2.13 - Newton's 3rd Law

Newton's third law and objects in equilibrium

Newton's third law: when two objects interact, the forces they exert on each other are equal in size and opposite in direction, and act on different objects.

A person pushing a wall

• When a person pushes on a wall, the wall pushes back on the person with a normal contact force of the same size in the opposite direction.
• These two forces are a third-law pair:
  • Same type of force (both contact)
  • Equal in size, opposite in direction
  • Act on different objects (one on the wall, one on the person)

A book resting on a table

• A book resting on a table is in equilibrium. Its weight pulls it down, and the normal contact force from the table pushes it up.
• These forces are equal in size and opposite in direction, but they are not a third-law pair:
  • They're different types of force (one gravitational, one contact)
  • They both act on the same object (the book)

The actual third-law pairs are:

• The Earth pulls the book down with gravity, and the book pulls the Earth up with an equal and opposite gravitational force.
• The table pushes up on the book with a normal contact force, and the book pushes down on the table with an equal and opposite normal contact force.