工人要将油桶搬运上汽车，常常用如图甲所示的方法，而且当高度一定时，采用长度不同的斜面，所用的推力也不同．小胡为了探究推力大小与斜面长度的关系，采用图乙所示方法即用弹簧秤拉动物体，使物体由斜面底部移到顶部来模拟实验．若斜面的高度为0.2m，物体重力为10N，某次实验中弹簧秤的示数和斜面的长度如图乙所示．试求：
（1）弹簧秤的拉力做了多少焦耳的功？
（2）该斜面的效率为多大？
（1）测力计的示数为2N即拉力F=2N；斜面的长度为1.2m，即s=1.2m；
则拉力做的功为W总=Fs=2N×1.2m=2.4J；
（2）此实验中，做的有用功为：W有=Gh=10N×0.2m=2J；
则斜面的效率为：η=
×100%=W有 W总
×100%=83.3%；2J 2.4J
答：（1）弹簧秤的拉力做了2.4焦耳的功．（2）该斜面的效率为83.3%．
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| A Man Named Lawrence This is an act of kindness that happened to me one Sunday morning. It was at two o'clock in the morning. We just came out of a club and were trying to find our 1 back to our city. As we were trying to 2 the bus stop, a man came to us and said he would show us 3 it was. Although we were a little 4 , we had no choice but follow him. He told us he was a homeless man and that his name was Lawrence. When we passed some city workers 5 the streets, they sprayed (喷溅) the homeless man. Lawrence told us that they always sprayed the 6 if they were "in the way". I didn't know why they would do that. They were human beings, 7 , not cattle or objects. This man had just gotten 8 in the middle of a cold night, but 9 shouting at the workers, he asked them, "How would you 10 if your father or grandfather were treated like this?" He then told us, "It's 11 use shouting at them, because they just get angry. You're better off saying something from the 12 ." It is 13 ! The world could be so different if only we put our anger 14 , lowered our voices and tried to speak from our hearts. He took us safely to the stop and on the way talked about everything. We 15 him and told him that we 16 have never found the place on our own. He said, "I have a son of your age, and I hope that if he were 17 , someone would help him." Not only did Lawrence clear out all the fear and distrust in our hearts, he helped us find our way, in more than one ways. 18 , this man taught us kindness. There was 19 we could really do to repay him. The only thing I hope to do is to 20 other people out. | ||||
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