Gravity and Universal Gravitation 43
Gravity and Universal Gravitation
In the narrowest sense of the term, gravity is the force of the earth attracting objects toward
itself. But the force of the earth comes from universal gravitation between all objects of
mass, not just the earth itself. Between two objects, there is an attractive force proportional
to the product of the objects’ masses and inversely proportional to the distance between
them, raised to the second power. This attractive force is universal gravitation, as discovered
by Newton. It’s called universal gravitation because it works on all objects with mass—it’s not
affected by the type of object. Its value only depends on the mass of objects affecting each
other and the distance between them.
As shown in the figure, when two objects with mass M and mass m are separated from
each other by distance r, a force of F attracts the two objects. The equation is as follows:
G is a universal constant referred to as the universal gravitational constant:
G = 6.67 × 10
(N × m
For an explanation of the unit newton (N), see page 92.
Universal gravitation satisfies the law of action and reaction, as it exerts a force on both
masses M and m. The equation above can be used to calculate the force on either object.
As their directions are obviously opposite, they satisfy the law of action and reaction. Thus,
we should note that forces working between objects at a distance from each other (not just
objects that come into contact with each other) also satisfy the law of action and reaction.
Universal gravitation is a very small force compared to electromagnetic force. While
electromagnetic forces may be attractive or repulsive depending on a combination of posi-
tive and negative charges, universal gravitation always works as attractive force—that is,
objects are always drawn closer to each other.
Because of universal gravitation, cosmic dust in outer space gathers into giant masses
over time—such as the earth or the other planets.
F G =