搜题请搜索小程序“爱下问搜题”
问题 问答题

有一个游泳池，水深5m，在距池底50cm高的侧壁上有一个100cm2的孔，要堵住这个孔至少需要多大的压力？取g=10N/kg．

答案

小孔受到的压强：

p=ρgh=1.0×103kg/m3×10N/kg×（5m-0.5m）=4.5×104Pa；

小孔受到水的压力：

F=Ps=4.5×104Pa×100×10-4m3=450N；

根据二力平衡的知识，要用堵住这个洞，至少需要的压力：

F′=F=450N．

答：要堵住这个孔至少需要450N的压力．

单项选择题 A1型题

香豆素与异羟肟酸铁反应的条件是（）

A.酸性

B.中性

C.碱性

D.先酸性后碱性

E.先碱性后酸性
查看答案
单项选择题

The United States government publishes guidelines for appropriate nutrient intakes. These are known as the Recommended Dietary Allowances (RDAs) and are updated regularly based on new research in nutrition. RDAs are suggested amounts of calories, protein, and some minerals and vitamins for an adequate diet. For other dietary substances, specific goals must await further research. However, for the U.S. population as a whole, increasing starch and fiber in one’’s diet and reducing calories (primarily from fats, sugar, and alcohol) is sensible. These suggestions are especially appropriate for people who have other factors for chronic diseases due to family history of obesity, premature heart disease, diabetes, high blood pressure, and high blood cholesterol, or for those who use tobacco.Snacks can furnish about one-fourth of the calorie requirements among teenagers. Those snacks should also provide much of the day’’s allowances for protein, minerals, and vitamins. Sandwiches, fruit, and milk make good snacks for active teenagers. Food from the food pyramid may be part of any meal. A grilled cheese sandwich or a bowl of whole-grain cereal is just as nutritious in the morning as it is at noon. In addition, a good breakfast consists of any foods that supply about one-fourth of the necessary nutrients for the day.

The reduction of calories in the diet is particularly good for people who suffer from

A．obesity

B．premature heart disease and diabetes

C．high blood pressure and cholesterol levels

D．all of the above
查看答案