如图所示为 一列沿x轴正方向传播的简谐波在某个时刻的波形图,下 列关于该列波的说法中正确的是( )
A:该简谐波的振幅是4 cm
B:质点P此时正向上运动
C:质点P此时正向下运动
D:质点P振动的振幅是2 cm
This is a photo of Mr Black's family. The man with glasses(眼镜)is Mr Black, the father. The woman is the mother. They have a son and a daughter. The son is behind(在……后面)Mr Black. His name is Jack. He's 14. Kate is Jack's sister. She is 12. Jack and Kate are in the same school, but they are in different(不同的)grades. Jack is in Grade Three and Kate's in Grade One. They are good students.
在水平地面上,用60N的力沿水平方向拉着重为200N的小车前进5m,拉力做功____ J,重力做功____ J.
下列叙述正确的是( )