①Alice what ten play ping-pong your friend a fat teacher
②Is she a tall girl? No,she isn't. She's a short girl.
使用下列简单机械费力的是( )
场源电荷Q=2×10﹣4C,是正点电荷.检验电荷q=﹣2×10﹣5C,是负点电荷,它们相距r=2m,且都在真空中,如图所示.求: