Newton's Law of Universal Gravitation

Isaac Newton is one of the greatest scientists that ever lived.  Among other things, he formulated a Law of Universal Gravitation that allows you to calculate the gravitational attraction between two objects.  With a little thought you can understand why certain variables appear in Newton's Law and why they appear in either the numerator (direct proportionality) or in the denominator (inverse proportionality).

You probably intuitively understand that the gravitational attraction between two objects (M and m in the figures) depends  on the distance between the objects.  The  gravitational force becomes weaker the further away the two objects are from each other.  The law of universal gravitation is actually an inverse square law, the gravitational attraction between two objects is inversely proportional to the square of the distance between the two objects.

In the bottom part of the picture above and in the upper part of the figure below we see that the attractive force also depends on the masses of the two objects.

The complete formula is shown at the bottom of the page above.  G is a constant.  On the surface of the earth G, M, and don't change.  The gravitational acceleration, Rg, is just the quantity  [G times Mearth divided by ( Rearth )2 ].  To determine the weight (on the earth's surface) of an object with mass m you simply multiply m x g

The figure below gives the Metric and English units of mass and weight.  You have probably heard of pounds, grams, and kilograms.  You might not have heard of dynes and Newtons.  Unless you've taken a physics course, you've probably never heard of slugs.