Pulleys for Fifth-Grade Dummies

Single-pulley systems

We've noticed that the nice folks  at Leaf Group have put a few of their more... prolific? freelancers back to work as some sort of cleanup team, rewriting a few of the older, more questionable posts. Good for them... or it would be good for them if the cleanup team members were good at their job. They aren't, necessarily, even though the sly foxes at Leaf have allowed them to craft a custom biography for each of the many niche sites at Leaf. Today's nominee, Laurie Brenner (aka Laurie Reeves) claims that she has a deep "interest in science which she now fulfills by writing for online science websites"; an odd hobby given that her other bio says she is a "contractor." Anyway, Laurie needed to go back to elementary school to get "How Does a Pulley System Work?" for Sciencing.com.

Brenner's version is an update of that written by Tara Dooley, for what it's worth. Dooley's version wasn't that well written, concentrating on the length of the ropes needed for multiple-pulley systems rather than the force enhancement. Then again, the change in "length of pull" is a proxy for force increase.
Brenner, however, had her own problems. Let's look at her initial description of a pulley:

"A single pulley hanging from a ceiling with a rope wrapped around its wheel allows you to lift a box on the floor up to a table or higher using only half the force it would take to lift it with your hands."

Wrong: a fixed single pulley has a mechanical advantage of 1; meaning that the force on the "pull" rope is equal to the downward force of the load. Oops. Needing "half the force" would require a mechanical advantage of 2; which can be attained with a single pulley only if the pulley moves up and down with the load (see image [b] above). In that case, the direction of the force applied is the same as the movement of the load, which is generally unacceptable.

Brenner muddies the waters even more in attempting to explain multiple-pulley systems when she says,

"While a single pulley allows you to move a load with half the force required, a system of pulleys increases the mechanical advantage by the number of pulleys and [bolding ours] the lengths of rope that support the load."

Wrong again, mainly because the word "and" (highlighted) makes it ambiguous. Brenner's next statement,

"The number of rope strands that support the load in a multiple pulley system basically correspond [sic] to the mechanical advantage of the system..."

...is physically if not grammatically correct (the word "basically" is unnecessary; and the subject of the sentence is "number," not "systems"), but by now the damage is done. Our Dumbass of the Day has contradicted herself at least once, and made enough basic errors that her physics teacher (if she ever took physics) would be aghast.

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