What does $\mathrm{ms}^{-1}$ mean?

Question

A baseball is hit with a velocity of $28.0 \ \mathrm{ms}^{-1}$. Should I just ignore this, or is it actually part of the question, what does it mean?

Answer 1

It means meters per second (recall that $s^{-1}=1/s$, so $m \;s^{-1}=m/s$).

Answer 2

It's never wise to ignore an element of a question. In the case of physical quantities the dimensions are required. Consider the alternative: a baseball is hit with a velocity of 28. 28 what? meter per hour? inch per second?

More interestingly, suppose that you had computed this answer, and it turned out that the resulting velocity is 28 meter per second per second (m/s^2). This is not the correct dimension for a velocity (it is for acceleration) and so it would alert you of an error in the computation.

This is an example of Dimensional Analysis.