Mark Twain was famous in his days as a public speaker. In his public speeches he always liked to tell funny stories. He also liked to listen to funny stories and play jokes on his friends.
One day one of his friends lost his wallet and asked Mark Twain to pay his train fare （车费）for him.
“But I don’t have enough money to pay both your fare and mine, ”Mark Twain said. The friend didn’t know what to do.
“We can do this, ”said Mark Twain. “We can get on the train and when the conductor(列车员) comes to check the tickets，you can hide(藏) under my seat. ”
Later, however, when the conductor came to check the tickets, Mark Twain gave him two tickets－one for his friend and one for himself . Then he explained in a loud voice. “My friend here is a strange man. When he travels on a train, he doesn’t like to sit on the seat. He prefers （宁愿）to lie on the floor under the seat. ”Of course everybody in the train looked at the poor friend and laughed at him loudly.
小题1:Mark Twain told him he couldn’t buy him a ticket because ______.
A．he didn’t have enough money
B．he wanted to play a joke(玩笑) on him
C．he wanted to save some money
D．he didn’t find his wallet, either小题2:When the conductor came to check the tickets, the friend was ________.
A．lying under the seat
B．sitting on the seat
C．lying on the seat
D．standing nearby小题3:Mark Twain spoke in a loud voice because ______.
A．the conductor was standing far away
B．his friend was deaf （聋）
C．he enjoyed doing so
D．he wanted all the people here to know there was a man under the seat
．现将导轨沿与水平方向成
角倾斜放置．在底部接有一个
的电阻．现将一个长为
、电阻
的金属棒自轨道顶部沿轨道自由滑下，经一段距离后进入一垂直轨道平面的匀强磁场中（如图12－17所示）．磁场上部有边界，下部无边界，磁感应强度
．金属棒进入磁场后又运动了
后开始做匀速直线运动，在做匀速直线运动之前这段时间内电阻R上产生了
的内能（
）．求：
时的加速度a的大小及方向；
方向为沿轨道向上；（2）32.5m
，又有
代入得：
负号表示其方向为沿轨道向上．
，金属棒做匀速运动时根据平衡条件得：
即
