Properties of action and reaction

1. There is no single force in the universe; action and reaction appear together and disappear together.
2. They are equal in magnitude, opposite in direction, and have the same line of action.
3. They have the same nature: if the action is a gravitational force, so is the reaction.
4. They do not cancel each other, because they act on two different bodies. (Forces cancel only if they act on the same body.)

Thinking corner: when a large truck collides with a small car, which receives the larger force? Both receive forces of equal magnitude. The car is damaged more, and gains a larger acceleration (a = F/m), because its mass is smaller.