问题 选择题

如图所示,一个木块放在固定的粗糙斜面上,今对木块施一个既与斜面底边平行又与斜面平行的推力F,木块处于静止状态,如将力F撤消,则木块(  )

A.仍保持静止

B.将沿斜面下滑

C.受到的摩擦力大小不变

D.受到的摩擦力方向不变

答案

设木块的重力为G,将木块所受的重力分解为垂直于斜面方向和沿斜面向下方向,沿斜面向下的分力大小为

Gsinθ,如图,在斜面平面内受力如图.力F未撤掉时,由图1根据平衡条件得,静摩擦力大小f1=

(Gsinθ)2+F2
.力F撤掉时,重力分力Gsinθ<
(Gsinθ)2+F2
.所以木块仍保持静止.由图2,根据平衡条件得f2=Gsinθ,所以木块受到的摩擦力变小.由图看出,木块所受的摩擦力方向发生了改变.故A正确,B、C、D错误.

故选A.

单项选择题
完形填空

Why is setting goals important? Because goals can help you do, be, and experience everything you want in life. __71 just letting life happen to you, goals allow you

to make your life happen.

Successful people __72 how their life should be and set lots of  goals. By setting goals you are __73 your life. It's like having a__74 to show you where you want to go. Think of it this way. There are __75 drivers. One has a destination (目的地) in mind (her goal) which can be found on a map. She can drive straight there without any __76 time or wrong turns. The other driver has no goal or destination or map. She starts off at the same time from the same place as the first driver, __77  she drives aimlessly around, never getting anywhere, just __78 gas. Which driver do you want to be?

Winners in life set goals and follow through on them. They decide what they want in life and then get there by making __79  and setting goals. __80 people just let life happen by accident. Goals aren't difficult to set—and they aren't difficult to reach. It's up to you to find out what your goals really are. You are the one who must __81 what to achieve and in what direction to aim your life.

Research tells us that when we write a goal down we are more __82 to achieve it. Written goals can be __83regularly, and have more power. Like a contract(合同) with yourself, they are harder to neglect or forget. Also __84  you write your goals in a particular way you are able to make yourself continuously __85  situations that will bring you nearer to your goal.

71. A. Because of      B. Instead of      C. In spite of       D. In addition to

72. A. imagine        B. discuss         C. experience        D. show

73. A. going on with                      B. taking control of

C. getting along with                 D. taking advantage of 

74. A. guide           B. friend          C. map               D. sign

75. A. two             B. such            C. some              D. goal

76. A. free            B. dull            C. fixed             D. wasted

77. A. or              B. for            C. but                D. so

78. A. thinking of     B. using up       C. turning off        D. paying for

79. A. plans           B. money          C. friends            D. maps

80. A. Lucky           B. Young          C. Ordinary           D. Unsuccessful

81. A. practise        B. change         C. admit              D. decide

82. A. willing         B. anxious        C. likely             D. clever

83. A. improved        B. achieved       C. set                D. reached

84. A. although        B. until          C. when               D. unless

85. A. aware of       B. worried about    C. familiar with       D. accustomed to