用分子热运动的观点解释下列现象，解释不正确的是（　　）

根据短文内容，选择正确答案。

    I am a student in China now. My name is Kelly Smith. I live with my parents, two sisters and a brother in Beijing. My parents teach English in No. 6 Middle School. I study in the same school. I am happy here, because I like my new school and the classmates. They are very kind to me. They like to play with me because I look different from them. I have blue eyes and long blond hair. They often say I look like a doll. Also I speak English well. so lots of students like to tam with me to improve (提高) their English.

如图所示，轻质木杆AB可以绕O点转动，OA的长度是OB的三倍，A端细线下所挂280N的重物静止在水平地面上，在B点用600N的动力竖直向下拉动，木杆静止不动，则重物对水平地面的压力为____ N，此木杆为____（“省力” “费力”“等臂”）杠杆。