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.)
