197
neighboring small ball toward us and while the small ball on the left would
be pulled away from us. The thread from which the small balls hung would
thus be twisted through a small angle, and by calibrating the twist of the
thread with known forces, the actual gravitational force could be deter-
mined. Cavendish set up the whole apparatus in a room of his house,
nailing all the doors shut to keep air currents from disturbing the delicate
apparatus. The results had to be observed through telescopes stuck through
holes drilled in the walls. Cavendish’s experiment provided the first nu-
merical values for G and for the mass of the earth. The presently ac-
cepted value of G is 6.67x10
-11
N
.
m
2
/kg
2
.
The facing page shows a modern-day Cavendish experiment con-
structed by one of my students.
Knowing G not only allowed the determination of the earth’s mass but
also those of the sun and the other planets. For instance, by observing the
acceleration of one of Jupiter’s moons, we can infer the mass of Jupiter. The
following table gives the distances of the planets from the sun and the
masses of the sun and planets. (Other data are given in the back of the
book.)
average distance from
the sun, in units of
the earth's average
distance from the sun
mass, in units of the
earth's mass
sun—330,000
mercury0.38.056
venus.72.82
earth11
mars1.5.11
jupiter5.2320
saturn9.595
uranus1914
neptune3017
pluto39.002
Section 10.5Weighing the Earth