This paper deals with an alternative approach to calculate the performance of a Pelton turbine. The discussion begins with an overview on the statement of Newton's second law for variable mass systems. Then the integral form of the Navier-Stokes equation is presented as a special case, which is useful to evaluate the force exerted by the water jet on the turbine buckets. Although the traditional approach based on the angular momentum theorem is more straightforward, the proposed method allows evaluation of the different contributions to the motive force by a suitable choice of control volumes. This in turn makes for a better understanding of factors that penalize overall performance when the ideal assumptions, under which the theory is usually developed, are no longer verified.