4.7 Newton's Third Law

Every force occurs as one member of an action-reaction pair of forces.
The two members of an action-reaction pair act on two different objects.
They point in opposite directions and are equal in magnitude.
These forces are always of the same type (e.g., both gravitational, both normal, both frictional). Importantly, because they act on different objects, they do not cancel each other out on a single object, unlike forces typically analyzed in Newton's Second Law.
Mathematically, if object A exerts force $\vec{F}<em>{A\;on\;B}$ on object B, then object B exerts force $\vec{F}</em>{B\;on\;A}$ on object A, such that $\vec{F}<em>{A\;on\;B} = -\vec{F}</em>{B\;on\;A}$ .
Action and reaction forces occur simultaneously; neither causes the other.

Interaction: The mutual influence of two objects on each other.
When identifying forces, the first subscript denotes the agent (source), and the second denotes the object on which the force acts.
It is crucial to distinguish between the forces (equal in magnitude) and the resulting accelerations (which depend on mass via Newton's Second Law). A smaller mass will experience greater acceleration for the same force.

Propulsion: Occurs due to action-reaction pairs.
Walking/Running: Your foot pushes backward on the ground (action), the ground pushes forward on your foot (reaction), propelling you. This involves static friction.
Cars: Tires push backward on the road, the road pushes forward on the tires, moving the car. (Static friction).
Rockets: Rocket expels hot gases backward (action), gases push the rocket forward (thrust, reaction). Works even in vacuum.
Snapping Turtle: Pulls water backward into its throat (action), water pushes the turtle's head forward (reaction).