Newton's Third Law
Newton's Laws & Momentum - OCR A-Level Physics
Key Definition
Newton's Third Law
If object A exerts a force on object B, then object B exerts a force back on object A that is the same size, the same type, and acts in the opposite direction. Forces always occur in pairs, called Newton's third law pairs.
If object A exerts a force on object B, then object B exerts a force back on object A that is the same size, the same type, and acts in the opposite direction. Forces always occur in pairs, called Newton's third law pairs.
The four required conditions
For two forces to be a third-law pair, all four must hold:
- Same size (magnitude).
- Same type (both gravitational, or both friction, or both normal contact, etc.).
- Opposite in direction, along the same line of action.
- Act on different objects.
$$F_{A \to B} = -F_{B \to A}$$
- Foot pushes back on the ground (friction force); ground pushes forward on the foot (friction force). This is how you walk.
- Rocket pushes hot gas downward (contact force); gas pushes rocket upward (contact force). Same type, equal size, opposite direction, different objects.
- Earth pulls the Moon (gravitational force); Moon pulls the Earth (gravitational force). Same size; the Moon's huge orbit is set by its small mass, not by a weaker force.
- Because the two forces act on different objects, they never cancel out on a free-body diagram for one object.
Common Mistake
HIGH
Wrong: Identifying weight and normal contact force on a book resting on a table as a third-law pair.
Right: Both forces act on the SAME object (the book), so they fail the "different objects" condition. They are also different types (gravitational vs contact). The third-law partner of the book's weight is the gravitational pull of the book on the Earth. The third-law partner of the table's normal force on the book is the normal force of the book on the table.
Right: Both forces act on the SAME object (the book), so they fail the "different objects" condition. They are also different types (gravitational vs contact). The third-law partner of the book's weight is the gravitational pull of the book on the Earth. The third-law partner of the table's normal force on the book is the normal force of the book on the table.
Examiner Tips and Tricks
- To identify a third-law partner, swap the two objects in the sentence. "Earth pulls book" pairs with "book pulls Earth", not with "table pushes book".
- The two forces in a third-law pair always have the same TYPE. If one is contact and the other is gravitational, they are not a pair.
- Conservation of momentum follows directly from Newton's third law (equal and opposite forces act for the same time, giving equal and opposite impulses).
Two side-by-side free-body diagrams of person and ground: arrow on foot pushing backward on ground; arrow on ground pushing forward on foot. Label as third-law pair.