A historical torsion-balance experiment was designed to compare the inertial mass and…
PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
A historical torsion-balance experiment was designed to compare the inertial mass and gravitational mass of a small test object. In the modern version of the experiment, two identical small spheres are suspended at opposite ends of a lightweight rod that can rotate about its midpoint on a thin fiber. Massive lead spheres are then brought near the small spheres to create a measurable gravitational attraction that twists the fiber. Rotational oscillations of the rod are monitored precisely.
(a) Explain how the design of this experiment allows the measurement of gravitational mass for the small spheres.
(b) Describe how the same apparatus, without the nearby lead spheres, can be used to measure the inertial mass of the small spheres.
(c) Discuss why the equality of the two measured masses is essential for interpreting everyday weight measurements as indicators of the amount of matter in an object.
(d) Suppose the gravitational mass of an object were found to differ from its inertial mass by 1%. Predict two observable consequences in common laboratory or astronomical settings, and justify each prediction using qualitative reasoning.