一座城市地图中两地图上距离为5厘米，表示实际距离30 千米，这幅地图的比例尺是____．

一卫星绕某一行星表面附近做匀速圆周运动，其线速度大小为v．假设宇航员在该行星表面上用弹簧测力计测量一质量为m的物体重力，物体静止时，弹簧测力计的示数为N．已知引力常量为G，则这颗行星的质量为（   ）

     A piano doesn't like a drum. It doesn't sound like a drum, either(也). So how can the two be alike(相似的) ?

     To play the drums, you use sticks. You hit the drums and the sound comes out. A piano is played in the same way. You hit the keys. The keys move hammers(锤) inside the piano. The hammers hit strings(琴弦) to make sounds. That's how a piano is like a drum.

下列说法中错误的是（）