Book: # 5

Newton's law of universal gravitation is F=Gm1m2/r*r where F is the force between two point masses m1 and m2 which are separated by a distance r.  The object of the experiment is to determine G.

Small lead balls, each of mass m, are supported on a T-shaped frame which is suspended by a fine wire to form a torsion pendulum.  Large lead spheres each of mass M are placed as shown, exerting a torque on the moving system.  The system is allowed to come to rest, and the equilibrium position is shown by a light beam reflected to a scale.

The large lead balls are then shifted to give an equal and oppositely directed torque, and the system comes to a new equilibrium position.  The approach to equilibrium is a damped SHM and requires over 1 1/2 hours.  The motion of the scale indicator is shown in time lapse in the final segment of the film.  The final measured position is just under 58.0 cm and the deflection is 57.95 cm - 51.8 cm = 6.15 cm on a scale 154 cm from the mirror of the moving system.

In the static method of analysis, the deflection S caused by shifting the large balls is measured between the two equilibrium positions.  If the scale is distant L from the mirror, then S/L is twice the angle T through which the system has turned.  To convert this angle into torque, the torsion constant (defined as torque per unit angular deflection) of the suspension wire is found indirectly from the suspended system's moment of inertia I and its period T when oscillating as a torsion pendulum. 

SEE DEMO BOOK 6 for description of equations.