My sister _____ the MP5 for a year. But it broke now.
A:has bought
B:has had
C:buy
以下说法中正确的是( )
如图所示的动滑轮,拉着物体M以0.5m/s的速度在水平面上做匀速直线运动,物体重200N,它受到的摩擦力是物重的0.2倍,水平拉力为25N.2s内拉力做的功是 ____J,该动滑轮的机械效率是 ____
设G为△ABC的重心, AG→ =2 AM→ ,则( )