问题 填空题

如图所示,在水平桌面上重为200N的A木块被B木块拉着,当B为100N时,A刚好开始移动,当B为80N时,A可保持匀速运动,则A与桌面间的动摩擦因数为______,如果B为50N,A受的摩擦力为______N,如果B为120N,A受的摩擦力为______N.

答案

当B为100N时,A刚好开始移动,知最大静摩擦力为100N,当B为80N时,A可保持匀速运动,根据平衡得,A所受的滑动摩擦力为80N,根据f=μFN=μmAg得,μ=

f
mAg
=
80
200
=0.4.如果B为50N,A保持静止,A受的摩擦力为静摩擦力,根据平衡得,A所受的静摩擦力为50N.如果B为120N,A做加速运动,所受的摩擦力为滑动摩擦力,大小为80N.

故答案为:0.4,50,80.

单项选择题

阅读下列短文,从每题所给的四个选项(A、B、C和D)中选出最佳选项,并在答题卡上将该项涂黑。

B

Grandparents Answer a Call

As a third generation native of Brownsville, Texas, Mildred Garza never pleased move away,. Even when her daughter and son asked her to move to San Antonio to help their children, she politely refused . Only after a year of friendly discussion did Ms Gaf finally say yes. That was four years ago. Today all three generations regard the move to a success,giving them a closer relationship than they would have had in separate cities.

No statistics show the number of grandparents like Garza who are moving closer to the children and grandchildren. Yet there is evidence suggesting that the trend is growing. Even President Obama’s mother-in-law, Marian Robinson, has agreed to leave Chicago and into the White House to help care for her granddaughters. According to a study grandparents com. 83 percent of the people said Mrs. Robinson ‘s decision will influence the grandparents in the American family. Two-thirds believe more families will follow the example of Obama’s family.

“in the 1960s we were all a little wild and couldn’t get away from home far enough fsst enough to prove we could do it on our own,”says Christine Crosby, publisher of grate manazine for grandparents .We now realize how important family is and how important”” to be near them, especially when you’re raining children.”

Moving is not for everyone. Almost every grandparent wants to be with his or her grandchildren and is willing to make sacrifices, but sometimes it is wiser to say no and visit frequently instead. Having your grandchildren far away is hard, especially knowing your adult child is struggling, but giving up the life you know may be harder.

What was the reaction of the public to Mrs. Robinson’s decision?()

A.17% expressed their support for it.

B.Few people responded sympathetically.

C.83% believed it had a bad influence.

D.The majority thought it was a trend.

问答题 简答题