小明在家里帮妈妈做饭时，把一枚鸡蛋放在盛有清水的盆中，鸡蛋沉在盆底．当他向清水中不断加盐并搅拌，鸡蛋又会悬浮然后漂浮在盐水中．对整个过程的分析正确的是（ ）
A．鸡蛋所受重力逐渐减小，所以上浮
B．液体密度一直增大，鸡蛋漂浮后继续加盐的过程中浮力增大
C．鸡蛋排开液体的体积先不变，后减小，鸡蛋所受浮力先不变，后减小
D．鸡蛋沉在清水中浮力最小，悬浮、漂浮状态下浮力相等
鸡蛋由沉底到悬浮的过程中，盐水的密度在变大，鸡蛋排开液体的体积不变，所以浮力变大，但鸡蛋的自身重力始终不变，故A、C错误；
鸡蛋在漂浮过程中，虽然盐水的密度在变大，但鸡蛋所受浮力始终等于自身的重力，也就是说鸡蛋所受浮力始终不变，故B错误；
鸡蛋沉在杯底时，浮力小于重力；鸡蛋上浮时，浮力大于重力；鸡蛋悬浮和漂浮时，浮力等于重力；因此当鸡蛋沉在杯底时，鸡蛋受到的浮力最小．故D正确．
故选D．
In China, very few children make pocket money, , in western countries, most kids make pocket money bythemselves. They make money in many different . When kids are very young, their parents help them sell the fruitsof their own trees to neighbours. Kids may also help do housework to make money at home. When they sixteen, they can make money by sending newspapers or by working in fast food restaurants, during the summerholidays. There are many of making pocket money by kids themselves. First of all, they learn the of money byworking hard so that they will not waste any. Secondly, they learn to money to buy things they need or want, suchas books, pencils, movies and even clothes they like. Thirdly, they learn to the daily life problems by helping theirparents or others. Making pocket money is for children when they grow up. That is why parents encourage theirkids to make pocket money.
|