part6

PART 6: FORCE ON THE ROCKS

      The family woke in the morning to a day that was wet and gray. Down at the eating tarp the pot full of water molecules performed another heating dance as the humans discussed how to spend the day.
      "I hauled my helmet and harness all the way up here," proclaimed Little Sister. "I want to do some climbing. I don't care if it's raining!"
      "I second that idea," said Father thinking of all the joules of energy he'd spent just carrying the heavy climbing ropes up the mountain. Perfectly good joules too. He could have saved them for something else. "We could go out to the boulder patch over that way. Climb, or if you want, practice repelling. That OK with everyone?"
      "Sure," said Brother. Mother nodded as well.
      Big Sister hesitated and then nodded too. She liked climbing OK, but wasn't too keen on the idea of repelling on slippery wet boulders.
      Breakfast was eaten in short order. The cooking site was cleaned up and the food bag once again hoisted up the side of the large boulder. Finally the intrepid family set off for the distant jumble of large interesting rocks across the valley. They had to stop several times to let Big Sister inspect some folded rock layers visible here and there through gaps in the tundra. The rocks showed signs of their ancient story.

Thin folds and thick folds. The arrows indicate the direction the stress originally came from.

They had been deposited as sand and mud millions of years ago on what had once been a beach. As more layers were dumped on top of them, the overlying pressure compacted and fused the individual grains into rock. Sideways stress from continental collision had slowly buckled the layers forming folds of rock. Although it was not visible to the eye, the internal fabric of the rocks had been deformed as the beds were crumpled. Along the outer part of the folds the rock was stretched apart. On the inner side of the folds the rock was compressed. The stress had been great enough to exceed the rock's elastic ability to return to its original form. It was left doubled up in folds and strained throughout its body.
"That's really cool," said Big Sister happily. "Look at how the thin layers are crinkled up in smaller folds that follow the shape of the bigger folds. See that? That's because those layers up here are a lot thicker. The thick ones are more rigid and make those smoother curves." She pointed up at them. "The thin ones can fold into smaller curves, but they also get pushed into the same overall curve as the big ones. Ohhh. I want to take a piece of this stuff with the little folds back with me! I wish I'd brought my rock hammer!"
"Yeah," quipped Brother. "Me too. We could have used it to pound in the tent stakes."

      The boulders that they came to were large with good places to practice climbing moves or repelling. Father set up belaying and repelling lines with anchors made from slings and carabiners. It was decided that they would try repelling for awhile first, down the steep side of a large boulder. Little Sister and Brother each repelled down the rock face quickly. Almost too quickly for Mother to take pictures. Big Sister hesitated at the top when it was her turn. She took a few deep breaths. She checked her harness and knots for the fifth time. She adjusted her helmet. She decided to wrap the control end of the rope behind her back for extra friction. She rechecked her harness. She sighed.
      "This is the part I don't like," she admitted, "getting onto the cliff face. Once I'm there I'm fine."
      "I know," Father agreed patiently. "It's understandable. You're asking your body to do something really stupid... to step off a cliff backwards. You just have to learn to trust the equipment. Trust the rope."
      "Yeah," grinned Brother. "Trust the Force!"

Big Sister demonstrates the physics of "the scary part". The arrows represent the forces acting on her body. The length of the arrows represents the size of the force.

The red arrow represents the force of gravity acting on Big Sister. She is exerting forces on the rope and cliff face (green arrows). The blue arrows represent the forces the rope and cliff exert back on her. The upward component of these forces keeps her from falling. (Photograph by Ruth Kircher)

      "Oh shut up," Big Sister growled. She moved her arm position to lessen the friction in the repel device and slowly tilted backwards. When she was almost halfway to horizontal, she stepped backwards and down, pressing her right foot against the rock face. She let out the rope a little more, tilted further back and stepped onto the cliff side with her left foot. When she moved her controlling hand back to its original place, the friction of the rope and repel device brought her to a complete stop. Her weight pulled down and backwards on the rope. The rope pulled on her, up and forwards, by the same amount. Her feet pushed down and forwards on the sloping rock face. The rock pushed backwards and up on her feet. Gravity still pulled down on her with the full force of her weight, but it was completely canceled out by the upwards forces of the rope and rock.
      She breathed a sigh of relief. "OK, now it's not so bad." She let out some rope and started moving backwards down the slope. "But, ugh, it's kind of slimy when it's all wet. Not a lot of traction."
      "You don't need it," Father called down to her. "Lean back so you're pushing straight into the rock, not at an angle. You want to go in caves, don't you? They're always wet and slimy. This is good for you. Builds character!"

      Big Sister laughed and continued backing down the boulder's side. Finally she was able to step onto relatively horizontal ground again. "Your turn," she said to Mother who was standing nearby with her camera.
       "I'm the photographer," answered Mother who had a great deal more common sense than anyone else in the family.
       Little Sister went again instead.
       Mother wandered around taking pictures of the repellers and a few other interesting things. She had a small automatic camera. Light entered through two small windows on the front of the camera. Mirrors behind the windows superimposed the two images. The camera's range finding device adjusted the mirrors until the two images appeared as one image. Because the two windows saw slightly different views, how much the mirrors needed to be titled to overlap the images depended on the distance to the object that Mother pointed the camera at. While it adjusted the mirrors, the range finder also moved the photographic lens. When the images from the mirrors were finally overlapped, the lens was also in place to produce a nicely focused image on the film.
       Mother could see her intended target through the camera's viewfinder window. When she pressed the button to snap a picture, a small round disk blocking light from the film was pulled aside. Light entered the photographic lens, was bent inwards, and hit the photographic film precisely at the point where the rays converged. There the light interacted with small light sensitive silver halide grains. Where the light was more intense, more grains got hit. Where it was less intense, less grains were hit. The silver halide grains that were struck by the light gave up their silver ions which formed little clusters marking the place light had been. The shutter slid back into place to once again shroud the film in darkness. It would be kept in the dark until it was time to develop the film. That process would turn the silver ion clusters black, wash away the remaining silver halide grains and bring the chemical reaction to a stop, forming a negative image of Mother's picture.
       The rain continued. Father and the three siblings repelled and climbed until their hands were too cold to enjoy it anymore. Mother wandered about repeatedly subjecting various sections of her film to a barrage of light. Puppy stayed curled up under the shelter of a rock, out of the wind and the rain. She glanced out to watch her family warily from time to time. Humans were sure weird, staying out in the wet like that.

[References used on this page: (Bloomfield, 1997), (van der Pluijm and Marshak, 2004)] See bibliography PREVIOUS PAGE MAIN PAGE NEXT PAGE