题目　用下列器材:弹簧秤、空矿泉水瓶(有瓶盖,瓶口系着便于称量的绳子,如图1所示)、足够的水和牛奶.试测量牛奶的密度.图1(1)写出简要的实验步骤(包括需要测量的物理量及其符号).(2)写出计算牛奶密度的表达式.分析　此题是广东省1999年中考物理试题(第33题)中一道实验题.根据 Subjects use the following equipment: spring scales, empty mineral water bottles (with caps, bottle ties for easy weighing of the rope, as shown in Figure 1), enough water and milk. Try measuring the density of the milk. Figure 1 (1) Write A brief set of experimental steps (including physical quantities to be measured and their symbols). (2) Write an expression for calculating milk density. Analyze this question as an experimental problem in the 1999 Guangdong Provincial College Entrance Examination Physics Test (33).