Cavendish experiment: estimating G

With a set-up known as the Cavendish experiment, students estimate the gravitational constant G from the attraction between masses. This experiment is currently being converted from a traditional “paper-handout” experiment, to a fully developed Lablet activity.

A pair of masses is suspended from a torsion balance. When two large masses are introduced to attract the masses on the torsion balance, the system oscillates until a new resting position is achieved:

A series of derivations guided currently by a paper hand-out — but under construction as a lab activity — shows how the observations of amplitude and phase of the oscillations lead to an estimate of G, the gravitational constant. These small oscillations are projected on a scale by a light reflecting from a mirror attached to the torsion balance.

Projection of light from the mirror on the torsion balance onto the scale

Projection of light from the mirror on the torsion balance onto the scale

Below is a video of the light on the scale, but played at an increased frame rate:

Until the advent of Lablet, this light was tracked manually and in real time (for hours) by students with a stopwatch. With the aid of the motion analysis module, and a newly developed auto-tracking module, the location of the light as a function of time is recorded, and can be automatically obtained:


Oscillations of the torsion balance in the Cavendish experiment, picked from the Lablet motion recording of light tracking on a scale.