Directions for building the Earthquake Machine

by Jeffrey S. Barker

Department of Geological Sciences, Binghamton University

Supplies:

1 sheet plywood, cut lengthwise into 9-inch widths (makes 5 "machines")

15 wood blocks (cut 4-5 inch lengths of 2x4s)

2 1-inch dowels (need a total length of about 40-50 inches)

1 1/4-inch dowel

1 1x4 about 6 feet long

2-3 popsicle sticks

screws, washers

30 eyelet screws

5 small rubber bands

15 weak springs (available at Lowes or Home Depot in packages of 3)

string (such as nylon fishing line)

30-40 sheets medium sandpaper

Weights (I use rocks)

1) Cut the plywood sheet lengthwise to make as many as five 9-inch wide boards. Most sandpaper sheets are 9-inches wide, so this is a convenient width for the boards. Make the length as long as practical, since the experiment with three blocks requires a substantial length. Mine are about 5-1/2 feet long, which is minimal. Use the entire 8 feet if using a new 4x8 sheet of plywood.

2) Staple sheets of sandpaper onto the boards, starting at one end and leaving about a foot uncovered. Butt the sheets tightly to each other, trying not to overlap. Staple toward the edge of the board. Avoid staples near the center, since these may cause the blocks to hang up, altering the results. Similarly, replace worn or torn sheets of sandpaper as needed.

3) Make reels by cutting the 1-inch dowels into 8-10 inch lengths. About 3-4 inches from one end, make 8 equally spaced marks around the circumference of the dowel. Use a router table with a straight bit to make notches in the dowel. I set the router bit to be at a height of half the dowel diameter, then push the dowel horizontally against the bit so that the cut is about 1/4 inch deep. The flat end of the cut will catch the ratchet lever. By the way, 8 equally-spaced notches in a 1-inch dowel result in almost exactly 1 cm of motion for each step of the ratchet.

4) Using the 1x4, make a U-shaped bracket to hold the dowel. The base of the U should be long short enough so that the dowel extends beyond each end, and the notches for the ratchet should fall outside one side. Drill 1-inch holes in the sides of the U, making sure the two sides are aligned (I clamp two together and drill them simultaneously). Sand any rough edges and make sure the dowel will turn freely when inserted through the holes. Drill pilot holes, glue and screw the sides to the base of the U. Finally, drill pilot holes and screw the base of the U-shaped bracket to the un-sandpapered end of the long board. Don't glue this down so that it can be removed for storage or transportation.

5) Fit the dowel into the holes in the bracket so that the notches fall just outside one side. Mark the dowel about 1/4-inch inside each side of the bracket. Remove the dowel and drill holes through the diameter of the dowel at the marks. These holes should be just large enough that short lengths of the small dowel can be hammered in without glue. Cut the small dowel into 2-inch lengths. Insert the large dowel into the bracket, then insert the small dowels into the holes to hold the large dowel in place within the bracket. These small dowels should keep the large dowel from moving more than about 1/4 inch, but should not be so tight against the bracket to impede the rotation of the large dowel.

6) Cut the popsicle sticks into 2-inch lengths. Attach to the outside of the bracket so that the cut (flat) end of the stick falls into the route notches on the large dowel, resting against the flat edge of the routed cut. The popsicle stick should allow the large dowel to be turned in one direction, but not the other unless the popsicle stick is lifted manually. Use one or two washers between the popsicle stick and the bracket to ensure that the stick moves freely and lands in the center of the routed notches. Add a small screw below the hinge of the popsicle stick. Attach a small rubber band between the center of the popsicle stick and this screw to apply a weak, constant force holding the popsicle stick into the routed notch. The ratchet assembly is now complete.

7) Attach sandpaper to the bottom of the wood blocks. I like to wrap the sandpaper around the side of the block and staple it in place on the side to avoid staples on the bottom. Insert an eyelet screw into the front and back of each block. Attach a spring onto one of these eyelets. You may want to use wire cutters to make the free end of the springs easier to attach to other blocks, but make sure one end is firmly attached to the block.

8) Measure a length of string (fishing line) the length of your board and attach (staple) one end to the center of the large dowel (in the middle of the U-shaped bracket). Turn the ratchet to wind this string onto the dowel. Tie the other end of the string onto the spring of one of the blocks. This will be the first block used in the experiment. By raising the popsicle stick, you should be able to unwind the string so that the block is resting at the far end of the board. Later, a second and third block and spring set will be added to this first block.

9) Finally, use permanent marker to mark and label a cm scale (metric, finally!) along the length of the board, in the center of the board, along the path the block will follow as it is pulled by the string. The students will use these marks to make their measurements.

10) If you have a set of weights available, students will place these onto the blocks. You may want to make additions to the blocks to hold these weights. I simply use rocks of various sizes as weights.

Questions or comments: jbarker@binghamton.edu

Last modified: November 1, 2001 (content), June 6, 2021 (reformatted and moved to Google sites)