6.2. Universal Law of Gravity http://www.ck12.org
6.2 Universal Law of Gravity
- Describe and give the formula for Newton’s Universal Law of Gravity.
- Using Newton’s Law of Gravity and the equations of uniform circular motion, solve problems involving the
force of gravity.
In 1798, Henry Cavendish designed and created an apparatus and experiment to determine the density of the planet
and the value of the gravitational constant,G. His apparatus involved a light, rigid rod about 2-feet long with two
small lead spheres attached to the ends. The rod was suspended by a thin wire. When the rod rotated, the twisting of
the wire pushed backwardsto restore the rod to the original position.
Force of Gravity
In the mid-1600’s, Newton wrote that the sight of a falling apple made him think of the problem of the motion of the
planets. He recognized that the apple fell straight down because the earth attracted it and thought this same force of
attraction might apply to the moon. It further occured to him that motion of the planets might be controlled by the
gravityof the sun. He eventually proposed the universal law of gravitational attraction as
F=G
m 1 m 2
d^2wherem 1 andm 2 are the masses being attracted,dis the distance between the centers of the masses,Gis the universal
gravitational constant, andFis the force of attraction. The formula for gravitational attraction applies equally to two
rocks resting near each other on the earth and to the planets and the sun. The value for the universal gravitational