0
$\begingroup$

A fixed disc free to rotate about its center (a flywheel) has a string wrapped around it, with a block attached to it. As the block falls, The tension in the string makes the disc rotate. Now, when the disc rotates by an angle $\theta$ (assuming the string doesn't slip), a length $r\theta$ of string unwraps. The length of the piece of string which is attached to the block is changing. Why is work done by tension (on block + disc system) zero?

I don't get it.

So if I'm using the energy theorem in this system that states:

Variation of energy = Work done by string tension (considering that the tension here is the only non conservative force) = 0?! Because work done by tension is zero?

[Here is the system]

(https://i.stack.imgur.com/knA8N.jpg)

(https://i.stack.imgur.com/lLS4M.jpg)

$\endgroup$

1 Answer 1

0
$\begingroup$

It does do work on each object (block and flywheel). But those two works cancel out. If only the block or only the disc had been considered, then sure the tension in the string does work. The work done on each object is equal, although with opposite sign. Here we consider them both as one system, and so the works are done internally and cancel out.

In general, work is always only done by external forces, never by internal forces - not because no work is being done, but because it cancels out (the energy stays within the system).

$\endgroup$
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.