Thread Tests

Linking the MTH challenge to the real world is a continual challenge. When do people need to lift heavy things or apply large forces (e.g., moving a heavy TV, raising a car to change a tire, trying to open a stuck jar lid)? Since students doing MTH are designing a model of a lifting system, the machines and the loads they work with are scaled down, making the challenge a bit harder to conceptualize. Making this aspect of the challenge clearer is the goal of Earl's lesson seen in MOVIE 1.

First, Earl conducts a demo where a student, who is grasping a pole held horizontally above her with both hands, is lifted off of the floor. She then holds the pole with one hand and is lifted. She cannot hold on. Why not? The analogy Earl makes is that sometimes to lift an object, you need to have multiples of a force (2 hands), just like multiples of a string are needed to lift the can.

Earl follows his demo with a lab. Students conduct tests to determine just how many strings (in parallel) are needed to lift the #10 can of food. The number of strings that they determine is needed is approximately the mechanical advantage that the device students will design must provide to meet the MTH challenge successfully. If 4 strings are needed to lift the can and 3 fail to do so, then the machine students create will need to provide an MA between 3 and 4. (Remember that knots subtract from overall cord strength, and so a somewhat lower MA is probably all that is needed.)