It's professor Dave, let's talk about
Newton's third law.
After learning about Newton's first law of
motion regarding inertia, as well as the
second law regarding the proportionality
of force and acceleration, we are ready
to round out the trio and look at
Newton's third law of motion.
This is most famously stated as follows:
for every action there is an equal and
opposite reaction.
While this is often quoted out of context
for metaphorical purposes, in physics it
means something specific and
quantitative. It means that if two
objects interact, the forces they impart
on each other will be equal in magnitude
and opposite in direction. Think of a car
hitting a brick wall. The car imparts
an action force on the wall from its
motion, and the wall imparts a reaction
force right back on the car, bringing it
to a quick stop. These vectors are equal
in length and point in opposite
directions.
What this also means is that there can
never be a single isolated force, as any
force is an interaction. Forces must
always exist in pairs. As you sit in your
chair right now, the force of gravity is
pulling you down towards the center of
the earth, but something called the
normal force points straight up with the
same magnitude, which is why you remain
perfectly still.
We will learn more about this force later.
The third law can sometimes be
confusing because it would seem to imply
that no object could ever move, since any
force accelerating an object must be
paired with another in the opposite
direction. For this reason we must
understand that when assessing the
motion of an object we consider only the
forces acting upon that object and not
the forces that it applies on other
objects. When hammering a nail into a
piece of wood, the hammer imparts force
onto the nail and the nail imparts
force onto the hammer.
But the nail is driven into the wood
because there is a net force acting upon
the nail from the motion of the hammer.
Likewise, the hammer has two forces
operating upon it, the downward force
from your arm and the upward force from
the nail.
That's why when you strike the nail the
hammer stops moving at roughly the
location where the impact occurs. So if
we are looking at the nail we only
consider the forces acting on the nail,
if looking at the hammer we only
consider the forces acting on the hammer,
these are different systems and this is
the context in which the third law is
best understood.
Let's look at another example. When a
rock falls to the ground the
gravitational force from the earth
causes the object to accelerate towards
the Earth, but it is also the case that
the earth is accelerating towards the
rock. This must be true
according to the third law. Of course we
don't notice this in everyday experience
because the acceleration of each object
is inversely proportional to its mass.
The rock is very light so the rock
accelerates quickly. The earth is very
heavy so earth's acceleration is
completely negligible. This may seem a
bit abstract but it is important to be
able to rationalize forces in this way.
So there you have it, the law of inertia,
F = ma, and action/reaction. That
wraps up our discussion of Newton's laws
of motion, so let's check comprehension.
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