On The Subject Of Gravity - Potential And Kinetic Energy

Let us assume that the operating principle of gravity is that matter is attracted to all other matter, and that the degree of attraction is influenced by mass and distance.  It's physical effects are demonstrated with every foot that reliably hits the ground whilst walking, and every Frisbee that consistently falls to Earth after being thrown at a basket by a Nerd on a Saturday morning.  

 

Still, it sounds pretty boring compared to the other "forces" that hold the universe together.  By comparison, electromagnetism, the strong nuclear bond and the weak nuclear bond are quirky, sexy, and intriguing: all the things that gravity is not.  In fact, gravity's primary virtue seems to be just showing up, and having its affect with a singular regularity.

 

I've been thinking about Gravity lately, particularly as it affects me, particularly when I'm reading in bed, get sleepy and drop my Kindle across the bridge of my nose.  That's the thing about Gravity: it works its magic on us at a personal level that the other Forces simply can't.  It is passive-aggressive, albeit insensate.

 

Or is it?

 

Gravity is with us every day, hour, minute and second.  I drop a Kindle on my Schnozz, wake up cursing, and try to decide what third party is responsible.  But what if my face had not been in the way?  Or the ground for that matter?  What precisely would happen if I drilled a hole straight through the middle of the planet, then dropped my Kindle into it?  Since this was not remotely practical, I dumbed the proposition down to an Asteroid and a Rock, with both of such uniform shape and composition that gravity would have a completely regular effect on both.  In this scenario, what would happen to a perfectly round rock that had been dropped down a perfectly drilled hole through said asteroid? 

My guess would be that it should fall some distance past the center of the Asteroid, based on the momentum it built up as it fell.  It also seems that the gravitational pull sustaining it would grow less strong in the direction of its flight the closer it got to the center, since progressively more of the mass that caused the rock to fall towards the center in the first place would be "behind" the rock as it fell.  This would exert an opposing force to the rock's momentum, thus slowing its rate of descent.  Of course, the term "descent" would be irrelevant once the rock passed the mid-point.

 

Also, the closer the rock got to the center, the more equal would be the gravitational influence of the entire mass of the asteroid upon the rock.  In fact, at the center of the asteroid, the pull of gravity should be exactly equal in all directions throughout a 360 degree axis perpendicular to the tube that it was falling through.  In theory, if you placed a rock at the exact middle of the tube that traversed the asteroid, it should just float there, equally attracted to all the mass that surrounded it.  Contemplating this rationale further, could we argue that the influence of the asteroid's mass upon the rock would be less than uniform because of the lack of mass directly in front of and behind the rock as it travelled through the asteroid?  We could not.  Even though there was no mass in the open tube to influence the rock coming and going, gravity would still uniformly affect the rock from every direction  because the mass surrounding the tube would exert an equal influence in every direction.

 

What eventually would happen to the rock?  Its momentum should carry it some distance past the center, but that momentum would be lost as the mass behind it became greater; eventually if should stop, and then fall back in the direction from whence it came.  Each successive trip past the center would cause it to shed the energy built up, the ever smaller iterations past the center depleting over some period of time, the energy "lost" would be in successively smaller increments until at the end the rock would appear to be doing nothing more than vibrating - it's last tiny bits of energy being bled into the asteroid - until it finally came to rest, floating and motionless, in the exact middle of the tube, and the exact middle of the asteroid.

 

It's interesting to note that the latent "energy" supposed to be in the rock before it was dropped and expended during its trip back and forth through the Asteroid might be nothing of the kind.  My teachers referred to this latent energy in every object at rest as "potential energy", so as to give some substance to the notion of why a rock, otherwise undisturbed on the ground,  would fall down a hole in the first place.  They also referred to the energy imparted to a moving object by some external force - say, the gunpowder in a bullet - as "kinetic energy".  But what if those two things were not true?  What if objects were entirely without the capacity to store energy at all?  For example, what if Potential Energy is nothing more than the attraction that every molecule in the universe exerts on another, and that if there is some mass that has a lot of them - my asteroid - somewhere in proximity to a mass that has less of them - my rock - that there might be some demonstrable attraction between the two: in this case, the rock appearing to "fall" out of my hand towards the asteroid, and then down through my perfect hole?

 

Those progressive swings back and forth through the tube, ending as I posit in stillness, represent not some potential energy in the rock itself, but the opportunity for two objects to allow Gravity to work its influence on both of them to some null state: in this case, the rock floating, and still, at the center of the asteroid.  But what are we to make of the fact that the Rock made it past the center of the Asteroid in the first place?  Surely it must have had some kind of kinetic energy built up in its mass that allowed it to travel beyond the mid-point, even though the entire mass of the Asteroid would be pulling on it equally. 

 

My conclusion is that the Rock - initially invested with movement by the effects of Gravity from the Asteroid - had only the appearance of some kind of stored energy enabling its flight.  As opposed to the conventional wisdom of Kinetic energy though, what if some other forces were at work?  What if the whole concept of Gravity involves not only attraction, but some element of repulsion as well?  Rather than believing that a moving object actually had stored energy that propelled it, wouldn't it make more sense that the moving object was simply influenced by a Gravity model more complex than the one we had constructed?

 

The notion of repulsion is not without precedent: electromagnetism demonstrates the principle every time two magnets are brought in proximity to one another.  If the positive pole of one magnet is presented to the positive pole of another, they are repulsed.  if positive is presented to negative, they are attracted to each other.  So, if every molecule in my Rock and Asteroid had a positive and negative property - and if the nature of those properties was effected by its proximity to any mass and expressed as movement, the Rock might conceivably "fall" past the mid-point, and then return in the opposite direction.

 

The notion that Kinetic energy is imparted to an object is based on the much-documented fact that the larger the source of energy employed to propel an object - say, the gunpowder charge in a bullet - the greater the effect on the object.  In this case, a bullet propelled by a larger charge would go farther than a bullet with a lesser charge.  But, what if the explosion, regardless of its size, didn't impart energy directly to the object, but merely altered its relationship to other objects, as affected by Gravity?  

 

One final thought: in this example, we have spoken exclusively of the effect that the Asteroid has on the Rock.  Given its much greater mass, this is substantially but not completely true: the Rock - even though tiny by comparison - draws the asteroid toward it just as surely as the Asteroid attracts the rock.  The only difference is a matter of degree.  We'll talk about that more in a later installment.