Module 4—Gravitational Force
Lesson Summary
In this lesson you explored the following questions:
- How does gravity affect planets, moons, and other objects in the solar system?
- What does the law of universal gravitation tell you about gravity?
- Is the force of gravity a constant?
Newton’s law of universal gravitation can be used to determine the following:
- an object’s weight at any point relative to the source of the gravitational field
- the acceleration due to gravity at any point relative to the source of a gravitational field
- the force of attraction between any two masses
The universal nature of this law is useful for understanding planetary and satellite motion away from the surface of Earth.
Newton’s law of universal gravitation states the gravitational force of attraction between any two masses is proportional to the product of the two masses and inversely proportional to the square of the distance between them. It is summarized mathematically as
Quantity |
Symbol |
SI Unit |
| force of gravity | Fg |
N |
| mass | m |
kg |
| separation between centres of masses | r |
m |
| gravitational constant | G |
 |
Applying Newton’s second law to the equation gives the following
expression for the acceleration due to gravity (a) at a distance (r)
from the centre of the mass (m2) creating the field.
Lesson Glossary
Newton's law of universal gravitation: a law that states the gravitational force of attraction between any two masses is proportional to the product of the two masses and inversely proportional to the square of the distance between them
It is summarized mathematically as
universal gravitational constant: the value G in the equation summarizing Newton’s law of universal gravitation 
.