Frictionless Pulleys

Engineering

Eddie sez:

There is a lot of math in engineering used to design you airplane that might surprise you. Designers can't know every single detail that goes into every component, so they have to make assumptions. I don't think I've ever been checked out in any airplane where we discovered "they" got it wrong, but so did every one else. But you can't really design anything without this kind of magic.

Last revision:

2020-04-18

An engineering approach that might surprise you

Consider the simple act of measuring the force multiplier of a simple pulley. We say a simple block and tackle pulley arrangement gives us a two-to-one mechanical advantage because the weight being lifted is being lifted twice, even though we are only pulling once.

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Photo: A simple "block and tackle" pulley system
Click photo for a larger image

But this ignores the friction in the pulley and the weight of the rope itself. That is why engineers tend to run thought experiments using “frictionless pulleys” and “massless ropes.” The resulting answer is inexact, but good enough to make design decisions.

So if you ever discover something about your airplane the designers didn't know, let them know. But understand that sometimes you have to make an assumption or two to get anything off the drawing board.


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