the most interesting aspect of this kind of special tools owned by the Old Physics Laboratory is the ability to highlight certain principles of physics in a curious way; in this case is the theorem of motion of the center of gravity. Other instruments with a strange behavior are, for example, the Double cone and the Cartesian diver.

To explain the motion expressed by this strange disc the students are encouraged to reach an acceptable explication based on laws and principles of physics discussed in classes. This is a mental exercise of enormous importance.