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How do you calculate pulley efficiency?

A 2:1 pulley rig will, in the frictionless world beloved of physistics, offer half the effort for twice the force, albeit with double the distance. So that means you pull in two metres of winch rope but you only travel one metre forwards…but instead of needing to pull say 1000kg, you need only pull 500kg.

Now in the real world there’s friction, so it’ll never be 2:1, or 50% of the pull. Perhaps it’ll be 550kg instead of 500kg, which is a 1.8:1 advantage calculated by 1000/550. Or, we could say it’s 550/1000 which is 55%, when it should be 50%. And as 55-50% = 5%, we could say there’s a 10% efficiency loss as 5% is 10% of 50%. There’s other ways to look at it too. Gets complicated very quickly. I don’t really know the best method to use, and I’ve pinged a few people who might know only to find there’s no agreement there either. So, use whichever method you like…so long as it is consistent, it’s comparative!

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