of a horizontal rod with a small sphere of equal mass at each end. The dumbbell-shaped rod was
suspended at the center by a sensitive twistable fiber. Large lead spheres of known mass were placed at
the ends of the rod, near the small spheres, in such a way that the gravitational pull between the small
and large masses caused the rod to rotate horizontally. Knowing the weights, the distances of the
masses, and the torsion of the twisted fiber, Cavendish computed the value of G to be 6.754 x 10-8 or,
written out, 0.00000006754 centimeter-gram-second (cgs) units. This constant is the force in dynes that
is exerted between two masses of 1 gram each with centers 1 centimeter apart. Cavendish's value is very
close to the latest accepted value of 6.673 x 10-8 cgs units determined in 1942 by Dr. Paul R. Heyl at the
National Bureau of Standards. Dr. Heyl used a refined version of Cavendish's device.
d. Using the following formula we can determine the acceleration (a) of mass m2 due to the
attraction of mass ml by dividing F, the force of attraction, by mass m2. (In Newton's second law of
motion, force is the product of mass times acceleration). Any force acting on a unit mass creates
acceleration, which gives a measure of the gravitational attraction at any point on the earth's surface.
e. The pull of gravity is usually expressed in terms of the acceleration of a freely falling body,
expressed as the rate of increase of velocity per unit of time. A G-force of one is the force required to
accelerate any freely movable body at the rate of about 32.16 feet per second per second. A G meter
measures the G force in aircraft. A G is equal to approximately one thousand gals, a unit of gravity
measurement named after Galileo. The gal has an acceleration of one centimeter per second per second
. The acceleration
of gravity over the
earth's surface ranges from approximately 983 cm/sec2
or 983 gals at the poles to 978 cm/sec2 or 978 gals at the equator. Theoretically, if you were to jump
from an airplane above the equator, you would fall faster and faster, gaining speed at the rate of 978 gals
(about 32 feet) every second. At the end of two seconds, you would be falling at the rate of
approximately 1,956 centimeters (about 65 feet) per second. After each succeeding second, the speed
increases at a rate of 978 gals per second.
f. A smaller unit of measurement used in gravity measurements is the milligal, or one-thousandth
part of a gal. It is used when dealing with variations in acceleration equal to one-millionth of one G.
Modern instruments go even beyond this, measuring acceleration changes of one-billionth of a G or to
one-thousandth part of a milligal.
g. Gravity can be defined as the attraction that the earth has for every particle on its surface. The
force of gravity holds all objects on the earth in place and prevents them from flying off into space as the
earth rotates. Gravity is what brings back to the ground a golf ball hit into the air and makes water flow
downhill. It is because of gravity that all things on earth possess weight. An average person will weigh
about 1 pound more at the poles where the attraction is greater than he will weigh at the equator where
the attraction is weaker. The variation of the earth's gravity, with respect to latitude, is caused by two
factors--the rotation of the earth and the earth's ellipsoidal shape. The