Physical Science / Unit 7: Motion and Forces
7.4 Newton's Third Law
Notes
Third law: For every action force there is an equal and opposite reaction force.
- The two forces act on different objects, so they do not cancel each other.
- Example: A rocket pushes gas down; the gas pushes the rocket up. A swimmer pushes water back and the water pushes her forward.
- The forces are equal in size even if the objects have different masses, but the smaller mass accelerates more because $\displaystyle a = \frac{F}{m}$.
Practice
When you push on a wall with 50 N, the wall pushes back on you with:
Action and reaction forces are equal.
A rocket moves upward because:
This is an action-reaction pair.
A small truck and a large truck collide. Which statement is true about the forces during the collision?
Newton's third law applies to every interaction.
Ready for more? Take the Unit 7 test.