School of Physics Lecture Demonstration Manual

Mk-1 Centre of Mass Rod

Published: Thursday 09 March 2006 - Updated: Thursday 24 March 2011


To demonstrate that the motion of the centre of mass of a body in projectile motion is a parabola.


Centre of mass rod


Click on pictures to enlarge.

Centre of Mass Rod Pic

Centre of Mass Rod Diagram


The apparatus consists of a rod with unequal masses attached at each end. A battery operated light globe may be slid along the rod to coincide with the centre of mass.

The lecture room is darkened and the device is thrown across the lecture room. If the globe does not coincide with the c. of m. the resulting motion of the globe can be quite complicated. Repeating the experiment with the globe at the c. of m. results in a parabolic path being traced by the globe.

Safety notes


top of page