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Is the definition of Work done by Torque a general result?

Physics Asked on May 17, 2021

I was going through this post.

In the post the person who asked the question mentioned a statement:

"Lastly, I am emphasizing that I’m am NOT saying that torque is incorrect, only that the work derivation of torque (which I believe is very common) is invalid and that there must be another explanation.Lastly, I am emphasizing that I’m am NOT saying that torque is incorrect, only that the work derivation of torque (which I believe is very common) is invalid and that there must be another explanation."

This got me to wondering. It is very common to find the definition of Work done by Torque in Undergrad books as
$$dW = vec tau . dvectheta tag1$$

However I think that the above definition is only valid or at least useful when the acceleration of centre of mass of a system is zero.

And in case where system’s centre of mass has an acceleration, it is better to use the general definition of Work as

$$dW = vec F . dvec r tag2$$

where $dvec r$ represents the displacement of point of application of force $vec F$.

However I am unable to understand that if equations $(1)$ and $(2)$ are synonymous.


Question:

  1. Is equation $(1)$ strictly a general result or only applies to certain special cases?
  2. Are equations $(1)$ and $(2)$ same?

One Answer

Equation (1) can be derived from equation (2) by considering $vec r$ in equation as a differential displacement along the circumference of a circle of radius $R$ (see Fig below) which is given by

$$dvec r=Rdvec theta$$

Then,

$$dW=vec F.dvec r$$

becomes

$$dW=Rvec F. dvec theta$$

Then, since

$$vectau =Rvec F$$

We have

$$dW=vectau .dvectheta$$

Hope this helps.

enter image description here

Correct answer by Bob D on May 17, 2021

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