Making and using the Bunjy cord Force Puller.
Most attempts to estimate the size of a force turn out to be quite subjective. When you are standing still and pulling on something, you always tend to pull harder than when the thing you are pulling on starts to move. The device described here should make it possible to supply the same pulling force on something whether it is moving or at rest.
Note from the illustration that the flexable bunjy cord is attached to a cord that does not stretch much. (Nylon works well but any strong cord that has a small amount of stretch compared to the bunjy cord will be fine.) When the ends of the bunjy cord are pulled, the nylon cord will come closer to touching the bunjy cord and the idea is always make the same distance between the two cords when applying the desired force. Before using the puller, it would be wise to adjust the initial droop of the nylon cord so that a comfortable and easy to maintain force is obtained when the nylon cord just touches (or is at a easily memorized distance from) the bunjy cord.
The clove hitch is an ideal knot to use to attach the nylon cord to the bunjy cord both because it is strong and because it is always easy to untie when it is necessary to make adjustments.
Bunjy cord can be purchased in most hardware stores in assorted strengths. Be sure to select a size which will fall in the range of your students force pulling ability.
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