There is a statement in my book i can't really understand. "To stretch a spring, we must do work. We apply equal and opposite forces to the ends of a spring and gradually increase the forces. We hold the left end stationary, so the force we apply at this end does no work.The force at the moving end does do work" ... How is it we apply force on both ends of a spring. Doesn't hook's law apply for the end we actually stretch. F=kx??? Kindly Clarify.. Thanks!