A practical wording of the definition of inertia is: A characteristic of any object that causes that object, whatever it is, to try to stay in the condition of rest or movement that it is already in. In other words, if the object is at rest, it tries to stay at rest. On the other hand, if the object is moving, it tries to continue moving in a straight line. So inertia might be considered to be two separate states: resting inertia and moving inertia. For our discussion, we are thinking in terms of the combined force of gravity and moving inertia.

Every day we see, and use, some trivial examples of the combined force of gravity and moving inertia to solve simple problems. One example of using this combined force is the farmer that comes from the barn with muddy shoes. To get some of the mud from his shoes, he stamps on the ground. The ground stops the shoe and the foot, but because of the combined force of gravity and inertia, the mud continues downward, thereby at least partially freeing the shoe of mud.

No doubt, you can think of many other examples.

Many years ago, before the development of commercial cold storage, folks would buy relatively large quantities of jumbo sweet onions at harvest time, place them in old discarded, nylon hose, and hang them from the ceiling so that each individual onion was "air cured." What makes this an especially good example is that the method used to get the huge onion to the foot of the hose is substantially analogous to getting a large stone down the ureter!

An onion is pushed a short distance down the hose, allowing the top of the hose to be gripped firmly. Even though the hose is extremely elastic, similar to the ureter in this regard, gravity by itself is not strong enough to pull the onion downward. However, when a sharp downward thrust is made with the hand, hose, and onion, and the hand and hose are abruptly stopped, the combined force of gravity and moving inertia causes the onion to continue downward for a short distance. After a few seconds of time and a few repetitions of the described movements, the onion is moved to the foot of the hose.

THE BEGINNINGS OF A URETER STONE PROBLEM

When a stone leaves a kidney (Ureterolithiasis)  and gets "hung" in the tiny ureter, partially or completely obstructing the normal transfer of urine to the bladder, urine will slowly accumulate above the stone. The urinary system responds to this small bulge of urine by increasing the frequency of peristaltic waves in an attempt to dislodge the obstructing stone. If the stone is not dislodged, the accumulation of urine will continue to slowly build. At some point, a sudden, distinct (but usually mild) pain is felt by the individual. In most cases, a considerable period of time elapses, sometimes several hours, before the distention of the obstructed ureter becomes so great as to cause extremely intense pain. For most individuals, this will be plenty of time to "prep" for the Jump and Bump routine. 

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