问题 选择题

如图所示,一倾斜的金属框架上放有一根金属棒,由于摩擦力作用,金属棒在没有磁场时处于静止状态.从t0时刻开始给框架区域内加一个垂直框架平面向上的逐渐增强的磁场,到时刻t1时,金属棒开始运动,则在这段时间内,棒所受的摩擦力(    )

A.不断增大

B.不断减小

C.先减小后增大

D.先增大后减小

答案

答案:C

题目分析:由于摩擦力作用,金属棒在没有磁场时处于静止状态.,则摩擦力沿斜面向上;从t0时刻开始给框架区域内加一个垂直框架平面向上的逐渐增强的磁场,穿过闭合回路的磁通量增大,闭合回路有收缩的趋势,金属棒受到沿斜面向上的安培力,金属棒运动前,受平衡力作用有mgsinθ=F+F,F安从0开始增大,F减小;F增大到等于mgsinθ时,F减小到0;F再增大,F会反向增大。C正确。

选择题
阅读理解

The H1Nl virus has been spreading world-wide,it is important for us to know how to control it.Please pay attention to the following points:

1.Don't touch your face.Above all,keep your hands away from your eyes,mouth and nose,all of which serve pathways(途径) for the viruses(病毒) to enter your body.

2.Wash your hands.If you have to touch your face,wash your hands,getting under the fingernails for 20 to 30 seconds with hot soap and water before.Soaps with surfactants(表面活性剂) can deal with flu viruses.

3.Cover your nose and mouth.When someone sneezes or coughs,liquid(液体) drops with flu viruses as far as three feet through the air and land on your nose and mouth,so it's best to keep at least an arm's length distance when talking to someone who shows signs of infection(感染).In order to protect others,cover your mouth and nose with a tissue(纸巾) when you cough or sneeze,and clean your hands afterward.Viruses can live for hours,or even longer when on the skin or other surfaces such as keyboards.

4.Consider buying a mask in case you need it in the future.Some research suggests that masks reduce the risk of getting the flu viruses by as much as 80%.But it also suggests that if you don't wash hands,the mask doesn't work well.So wash your hands and use the mask,especially wearing face masks in crowded places is also useful.

根据短文内容,选择最佳答案。

小题1:The article mainly tells ________.

A.the H1N1 virus has been spreading world-wide

B.how to control the H1N1 virus

C.cover your nose and mouth

D.everyone should buy a mask in the future小题2:How does the viruses enter your body easily?

A.Get though your mouth.

B.Get though your nose.

C.Get though your eyes.

D.All of the above.小题3:What can we do with the flu viruses?

A.Water.

B.Masks.

C.Soaps with surfactants.

D.Tissues.小题4:How long can the viruses live on the skin or other surfaces such as keyboards?

A.For an hour.

B.For several hours.

C.For two weeks.

D.For a year.小题5:Which of the statements is Not True?

A.Wash your hands before meals.

B.When you cough or sneeze,you should cover your mouth and nose with a tissue.

C.You shouldn't often touch your mouth,eyes and nose.

D.When you put a mask,you can often go to the crowded places.