In my physics textbook (Tipler et al.), the following equations were given as a solution to a problem. I am slightly in doubt of their equations. I think the answer should be $\frac{m_2}{m_1+m_2+(I/R^2)}g$, with the square inside the parenthesis. Because the units don't seem to work out in their answer (you can't add kilograms to kilograms squared times meters squared). Am I correct, or am I forgetting/missing something?

textbook screenshot

  • 2
    $\begingroup$ $\uparrow$ Which page? $\endgroup$ – Qmechanic Jan 20 at 12:36
  • 6
    $\begingroup$ I think it would be better if you provide more context regarding this equation like what the original question was, How it was solved, etc. $\endgroup$ – Johan Liebert Jan 20 at 14:10
  • $\begingroup$ @JohanLiebert, I edited my previous reply and included the complete problem. $\endgroup$ – David White Jan 20 at 20:01

I taught out of Tipler's book for several years. I found very few errors, but the excerpt noted above is definitely dimensionally inconsistent. If you look carefully at the dimensions in the problem, you will note that the term in parentheses should be $(I/R^2)$.

Johan Liebert, based on your request for more context, here is the info:

Example 9-13, p. 307, Physics For Scientists and Engineers, 6th edition, Paul A. Tipler and Gene Mosca, W. H. Freeman and Company, New York, 2008

enter image description here


If $m_1$ and $m_2$ are masses, $I$ is a moment of inertia, and $R$ is a radius, as one would expect from the notation, then you are correct that these equations are dimensionally inconsistent and therefore wrong. The authors should have done a better job of proofreading.


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