如图所示，在光滑水平面上以水平恒力F拉动小车和木块，让它们一起做无相对滑动的加速运动，若小车质量为M，木块质量为m，加速度大小为a，木块和小车间的动摩擦因数为
.对于这个过程，某同学用了以下4个式子来表达木块受到的摩擦力的大小，下述表达式一定正确的是
A． F-Ma
B．(M+m)a
C．μmg
D． Ma
答案：A
：它们一起做加速度大小为a的加速运动，隔离木块，由牛顿第二定律，木块受到的摩擦力的大小为f=ma；对小车和木块整体，应用牛顿运动定律，F=(M+m)a，解得ma= F-Ma，选项A正确BCD错误。
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| Bill was a newspaper boy. He went from house to house 1 the day's papers to the neighbors. Each week he went to get his 2 . Bill kept a little book. In the book he wrote down who 3 him and who did not. Bill was keeping a record (记录) Men have been keeping records 4 very early time. How did they do this before they learned to write? Sometimes they made knots (打结) in different colored strings (绳子).Indians in South America kept records in this 5 . Today we have machines for 6 and keeping records. Do you go to the grocery store? Does it have a cash register? The cash register keeps records. It adds up the things you 7 . It tells how much is sold in the store everyday. Many places have machines called computers. They 8 computers to keep all the records. Computers are 9 than knots in strings. Someday men may use _10 even better than computers. | ||||
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