题目

One day when Jack was walking inthe park, he saw a woman he knew sitting on a bench(长椅)with a dog beside her. The dog was looking up at the woman. Jack walked up to the woman and said, “Hello. Sue, how are you? May I sit and talk with you for a while?”“Of course, please sit down,”Sue said.Jack sat down next to Sue on the bench, and they talked quietly together.The dog continued to look up at Sue, as if waiting to be fed. “That's a nice dog”Jack said, pointing(指向)at the animal. “Yes, he's handsome. He's a bit of a mixture(混合) but that's not a bad thing.He's strong and 如图所示的电路中，两平行金属板A、B水平放置，两板间距离d＝40 cm，电源电动势E＝10 V，内电阻r＝1 Ω，定值电阻R＝8Ω。闭合开关S，待电路稳定后，将一带负电的小球从B板小孔以初速度v0＝3 m/s竖直向上射入板间．已知小球带电荷量q＝1×10－2 C，质量为m＝2×10－2 kg，不考虑空气阻力。(g取10 m/s2)（1）要使小球在A、B板间向上匀速运动，则滑动变阻器接入电路的阻值为多大？（2）若小球带正电，只改变滑动变阻器划片位置，其它量不变，那么，A、B板间电压为多大时，小球恰能到达A板？此时电源输出功率是多大？