8th Grade Science/Section 3: Newton's Third Law
Newton’s third law states that when two bodies interact with one another, they apply forces to one another that are equal in magnitude and opposite in direction.
The third law is also known as the law of action and reaction. This law is important in analyzing problems of static equilibrium, where all forces are balanced, but it also applies to bodies in uniform or accelerated motion.
For example, as shown in picture at the right,a book resting on a table applies a downward force equal to its weight (mg) on the table. According to the third law, the table applies an equal and opposite force to the book (N). This force occurs because the weight of the book causes the table to deform slightly so that it pushes back on the book like a spring.