General > General Technical Chat
Load force vs. Effort force
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james_s:

--- Quote from: bdunham7 on April 01, 2022, 05:48:47 pm ---If I understand correctly, what they are trying to get the students to understand is simply the principle of force multiplication by simple machines.  In order to teach that to an elementary student, you have to oversimplify quite a bit.

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The best way to teach that is to do it. I remember going to a science museum when I was a kid and they had an exhibit with pulley systems. A bunch of identical weights were attached to single and multiple pulley arrangements. With the single pulley you could barely lift the weight but with multiple pulleys it was easy to lift but you had to pull a lot more rope.
IanB:

--- Quote from: edy on April 01, 2022, 03:49:41 pm ---However, I don’t like this explanation to kids as I’d rather they see generally that the force also is related to distance and really has to do with work conservation. The above is a static example of force distribution. However, ultimately the setup means that for every 10 cm you move the effort rope, the load only travels 5 cm against the load force. The mechanical advantage is that the distance x force is equal on both load and effort sides. You can make your effort less by moving a greater distance with less effort.

This analogy is better translated to many other examples including torque wrenches, levers, gears, ramps, and many other simple machines. The above explanation of the ceiling taking “half the weight” makes no sense to my 9 year old and ultimately nothing about the fact that the load is travelling a half the distance (for each pull of the “effort” rope) is mentioned anywhere.
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Here's a counter-example to that (work conservation rather than static force distribution).

In the setup pictured below, the blue rope is taut and the red and green ropes each have about one inch of slack.

If we cut the blue rope, how far will the weight fall before it reaches a new equilibrium?
edy:
This is interesting…. One would at first think the weight falls. But if you look at the situation you first have 2 springs sharing the load and stretching by a certain amount each (half the load “yield”). It is as if you have one long spring really (you can ignore the blue rope or pretend it is at the bottom or top, not in the middle). After you cut the blue rope, you still have 2 springs but now they are in parallel, not serial. In effect their modulus is half, since each spring will stretch less as the weight tension is distributed between them… so counter-intuitively the weight will not drop and will probably go up!
IanB:
Yes, that is indeed what happens:

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