Physics / Unit 3: Forces and Newton's Laws
3.5 Newton's Third Law
Notes
Newton's third law: when object A pushes on object B, B pushes back on A with a force equal in size and opposite in direction.
- The two forces in a pair act on different objects, so they never cancel each other.
- They are the same type of force (contact with contact, gravity with gravity).
- Equal forces can give different accelerations if the masses differ.
- Example: a swimmer pushes water backward; the water pushes the swimmer forward.
Practice
A truck collides with a small car. How do the forces they exert on each other compare?
Third-law pairs are always equal and opposite, whatever the masses.
Earth pulls a falling apple down with 2 N. What is the reaction force?
The pair is gravity between the same two objects, Earth and apple.
Why don't the action and reaction forces cancel out?
Forces only cancel when they act on the same object.
Ready for more? Take the Unit 3 test.