问题 选择题

如图所示,水平恒力F拉着质量为m的木块在水平放置在地面上的长木板上向右匀速运动,木板保持静止.若木板质量为M,已知M>m,则关于木板与地面之间的摩擦力大小的说法正确的是(  )

A.也是 F

B.大于F

C.由于木板没有运动,所以没有摩擦力

D.由于木板和地面的粗糙程度不知道,所以无法判断

答案

木块相对木板运动受到滑动摩擦力,当木块在拉力F的作用下做匀速直线运动时,而木板与地面则是相对静止,所以它们之间存在静摩擦力,由平衡可知,静摩擦力大小等于F,故A正确,BCD错误;

故选A.

阅读理解

阅读理解。

     When Julia Samberg eats her favorite food, she feels bad. She knows that chocolate can have a lot of fat

and sugar. But Julia says she loves chocolate so much-once she starts eating it, she can't stop.

     Julia isn't the only one who loves chocolate. It is a favorite food for people all over the world. People prefer

chocolate over ice cream, cakes, and cookies.

     The idea of eating chocolate didn't begin until the 19th century. Before that, people drank chocolate. The

habit began in Central America where the Aztecs drank bowls of chocolate to stay watchful. When the liquid

(液体) chocolate was brought to Spain in the 1500s, people thought it was medicine because it had a medicine

taste. In fact, the people who made chocolate into drinks were doctors.

     Then people discovered that mixing chocolate with sugar made a wonderful drink. King Ferdinand of Spain

loved this drink so much that he put out an order: anyone who talked about chocolate outside the court (法庭)

would be killed. For about 100 years, chocolate was a secret in Spain.

     Finally, people found out about chocolate, and it became a popular drink in Europe. Later, the Swiss mixed

milk and chocolate. Today, most Americans prefer milk chocolate, while most Europeans prefer dark chocolate.

     New research shows that chocolate is actually good for us." Chocolate has different kinds of vitamins,"

says a researcher in France." It has more than 300 different chemicals. One chemical works on the part of the

brain (大脑) that feels pleasure. People who feel good when they eat chocolate are actually healthier. Feeling

pleasure is important for health and can protect against illness."" Good chocolate doesn't have much fat or

sugar. You can enjoy it if you eat a little at a time!" says Tara Bearish, another chocolate lover.

1. _______    Doctors made chocolate into drinks.                         

         1         The Aztecs drank chocolate to stay watchful.               

2. _______    Chocolate was a secret in Spain for 100 years.             

3. _______    Liquid chocolate was brought to Spain from Central America.

4. _______     Sugar was mixed with chocolate to make a sweet drink.       

         7           Research has shown that chocolate is good for us.          

5. _______    The Swiss put milk into the chocolate mixture.             

单项选择题