Feynman mentions in his book, The Feynman Lectures on physics

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, and if we used various pulleys, etc., we would be lifting our one ton with an unknown smaller weight W applied to the end of the handle. So we find out that W is about 1.6 pounds. This is a result of the conservation of energy.

I dont understand the meaning of the bold part of the text. How it's connected with the example he's carrying on ?

  • $\begingroup$ Conservation of energy often is a useful shortcut that you can use to get answers to questions that could have been solved in other, more complicated ways. You could draw diagrams, and do vector math to figure out the relationship between force on the top of the jack and force on the end of the handle, but Feynman blew past all that by noticing that if the jack was perfectly frictionless, then any value other than 1.6 pounds would lead to a violation of conservation of energy. $\endgroup$ Commented Apr 8, 2018 at 20:51

1 Answer 1


Energy (work done = force $\times$ distance) is conserved so $1.6 \times 1260 \approx 2000 \times 1$

Which is the applied force on handle $\times$ the distance moved by the applied force is equal to the load $\times$ distance moved by the load

  • $\begingroup$ So, the measure of 1.6 pounds ( which i didnt understand where it came from) is simply a parallelism with the force needed to move the handle ? $\endgroup$
    – Qwerto
    Commented Apr 9, 2018 at 5:45
  • $\begingroup$ @Qwerto The pound is taken to be a unit of force and the 1.6 pounds was evaluated by assuming that the system was perfect (no friction etc) so work done by force on handle is equal to work done on load. $\endgroup$
    – Farcher
    Commented Apr 9, 2018 at 5:49
  • $\begingroup$ Then, if a person want to lift the screw using the handle it have to apply a constant force of 1.6 pounds for all the 1260 inches ? $\endgroup$
    – Qwerto
    Commented Apr 9, 2018 at 5:53
  • $\begingroup$ @Qwerto And in theory that will lift a 2000 pound load one inch. $\endgroup$
    – Farcher
    Commented Apr 9, 2018 at 5:54
  • $\begingroup$ This isn't related to what the OP asked, nevertheless, I would like to know why it goes 126 inches by turning around once. Is it a given in the problem? $\endgroup$ Commented Oct 24, 2021 at 7:53

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