In a standard Atwood machine physics problem, the string going over the pulley is considered massless. So does that imply mass = 0 or mass = dm? General question: what is the difference between 0 and an infinitesimal number?

  • $\begingroup$ Possible duplicates: How to treat differentials and infinitesimals? , Rigorous underpinnings of infinitesimals in physics and links therein. $\endgroup$
    – Qmechanic
    Jul 11, 2020 at 15:08
  • $\begingroup$ You will understand the difference between zero and infinitesimal by learning rigorous mathematical analysis. In short, zero $0$ is specific a real number, while infinitesimal is the process of taking limit approaching zero, which is not a number. $\endgroup$
    – Valac
    Jul 11, 2020 at 15:13
  • $\begingroup$ If you say the string has a mass dm and solve the problem, the result will depend on dm. But as you take the limit dm -> 0 in that solution, the result should match the mass = 0 solution. (That no doubt assumes certain mathematical properties of the solution with dm which I'm not qualified to comment on). $\endgroup$ Jul 11, 2020 at 15:20

1 Answer 1


A big difference is the fact that you can divide a number by an infinitesimal number. You can't divide a number by zero. So certain terms are sometimes excluded in certain equations because of this fact.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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