例1 水稻丰收后,带壳的稻谷放在场院上堆成圆锥形的谷堆.给你器材是:量程为20米的卷尺,一个不太大的薄铁皮桶,一个量程为15千克的杆秤,还可以利用一些日常生活中常用的东西.依据所学过的物理知识和数学知识,请你说出测出稻谷密度的方法,并写出计算密度的公式. Example 1 After the rice harvest, the shelled rice is piled into a conical heap on the yard. The equipment for you is: a measuring tape with a measuring range of 20 meters, a not too large thin metal barrel, and a steelyard weighing 15 kilograms. , you can also use some of the things that are commonly used in daily life. Based on the physical and mathematical knowledge that you have learned, please tell us the method for measuring the density of rice and write a formula for calculating the density.