Topic 3 of 7
Pairs of forces
When two bodies interact, each exerts a force on the other. The two forces have equal magnitudes and opposite directions.
A free-body diagram contains forces on one body. An interaction pair spans two bodies, so its members belong on different body diagrams.
These are often called action-reaction pairs. The forces occur together; "reaction" does not mean a later response. Identify the pair by naming both bodies and then reversing their roles.
One contact interaction, two different bodies. Only the paired contact forces are shown.
The table pushes the book upwards
Selected body: book
The book pushes the table downwards
Selected body: table
Balanced forces and interaction pairs answer different questions
| Two forces that balance on the book | A contact interaction pair |
|---|---|
| Table on book, upwards. Earth on book, downwards. | Table on book, upwards. Book on table, downwards. |
| Both act on the same body. Their resultant on that body is zero. | They act on different bodies. They belong to the same book-table interaction. |
The table's support and the book's weight can be equal and opposite, but that alone does not make them an interaction pair. One comes from contact with the table; the other comes from gravitational attraction by Earth.
The partner of Earth's gravitational force on the book is the book's gravitational force on Earth. These also have equal magnitudes and opposite directions. Earth's very large mass means its resulting acceleration from this interaction is extremely small.
Worked explanation
A hand pushing a wall
A hand pushes horizontally on a wall with a force of 12 N. The wall exerts a 12 N force on the hand in the opposite direction.
On a diagram of the hand, include the wall's force on the hand. The hand's force on the wall acts on the other body, so it does not belong on that diagram. The pair does not cancel within the hand's force diagram.
Equal forces do not require equal accelerations of the two bodies. Their masses and the other forces acting on each body matter.
Name the interaction, not just the arrow directions. "Force on the book by the table" pairs with "force on the table by the book". Swapping the bodies keeps you from confusing this pair with the book's weight.