Friday, September 11, 2015

Newton's Third Law: "I Can't Stop Pushing Back!"

At some point in my thirties, I injured my shoulder playing soccer; it slammed to the ground after a nasty tackle in my 'friendly' soccer league.  After some nagging from my wife and a few of my students (who got to see what my left hand writes like at the board), I went to get some X-Rays of my right shoulder region taken.

Once in the room with the young radiologist, I was shown where to stand, somewhere between an emitter and receiver of the X-Rays.  She needed to make slight adjustments to my position between exposures.  She would place her hand on my shoulder and/or hip, and push or pull me to the right spot.  And each time it was time to move me, she would state, "Sir, please let me move you."  Eventually, this became, "Sir, please relax and let me move you."  Her frustration seemed to grow as she finally uttered, "Sir, please stop resisting being moved."

I must say, throughout all of this, it took every ounce of my patience to not say: "Enough! I can't take it anymore! Let me explain some basic laws of mechanics to you..."

First off, you cannot push or pull me without being pushed or pulled yourself.  That is Newton's third law, which, if ever violated, would send shock waves through the scientific community.  So, if you expect to push or pull me without getting pushed or pulled back, you must be new at this job.

Second, the extent to which I push back when you push me is related to my inertia.  The amount that you accelerate me from rest with your force has a limit.  It is not a matter of "relaxing my muscles" - the only way you will feel no resistance is if I have no mass.  By saying "Stop resisting being moved," you are essentially telling me to "Stop having mass."

In the end, I thought it would be smart not to lecture the radiologist about Newton's Laws of Motion.  I held back my urge to scream "Have you never heard of Isaac Newton?"  After all, she was in charge of the amount of high-frequency electromagnetic waves that would be periodically travelling through my body.  She could have responded, "Have you never heard of James Maxwell?"