A:中学生的质量约500kg
B:人体感觉最舒适的温度约为37 ℃
C:中学生跑完50m用时约2s
D:教室课桌的高度约0.8m
— Are you relaxing yourself here?
— Yes. It feels ________ to walk slowly along the river.
——Would you mind if I sit here?—— It's for Miss Zhou.
The boy is____ ____ ____go to school.
如图,在水平面上放置着两根相距1m的平行金属导轨,导轨的电阻不计,在导轨的左端有滑动变阻器R、电键S、电源与导轨相连,电源电动势E=10V,内阻r=1Ω.导体棒MN长l=1m,质量m=1kg,其电阻不计,与导轨接触良好.整个装置处于方向竖直向上的匀强磁场中,磁感应强度B=0.5T.现将滑动变阻器R调到4Ω,闭合电键S,导体棒刚好能处于平衡.(设最大静摩擦力等于滑动摩擦力,g=10m/s2)求: