Mechanics · Forces & Newton's laws
Chapter 1 · 3
The idea
Pulleys
Why a light inextensible string over a smooth pulley gives the two masses the same tension and the same size of acceleration in opposite directions, the two-equations-add method for finding a and T, and the classic twist — when one mass lands the string goes slack and the other flies on freely under gravity.
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Mechanics · Forces & Newton's laws
Pulleys
Why a light inextensible string over a smooth pulley gives the two masses the same tension and the same size of acceleration in opposite directions, the two-equations-add method for finding a and T, and the classic twist — when one mass lands the string goes slack and the other flies on freely under gravity.
Why it works
A pulley redirects the pull
A pulley just changes the direction of a string's pull. Over a smooth (frictionless) pulley, a light, inextensible string keeps both connected-particle properties, with one new wrinkle:- Same tension throughout the string — the pulley being smooth means it doesn't grip the string, so is the same on both sides.
- Same size of acceleration — inextensible string, so as one mass goes down the other comes up by the same amount, at the same rate. But the accelerations point in opposite directions: if the heavier side accelerates down at , the lighter side accelerates up at .
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