is an exercise that is frequently met in quantum mechanics textbooks. Recently，it also attracts interesting in the

study of quantum nonequilibrium processes. In this paper，a force operator is proposed and is utilized to re-obtain

previous result. Its correctness is as well proved in the case of one wall moving at constant velocity. The advantage

of the operator is that a process of taking limit does not need，where one has to calculate the mean force for a finite

square-well potential first and to take the potential-depth be infinite later. The computations show that，in quantum

regime using a time-dependent canonical transformation to define a force operator is neither correct nor essential.