题目

Most drink stating that they are fruit-flavored（水果味道的）contain no fruit at all,while most of the rest contain only a small quantity of fruit, according to a study carried by the British Food Commission. “Shoppers need to check t he labels (标签) before buying drinks, though sometimes the actual content can be non-existent,” said Food Commission spokesperson Ian Tokelove. “Food production is highly competitive.  36   It will increase profits, and consumers won’t always realize they are being tricked.”      Flavorings are focused on the flavors of natural 如图所示有黑白两条毛巾交替折叠放在地面上，白毛巾的中间用绳与墙壁连结着，黑毛巾的中部用手将它拉住．欲将其分离开来，若两条毛巾的质量均为m，毛巾之间及其与地面之间的动摩擦因数为m ，问：将黑毛巾匀速拉出需加多大的水平力？如果有n对白、黑毛巾交替折叠放置着，要将n对黑毛巾一起匀速拉出，要多大的力？