我校机器人协会的同学利用如图所示的装置探究作用力与反作用力的关系。如图甲所示，在铁架台上用弹簧测力计挂住一个实心铁球，在圆盘测力计的托板上放一烧杯水，读出两测力计的示数分别为F1、F2；再把小球浸没在水中（水不会溢出），如图乙所示，读出两测力计的示数分别为F3、F4。
（1）请你分析弹簧测力计示数的变化，即F3_______F1（选填“>”“=”“<”）。
（2）水对铁球的浮力大小为_________，若___________，就说明了作用力与反作用力大小相等。
（1）<（2）F1-F3或F4-F2；F1-F3=F4-F2
题目分析：（1）开始时弹簧测力计对小球的拉力F=mg，当小球浸没在水中后，小球受到竖直向上的浮力的作用，则：F′+F浮=mg，F′=mg-F浮，故弹簧测力计示数变小，即F3<F1；（2）水对铁球的浮力大小为F1-F3或F4-F2，若F1-F3=F4-F2，就说明了作用力与反作用力大小相等。
| I graduated from a small public school in Michigan. One day, Mr. Ledy, our football coach, asked me to play football. I couldn't help__1__: me, play football? I was six feet tall and was not much__2__a hundred pounds in weight. I thought that they might just need__3__fool to increase the number of the players. However, he was persistent(坚持的) and__4__I joined the football team. That__5__to be one of my best decisions as a kid. Our worst game__6__was the one against Crystal Falls. They beat us 56 to 6. Yeah! That was a football not a basketball__7__. We played them again when we were seniors. They__8__us again but the score was much more__9__, 30 to 20. Coach Ledy would not let that __10__again. He encouraged us to beat them, to pay them back. And we did so in the last game we played before__11__. I had scored 6 points alone in the game. Coach Ledy praised me. It was a huge__12__for me. Was I a football hero? Ha! Ha! No, I couldn't__13__it without our team. The thing I learned through football was__14__. We won as a team and we__15__as a team. I think Mr. Ledy would be__16__of me if he knew that I had broken the athletic__17__at the University of Michigan. I'm__18__to him as I would not have today’s honor without him. I'll __19__life's final war with pain to win the victory; I'll seek the lights of glory wholeheartedly and I'll__20__working hard so as not to let him down. | ||||
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