【简答题】如图所示,匀强磁场的磁感应强度为 B ,方向竖直向下,在磁场中有一边长为 l 的正方形导线框, ab 边质量为 m . 其余边质量不计, cd 边有固定的水平轴,导线框可以绕其转动,现将导线框拉至水平位置由静止释放,不计摩擦和空气阻力,导线框经过时间 t 运动到竖直位置,此时 ab 边的速度为 v . 求: (1) 此过程中线框产生的平均感应电动势的大小; (2) 线框运动到竖直位置时线框感应电动...
【单选题】Once found almost entirely in the western United States and in As. ia, dinosaur fossils are now being discovered on all seven continents. A host of new revelations emerged in 1998 that promise to resh...
A.
Tyrannosaurus Rex had lips and Triceratops had cheeks.
B.
dinosaurs might have looked like mammals such as sheep.
C.
dinosaurs might not have looked like what we thought.
D.
dinosaurs must have looked like birds or crocodiles.