What is a thread? How does he know you need to turn the handle "around" 10 times? From where does the 126 inches come from? I thought Feynman explanations were easy...

Let us now illustrate the energy principle with a more complicated problem, the screw jack shown in Fig. 4-5. A handle 20 inches long is used to turn the screw, which has 10 threads to the inch. We would like to know how much force would be needed at the handle to lift one ton (2000 pounds). If we want to lift the ton one inch, say, then we must turn the handle around ten times. When it goes around once it goes approximately 126 inches. The handle must thus travel 1260 inches.

No need to say I don't speak English very well but the Spanish version of the book is horrible and confusing.

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    $\begingroup$ Google "10 threads to the inch" first and use this link to conver between TPI to pitch which equates to how far along the screw moves axially for one turn. $\endgroup$ – John Alexiou Sep 21 '19 at 1:32
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    $\begingroup$ $2\pi$ times 20 inches is about 126 inches. It’s the circumference of a circle with radius 20 inches. $\endgroup$ – G. Smith Sep 21 '19 at 2:17
  • $\begingroup$ So the distance calculated is at the top of the handle only? Where the force is applied? $\endgroup$ – Esteban Soto Montijo Sep 21 '19 at 2:19
  • $\begingroup$ At the end of the handle, where you can exert the greatest torque for a given force. $\endgroup$ – G. Smith Sep 21 '19 at 2:25

Threads refer to the helical ridge that wraps around a screw—tornillo in Spanish. Each complete rotation of a screw with 10 threads per inch advances it one tenth of an inch.

As mentioned in the comments, 126 inches is the circumference of the circle swept out by the handle—the distance you move your hand to put the screw through one full rotation.


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