如图所示，一物体置于水平地面上静止不动，若用水平向左的力F="5.0" N拉物体，物体仍静止，下述结论正确的是（ ）
A．物体受到的合外力增加5.0 N
B．物体对水平面的压力增加5.0 N
C．物体对水平面的作用力大小增加5.0 N
D．物体受到水平面的静摩擦力是5. 0 N
答案：D
物体一直处于静止平衡状态，受到合外力为零不变，物体对水平面的压力大小不变（等于物体受到的重力的大小），由平衡条件可知物体受到的静摩擦力与水平方向的拉力大小相等，为5 N，A、B均错，D对；物体对水平面的作用力有压力与静摩擦力，方向互相垂直，即物体对水平面的作用力是这两个力的合力，不在同一直线上的力应用矢量平行四边形定则计算，而不能用代数方法运算，C错。
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| We often talk about ourselves as if we have permanent genetic defects(缺陷)that can never be changed."I'm impatient.""I'm always behind.""I always put things off!"You've surely heard them.Maybe you've used them to describe yourself. These comments may come from stories about us that have been __1__ for years-often from as far back as childhood.These stories may have no __2__ in fact.But they can set low expectations for us.As a child,my mother said to me,"Marshall,you have no mechanical skills,and you will never have any mechanical skills for the rest of your life."How did these expectations __3__ my development?I was never __4__ to work on cars or be around __5__.When I was 18,I took the US Army's Mechanical Aptitude Test.My scores were in the bottom for the entire nation! Six years later,__6__,I was at California University,working on my doctor's degree.One of my professors,Dr. Bob Tannbaum,asked me to write down things I did well and things I couldn't do.On the positive side,I __7__ down,"research,writing,analysis,and speaking."On the __8__ side,I wrote,"I have no mechanical skills." Bob asked me how I knew I had no mechanical skills.I explained my life __9__ and told him about my __10__ performance on the Army test.Bob then asked,"Why is it that you can solve __11__ mathematical problems,but you can't solve simple mechanical problems?" Suddenly I realized that I didn't __12__ from some sort of genetic defect.I was just living out expectations that I had chosen to __13__.At that point,it wasn't just my family and friends who had been __14__ my belief that I was mechanically hopeless.And it wasn't just the Army test,either.I was the one who kept telling myself,"You can't do this!"I realized that as long as I kept saying that,it was going to remain true.On the contrary,if we don't treat ourselves as if we have incurable genetic defects,we can __15__ well in almost anything we choose. | |||
| ( )1. A. said ( )2. A. basis ( )3. A. lead ( )4. A. encouraged ( )5. A. means ( )6. A. therefore ( )7. A. settled ( )8. A. passive ( )9. A. experiences ( )10. A. unexpected ( )11. A. complex ( )12. A. arise ( )13. A. believe ( )14. A. weakening ( )15. A. do | B. spoken B. plot B. improve B. demanded B. tools B. somehow B. turned B. active B. trips B. poor B. advanced B. separate B. suspect B. strengthening B. go | C. spread C. cause C. affect C. hoped C. facilities C. instead C. took C. negative C. roads C. excellent C. common C. suffer C. adopt C. abandoning C. work | D. repeated D. meaning D. change D. agreed D. hammers D. however D. got D. subjective D. paths D. average D. primary D. come D. receive D. accepting D. get |