Mechanics 2: Linear and Rotational Dynamics - 3D Math Primer for Graphics and Game Development - page 560

rule: they are the reason why a reference frame fixed to Earth's surface is
not quite an inertial frame. Experiments such as Foucault's pendulum can
detect the difference, even though it is slight.)
In summary, if a reference frame is accelerating or rotating, the motion
of objects described using that reference frame will not be consistent with
mechanical laws. An inertial reference frame must be stationary or moving
at a constant linear velocity.
12.1.3 Newton's Third Law
Newton's third law is often misunderstood in spite of being the one most
often quoted. It has a certain zen-like justice to it. 5
Newton's Third Law
To every action there is always an equal and opposite reaction. Or, the
forces of two bodies on each other are always equal and are directed in
opposite directions.
This law basically says that there is no such thing as a single unilateral
force. If object A pushes or pulls on object B, then object B always pushes
or pulls back on object A with a force of the same magnitude but opposite
direction. If gravity is pulling me towards Earth, it's also pulling Earth
towards me! A force is always part of an interaction between two bodies.
In diagrams, we often draw a force as an arrow, since it is a vector. But
really, these diagrams would be more accurate if both ends of the arrow had
arrowheads. When we leave off the other side of the arrow, it's because it
is acting on an object in which we have no interest. When you see a single-
sided arrow that represents a force in a diagram, you can always fill in the
other half in your mind.
One source of misunderstanding of Newton's third law is the word “re-
action.” The purpose of this word is to describe the forces as being in
opposition to one another. It is not meant to imply a causal link between
them; neither force is a “cause” or “effect.” The two opposing forces act
simultaneously and, so far as the laws of physics are concerned, have equal
status.
But aside from this mistaken inference of cause and effect, the third
law is just plain counterintuitive. Let's say a guy named Moe pushes a
5 One could easily imagine Yoda saying “To every action, always an equal and opposite
reaction, there is.”