在细线拉力F作用下，质量m=1.0kg的物体由静止开始竖直向上运动，

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问题问答题

在细线拉力F作用下，质量m=1.0kg的物体由静止开始竖直向上运动，速度-时间图象如图所示，取重力加速度g=10m/s²，求：

（1）在这4s内细线对物体拉力F的最大值、最小值

（2）在F-t图象中画出拉力F随时间变化的图线．

答案

（1）t=0至t=1s内，物体加速上升，产生超重现象

加速度大小a1=

△v1

△t1

=5m/s2，方向向上

由牛顿第二定律，有 F₁-mg=ma₁

拉力最大值F₁=mg+ma₁=15N

t=1s至t=2s内，物体匀速上升，拉力F₂=mg=10N

t=2s至t=4s内，物体减速上升，产生失重现象

加速度a3=

△v3

△t3

=-2.5m/s2，负号表明

方向向下

由牛顿第二定律，有 F₃-mg=ma₃

拉力最小值F₁=mg+ma₃=10N+1.0×（-2.5）N=7.5N

（2）如图所示

答：（1）在这4s内细线对物体拉力F的最大值为15N，最小值为7.5N；

（2）如图所示．

阅读理解

A nine-year-old kid was sitting at his desk when suddenly there was a puddle （/JC^C）between his feet and the front of his trousers was wet．He thought his heart was going to stop because he couldn't possibly imagine how this had happened．It had never happened before, and he knew that when the boys found out he would never hear the end of jt. When the girls found out, they would never speak to him again as long as he lived．

He prayed this prayer, "Dear God, I need help now! Five minutes from now I'm dead meat!" He looked up from his prayer and here came the teacher with a look in her eyes that said he had been discovered．As the teacher was walking toward him, a classmate named Susie was carrying a goldfish bowl full of water．Susie tripped （绊倒） in front of the teacher and dumped （倒） the bowl of water in the boy's lap．The boy pretended to be angry, but all the while was saying to himself, "Thank you．Lord!"

Now all of a sudden, instead of being the object of ridicule, the boy was the object of sympathy．The teacher rushed him downstairs and gave him gym shorts to put on while his trousers dried out．All the other children were on their hands and knees cleaning up around his desk．The sympathy was wonderful．But as life would have it, the ridicule that should have been his had been transferred （转移） to someone else—Susie．She tried to help, but they told her to get out．

When school was over, the boy walked over to Susie and whispered, "You did that on purpose, didn't you?" Susie whispered back, "I wet my trousers once, too!"