Teach Fzx - Newton's Third is Impossible

Newton, 1700, Genius

Newton used multiple choice to arrive at his Third Law of Motion.

Galileo had already proposed the concept of inertia more than half a century earlier and Newton was working forward from that idea.  Newton knew that an object with smaller inertia (mass) accelerates proportionally more than an object with larger inertia if the same net force is applied so he considered collisions between two objects and only had a few choices.

A.   The amount of inertia (mass) determines which object applies a larger force to the other.

B.    The speed of an object determines which applies a larger force to the other.

C.    The momentum of an object determines which applies a larger force

D.   Both objects will always apply the same size force to each other no matter what.

My students can finish the sentence "For every action, there is ...."  
("an equal and opposite reaction.")

Aristotle - 350 BC - Fzx Moron

But we all agree with him

These same students will answer that the larger mass object will apply a larger force (A).  They get a bit confused when the smaller mass object is moving fast (B), but the answer that makes sense to my students is the same that Aristotle would pick (A).  He was also dead wrong on everything else physics.

Isaac Newton was the Einstein who finally figured out that we always misinterpret what we see in collisions and other force interactions.  It’s not a problem of observation - it’s a problem of interpretation.

We see the baseball rocketing off the bat.  We observe the small car crushed after the crash with the barely damaged SUV.  The soccer ball flies off the foot of the goalie to the other end of the field.  The bullet has a huge velocity and the rifle only recoils a little.  We interpret these situations by proposing a difference in the force applied to the two objects.  We’re wrong.

Equal magnitude forces on each

(goo.gl/9GskLN)

The difference we see, as Newton in his genius realized, does not result from a force difference on the two objects, but only a difference in mass (inertia). 

Newton’s process was probably like one of Einstein's famous thought experiments on relativity.  Since he couldn’t test it directly, Newton had to consider the options and pick the best answer – multiple choice and the process of elimination.

His answer?  There is no reason to conclude that our universe prefers a larger mass or a larger speed or a larger momentum.  Experiments, in fact, eliminate these possibilities.  The only conclusion possible is that

Any two objects will always apply equal magnitude opposite direction forces to each other.

No matter the speed.  No matter the mass.  No matter the situation.

Invariably.  A law of our universe.

Same size forces.
Different effect.

(goo.gl/v3lhPM)

It's connected to the Law of Conservation of Momentum.  It's connected to the other laws of motion.  Newton's Third Law is a simple idea, but an extremely difficult concept to grasp when applied to real situations.  

I taught Newton’s 3^rd Law for a few years before I finally came to understand it and I still get it wrong sometimes.  Imagine the difficulty a physics student has.  It is rocket science after all.