# Line integral

There are line integral with respect to arc length and line integral with respect x/y.

I know $\int_C Pdx+Qdy$ is useful to calculate the work. When do we need the line integral with respect to arc length?

I know $\int_C Pdx+Qdy$ is useful to calculate the work. When do we need the line integral with respect to arc length?
$$M =\int_C \delta(s)\,ds$$