问题 问答题

某小组做“测滑轮组机械效率”的实验.

(1)利用图所示的滑轮组成滑轮组,要求弹簧测力计向上拉,在图中画出滑轮组的绕法.

(2)使钩码升高的过程中应向上______拉动弹簧测力计;记下钩码及弹簧测力计的位置.拉动弹簧测力计使重为G的钩码升高,读出弹簧测力计的示数F,用刻度尺分别测出钩码提升的高度h和弹簧测力计移动的距离S.

(3)用公式η=

Gh
(mg+G)h
计算机械效率(动滑轮质量为m),比用η=
Gh
Fs
计算出的结果______(填“偏大”、“相同”或“偏小”).

(4)保持此装置不变,将提升的钩码增加为2G,滑轮组的机械效率将______(填“增大”、“不变”或“减小”).

答案

(1)绳子自由端的弹簧测力计向上拉动,反方向绕过动滑轮,定滑轮,最后连接在动滑轮上.如图.

(2)实验中要竖直向上匀速拉动弹簧测力计,弹簧测力计示数不变,便于读数.

(3)η=

Gh
(mg+G)h
,只考虑动滑轮的重,没有考虑绳重和摩擦力,η=
Gh
Fs
,同时考虑到动滑轮重、绳重、摩擦,提起物体时有用功相同,根据η=
Gh
(mg+G)h
计算时总功偏小,机械效率偏大.

(4)同一滑轮组提起钩码的重增加,有用功在总功的比值增大,额外功在总功中比值减小,所以同一滑轮组提起的物体越重,机械效率越高.

故答案为:(1)见上图;(2)匀速直线;(3)偏大;(4)增大.

改错题

第二节短文改错(共10小题;每小题1.5分,满分15分)

此题要求改正所给短文中的错误。对标有题号的每一行做出判断:

如无错误,该行右边横线上画一个勾(√);

如有错误(每行只有一个错误),则按下列情况改正:

此行多一个词:把多余的词用斜线(\)划掉,在该行右边横线上写出该词,并也用斜线划掉。

此行缺一个词:在缺词处加一个漏字符号(∧),在该行右边横线上写出该加的词。

此行错一个词:在错的词下划一横线,在该行右边横线上写出改正后的词。

注意:原行没有错的不要改。

Mother’s Day was around the corner. My good friend                         76. __________

Alice, Jenny and I get together, discussing what we should do to      77. __________

our respect for. Alice was planning to buy some flowers with            78. __________

his pocket money in honor of her mother. Jenny was going to buy     79. __________

vegetables to cook delicious dinner for her family on that day, because80. __________

she wanted to experience how many her mother does for her family  81. __________

every day. As for me, I’d like to make a greeting card, said                     82. __________

“Happy Mother’s Day”, in which I would also tell my mother           83. __________

to pay attention to health or promise her to study harder.                  84. __________

We are looking forward to Mother’s Day.    85. __________

单项选择题