NEWTON'S LAW OF UNIVERSAL GRAVITATION

 motion: TOC for Knowledge Concepts, Exercises, and Solutions

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F = GMm/r^2   (gravitational force = G * mass of primary * mass of secondary

a = GM/r^2    (gravitational acceleration caused by an object = G * mass
of that object / orbital radius squared)

When a primary object attracts a secondary object, the force between them
follows an inverse square law with distance, and is proportional to the
product of both masses.  The constant of proportionality, called the
"gravitational constant," is 6.67 * 10^-11  m^3 kg^(-1) s^(-2).
Note that, since F = ma for all systems, the acceleration caused by gravity
can be derived from this formula.

The gravitational force of an object is known as its weight.  This can be
confusing, because in most of the world we refer to our weight in mass units (for
example, "I weigh 66 kg; I've lost 2 kg since I started my diet").  In the United
States, we have the opposite problem: weight is normally given in pounds, which is
a unit of force, but is commonly used as a unit of mass (for example, "I weigh 145
pounds; I've lost 5 pounds since I started my diet").  Avoid befuddlement; in your
mind, keep the vernacular weight straight from the gravitational force weight!

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