part 1:  WARM-UP STRETCHES

 
      The day started out sunny enough, but the forecasters had ranted ominously about rain over the weekend.  The Physics family was never much deterred by rain however.  They were tough.  Most people who knew them would say so.  Most people might also quietly mutter that they clearly had no common sense at all, starting out on a three day backpacking trip with bad weather almost a certainty.  The family liked camping though.  They had made plans.  And if plans were made, rain or no rain, there was no turning back. 
      Father was outside, making a last minute inspection of the van's engine.  He also checked that there was a spare tire in the back.  The elderly vehicle would have to carry five people, a dog, and everyone's heavy packs up a steep road before the family continued their journey on foot.  A breakdown in the middle of nowhere could ruin a pleasant trip. 
      Indoors, Brother and Big Sister were still trying to stuff their warm clothes, rain gear, and sleeping bags into their sturdy frame packs.  Little Sister had already packed her own backpack and was gathering the leash, dog food and water dish for Puppy.  The dog, old enough now to no longer really warrant such a name, crouched uneasily at her feet trying to figure out whether all the commotion meant she would be going along or left behind.  Mother stood in the kitchen, surrounded by boxes and plastic bags.  She had been designated the daunting task of calculating how much potential chemical energy, currently stored in the form of food, would be needed to keep her family active, warm, and in good spirits for three days. 
      When the food had been chosen and neatly rationed out for individual meals it was bagged and added to the packs.  Father had just come back inside as Brother and Big Sister dragged all the backpacks into the dining room and lined them up to be weighed.  Brother commented that he'd actually rather not know how heavy his pack was, but Father felt it was important to check.  This way, he pointed out, if any of them had more (or less) of a burden than they should, it could be remedied before the trip started.
 
      Father hung up his trusty spring scale.  He had installed a ring in a ceiling beam for exactly this purpose.  Grunting, he and Brother lifted his massive backpack up and hung it from the hook at the bottom of the scale.  The gravitational pull of the Earth acted on the mass of the pack exerting a strong downward force on it.  The coiled spring inside the scale was stretched out of its comfortable equilibrium position.  Determined to retain its natural shape, the spring resisted the force pulling down on it.  The further it was stretched the more stubbornly it pulled back.  The weight of the pack was great though, and the poor spring was stretched quite a lot before its own upwards restoring force equaled the downward gravitational force.  The size of this force was indicated by a little pointer attached to the bottom of the spring.  As the spring stretched, the pointer was moved down a vertical scale marked in pounds.  As the spring reached a new equilibrium that included the pack as part of the system, the pointer stopped at the mark labeled "sixty-four pounds": the equivalent of about two hundred eighty-five newtons of force  
                        The spring scale at initial equilibrium.  
 
 
           The spring scale at its new equilibrium. 		       "Ugh," Father muttered as he and Brother hefted the pack back down to the floor.  "I wish we were going camping on the moon!"  The moon, having a smaller mass than the Earth, would exert a weaker gravitational force on the pack.  This meant the pack would weigh less. 
      One by one the other packs were weighed.  The spring snapped back to its original equilibrium each time a pack was removed from the hook.  Too great a force could have permanently stretched the spring so that it would never recover.  None of these packs, however, exceeded this elastic limit so the spring scale could be used again and again.  Father joked that his own pack would probably exceed the elastic limit of his back, not to mention his patience, and transferred several items to Brother's pack which had only weighed forty-one pounds.  "That's exactly why I didn't want to weigh them," sighed Brother, but he gamely took the extra things and stuffed them into his pack's side pockets. 
     "Are we ready to load up?"  Father asked glancing around.  He was answered by the nodding of four human heads and the prancing feet of Puppy who had just noticed that her doggy pack was among the other packs and had concluded that she would not be left behind after all. 
 
      Fitting the packs into the back of the van was a three dimensional puzzle.  As usual, only Mother was able to figure it out successfully.  Once they were in place, Father strapped several bungee cords around them to fasten them to the back seat and van walls.  The rubber of the bungee cords responded to being stretched in the same manner that the spring scale had.  The more force he exerted on them with his hands, the more the cords pulled back trying to return to their original length.  This made them fit snugly around the packs, preventing them from toppling over. 
      Puppy needed no command to jump eagerly into the van.  The house was locked, and then locked once again after three people ran back to use the bathroom.  Finally the Physics family piled into the car as well.  Father turned the key in the ignition, completing a small circuit connected with the van's battery.  The battery put out a measly 12 volts, sending a current through a coil of wire in the engine and building up a strong magnetic field.  After a few seconds a switch broke the circuit, shutting off the current suddenly and collapsing the magnetic field.  The rapid demise of the field induced a current in a second coil of wire with more than a hundred times as many loops as the first.  A voltage almost three thousand times stronger than the original voltage was made sending a well timed spark arcing across the waiting prongs of a spark plug.  Fuel was ignited.  The engine roared to life, turning gears, driving pistons, and poising itself to power everything from the wheels to the CD player.  Father engaged the wheels by shifting the engine out of neutral, and pressed the accelerator pedal.   The raging mechanical beast allowed itself to be steered down the driveway and out to the open highway.
 
[References used on this page: (Bloomfield, 1997),  (Macaulay and Ardley, 1998),  (Bordoff, 2004)]            See Bibliography
 
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