1. Newton's third law is about a pair of forces in a single interaction: if object A exerts a force on object B, then object B exerts a force on object A.
2. The two forces are the same type, such as both contact forces or both gravitational forces.
3. They are equal in magnitude and act in opposite directions.
4. They act on different objects.
5. For example, when a person pushes on a wall, the wall pushes back on the person with an equal force in the opposite direction; the person's force acts on the wall and the wall's force acts on the person.
6. Equal and opposite does not mean equal effect: the person may move back while the wall does not noticeably move, because the wall is joined to the building and has a much larger mass.

Newton's third law and equilibrium

1. In equilibrium the forces acting on that one object are balanced, giving a resultant force of zero.
2. Consider a book resting on a table: the upward contact force from the table on the book is equal to the downward weight of the book.
3. These two forces act on the same object, the book, so they are the balanced forces that keep it in equilibrium.
4. The upward contact force of the table on the book has its own third-law pair: the book pushes down on the table with an equal and opposite contact force, and these two act on different objects.
5. In the same way, a person standing still is in equilibrium because the ground's upward push balances their weight, while the ground pushes up on the person and the person pushes down on the ground with equal and opposite forces.

Comparing balanced forces (first law) and third law pairs

1. Both ideas involve two forces that are equal in size and opposite in direction, which is why they are so easily confused — but they describe completely different situations.
2. Newton's first law describes balanced forces acting on a single object. When they are equal and opposite, the resultant force is zero and the object stays in equilibrium (at rest or moving at constant velocity).
3. Newton's third law describes a pair of forces on two different objects that appear whenever the objects interact, whether or not either object is in equilibrium.
4. The key differences are:
   1. Number of objects — balanced forces (first law) act on the same one object; a third law pair acts on two different objects.
   2. Type of force — balanced forces can be different types (for example a weight balanced by a contact force); a third law pair is always the same type (both contact, or both gravitational).
   3. When they are equal — balanced forces are only equal when the object is in equilibrium; a third law pair is always equal and opposite, even when the objects are accelerating.
   4. Cancelling out — balanced forces cancel to give a zero resultant on that object; a third law pair can never cancel, because the two forces act on different objects.
5. For a book resting on a table, the book's weight (the Earth's gravitational pull on the book) and the table's upward contact force on the book are balanced forces (first law): both act on the book and are equal only because the book is in equilibrium.
6. The third law partner of the table's upward push on the book is the book's downward push on the table; the third law partner of the book's weight is the book's gravitational pull upward on the Earth.

Identifying force pairs correctly

1. A common mistake is to call the weight of the book and the upward force from the table a third-law pair.
2. This is wrong: both of those forces act on the book, so they are balanced forces that put the book in equilibrium, not a third-law pair.
3. For the book on the table, the upward force of the table on the book pairs with the downward force of the book on the table.
4. The Earth's gravitational force on the book pairs with the book's gravitational force on the Earth.