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Inca Engineering

 

Overview

About 90 years after Spaniards conquered the Inca Empire, Shakespeare wrote that "our life ? finds books in the running brooks and sermons in stones." Stone structures, road systems, agricultural terraces, and irrigation systems built in 1450 - 1540 A.D. by Incan engineers speak eloquently of a highly developed culture. Without machines, metal tools, or wheels, the Incas built structures that harmonize with natural outcroppings of stones found on the mountain tops. The precisely carved, carefully fitted stones required no mortar, yet have withstood 500 years of weather and earthquakes. The Incas used simple, labor-intensive technology to help carve and move the stones harvested from nearby rockfalls. Many weighed more than 100 metric tons. Stonemasons shaped the blocks using a simple, effective method called flaking and a neolithic tool called a hammer stone. Made of granite, quartzite, or olivine basalt, hammer stones have a hardness of at least 5.5 on the Mohs scale, about the same hardness as the larger stones. Striking at a 15- to 20-degree angle, stonemasons could chip off pieces of the rock; alterations in the angle and force of the blow determined the size of the chips. Twenty quarry workers took about two weeks to dress four sides of one stone measuring 4.5 by 3.2 by 1.7 meters (14.8 by 10.5 by 5.6 feet). Inclined planes, rope fashioned from the ourcroya andina plant, and gravity helped transportation crews move the stones. They moved the massive blocks across several kilometers of valley, through a shallow river, and up the mountain face to 2,400 meters (7,875 feet) above sea level, where their buildings still stand. In his account, Garcilaso de la Vega reported that "they moved them, dragging them with muscle power using thick ropes; nor were the roads along which they hauled them level, but very rough mountains with steep slopes over which they were moved up and down with sheer human strength." Excellent organization and management were required to accomplish this. Labor rather than money served to pay taxes, so a large labor force was seasonally available. Researchers estimate that 1,800 laborers were required to drag one 100-metric-ton block. This is equivalent to each laborer pulling approximately 54 kilograms (120 pounds). Evidence on or near the ramps suggests that the Incas may have used wet clay or gravel to reduce friction, which could have decreased the number of laborers needed to pull each stone. Incan engineers worked with their environment, economy, and people to fashion "sermons in stones" that have withstood centuries. Today's challenge is to withstand the thousands of visitors that come to marvel at their accomplishments.

Activity

The Incas realized that by using wet clay or gravel on their roads, they needed fewer people to move huge stones. Do differences in the surface really reduce the friction? What effect do rollers or skids have on friction? Predict which method will work best, then try all of them.
Materials
  • one 10-lb bag of flour, wrapped in a plastic bag
  • one 4'-long, two-by-six board
  • 5 bricks
  • gravel
  • fine sand
  • mud
  • spring scale
  • four 8" dowels, 1/2" in diameter
  • four 8" dowels, 1" in diameter
  • two 4" dowels, 1/2" in diameter
  • two 4" dowels, 1" in diameter
  • paper
  • pencils
  • concrete block
  • 2 pieces of twine, 2 meters each (61/2')
1. Make a stack of bricks about 5 bricks tall. 2. Lean the board on the stack to create a ramp. 3. On paper, create a table to keep track of which variable you alter each time you move the bag down the inclined plane. Alter only one variable at a time. Include a column for each variable. If you try other variables, be sure to create columns for them. Include rows for predictions, observations, and conclusions about what happens on each ramp. 4. Decide who will record your predictions and observations, who will move the bag or block on the ramp, and who will time each run. 5. Tie a piece of twine around the bag of flour and attach the other end to the hook on the spring scale. 6. Release the bag down the board. Record the force it takes to first lower, then raise the bag at a constant speed. Try replacing the flour bag with the concrete block. Spread sand, gravel, or mud on the ramp, and then try using skids and dowels. Each time you change the variable, first lower, then raise the block or bag at constant speed, carefully measuring the force necessary to do so. Questions
  1. What conclusions can you draw about the best way to reduce friction?
  2. What kinds of companies or professions might be interested in finding ways to reduce friction to move things?
  3. What factors did not affect the amount of force necessary to move the bag or block? Why?

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