part8

PART 8: FINAL STOP

Enjoying the sun, the family took their time hiking back. They stopped to admire the view, look at interesting rocks, take pictures, eat lunch, and inspect the old ruins of a miner's cabin. They reached the van by mid afternoon. Happy and exhausted, they loaded their packs into the back, securing them once more with the elastically resisting bungee cords.
Driving home always seemed shorter to the family than driving out somewhere. They talked and slept and sang with the CD player. The van wound its way down the steep dirt road and back out to the highway. The moon still tugged at the water making tides along the shores. The sun still sent its energy to the planet and illuminated their path. The Earth still spun on its axis. None of it seemed to care that five people had been missing from society for three days.

Approaching a curve in the road, Mother applied a small torque to the steering wheel causing it to rotate. Because the steering wheel had a much greater radius than the steering shaft, she barely had to supply much force at all. Her small force was applied over a long distance as she turned the wheel rim. The force was increased at the shaft in the center because it did not have to move as far. This amplified force was used to turn a gear at the other end of the shaft.
The teeth of the gear fit into a rack of teeth on a horizontal bar running between the steering arms of the front wheels. As the gear turned, it moved the rack to the side. The rack moved the bar which pushed on the steering arm lever of the right wheel and pulled on the lever of the left wheel.

                    The width of the arrows
               represents size of force. The
                  length represents distance.

The front tires pivoted around to the left. The tires continued to spin, exerting a backwards force on the road. The road continued to exert a forward force on the tires. Now the tires were pointed in a new direction however and so the direction of the force acting on them was at an angle to the van's original motion. Friction kept the vehicle from continuing on its original straight path. It was easier for the tires to roll forward on the road than to slide sideways. Without the friction, the van could not have changed directions. Inside the van, the people felt themselves pressed towards the outside of the curve as their bodies tried to keep going in a straight line.

Rack and pinion gears.

With no air resistance, no friction of the road, no friction in car's mechanisms, and no obstacles to run into, the family could have traveled straight on forever. With no force to change their speed or direction they could have serenely continued the way they were going for the rest of eternity. But that's not the way it was. The world supplied no end of forces. Air resistance pushed against them and slowed them down. Friction of the tires on the road made the van change direction as its wheels were turned. Metal rubbed against metal in the gears, cranks, wheel axles and pistons losing some of the energy as heat and noise. Mother had to continually press the gas pedal to supply fuel to the engine otherwise the forces acting against it would have made further progress impossible.

Disk brake. Most cars have these on the front wheels.

In the end friction won, but that was what she wanted. As the van pulled into the garage of the Physics family, Mother pressed the brake pedal. The pedal pushed a plunger in a cylinder full of brake fluid. Like all liquids, the brake fluid refused to be compressed. So it was forced out through tubes which ran to the wheels. Her force on the pedal was transmitted to plungers in the braking devices by the fluid. The plungers pinched stiff pads against metal disks on the front wheels. They pressed pads on the inside of metal drums on the back wheels. The friction between the pads and the metal brought the van to a gentle stop inside the garage.

Drum brake. Most cars have these on the back wheels.

      "The funny thing is," said Brother as they got out of the van, "that we're back where we started. So our total displacement is zero. If you look it that way, we've gone nowhere."
      "I suppose," mused Father, as he opened the van's back door to let Puppy out so she could bound excitedly around the yard sniffing things, "that it just goes to show... the journey matters more than the destination."
      With that thought, they gathered their belongings and headed for the house.

THE END [References used on this page: (Macaulay and Ardley, 1998), (Griffith, 1992), (Ofria, 2000), (Parker, 2003)] See bibliography PREVIOUS PAGE MAIN PAGE