题目

阅读理解 Facebook is an Internet-based social network site that lets people get in touch with family and friends and reach out to people with common interests around the world, all through computer. It is very popular, with more than 900 million users. If Facebook were a country, it would have about three times the population of the United States. Documents filed with US. financial regulators say Facebook has hundreds of millions of active users who send billions of messages each day and upload 250 million photographs on their personal pages. Facebook users have registered 100 billion “f有一个合金块，质量10kg，全部浸没在水中时，需用80N的力才能拉住它，求：此时合金块受到的浮力多大？