part3

PART 3: PUTTING A FOOT FORWARD

       Clouds were beginning to form over the mountain peaks as the Physics family unloaded their gear from the back of the van. Puppy bounded happily around the parking lot until Little Sister cornered her and buckled on the doggy pack. The bags hung over Puppy's shoulders, so that the force of their weight was supported by her strong front legs rather than her spine.
       The family members helped each other lift and strap on their packs. The external frame packs raised each person's center of gravity, but were designed to keep it over their feet so the hikers would not have to lean forward a lot to counteract the weight added to their backs.
      "All saddled up?" Father asked.
      "Looks like it," responded Big Sister distractedly while trying to adjust her pack straps to more evenly distribute the pressure on her shoulders.
      "We're locked, cocked, blocked and ready to rock!" Brother chimed in. Little Sister groaned at his way of constantly quoting computer games. Mother wandered around the van double checking all the door locks.

Modern external frame packs are designed to keep the center of gravity high up and close to the body. The weight of Puppy's pack is over her front legs. These designs make the packs safer and more comfortable for the wearers.

      The family started up the trail. Puppy ran in excited circles around them sniffing at everything they passed. "I think her pack isn't heavy enough," mused Father.
      The narrow dirt trail dipped gently down at first, headed for a small river crossing. Step after step, the family members applied a backwards force to the ground with their feet. Friction kept their feet from slipping and the ground pushed back on them with equal force. A human's mass, even with a heavy pack, is far less than the mass of the Earth however, and so it was the people who were moved forward. Their footsteps did not affect the position, rotation, or orbit of the Earth in any way that could be noticed.
      Their feet collided with the ground over and over. Inside their hiking boots, a complicated system of bones, tendons and muscles acted first as a cushion to absorb the force and then as a lever to move its owner forward. As a foot hit the ground its curved arch flattened and expanded under the downward pressure, storing energy to be released again as it was lifted. As it left the ground again for the next stride it rolled slightly to the outside, tightening tendons and locking bones into an effective lever. Coming back in contact with the ground it rolled slightly inwards, loosening the tendons, unlocking the bones, and once again becoming a squishy pad to absorb the force of the impact.

Little Sister and Puppy at the beam bridge.

At the river, they crossed on a bridge consisting of narrow wooden boards that were suspended from the two riverbanks. As they crossed, their weight was supported directly by the riverbanks at each end of the boards. The top of the boards were compressed slightly and the bottom stretched as they sagged under the hikers' weight. Their internal strength was great enough to keep the bridge from breaking though, even when Father crossed with his overloaded pack. Puppy wanted to swim across, but Little Sister grabbed her collar and forced her to walk across the bridge so as not to get her pack contents wet.

The green and red arrows are the forces acting on the bridge. Their thickness represents the size of the force.

The work involved in moving their bodies and heavy packs over the great distance they were traveling caused them to expend their chemically stored energy at a furious rate. Within a few hours both sisters were complaining of being hungry, and the others did not try to argue when Mother suggested that they stop for lunch.
"Eat up. Eat up," said Father passing around the bag containing the lunch. "The more you eat, the less I have to carry."

Making switchbacks. The red arrow points in the direct
uphill direction. The blue arrows show the family's path.
The thickness of the arrows represents the size of the
force. The length of the arrows represents the distance
over which the force is exerted.

Just beyond their lunch spot, the slope steepened sharply. The family trudged slowly up the hillside making a zigzag path. The actual energy expended would be the same if they went straight up, but going at an angle to the slope lengthened the distance which allowed them to use a much smaller force to do the same amount of work. Puppy started to run straight up the hill but quickly discovered that applying a very large force over a short distance was no fun and turned to follow her humans.
They climbed slowly, stopping often for drinks of water. The clouds overhead thickened and grew dark in a way that did not look too friendly. Mother scanned the sky and commented that they'd better keep moving if they wanted to set up camp before it started raining. Feet obediently levering against the ground over and over, they pressed on.

[References used on this page: (Weiss, 1991), (Epstein, 1994), (Morrissey, 2004), (Bloomfield, 1997)] See bibliography PREVIOUS PAGE MAIN PAGE NEXT PAGE