一定质量的导体棒放在光滑的倾斜导轨上，如图3（
）．若给导体棒通电，并在此区域加上强度一定的匀强磁场，图3（
）中，有可能使导体棒保持静止状态的有（ ）
ABC
根据左手定则可以知道四个选项中导体棒受到的安培力方向分别为：水平向右、竖直向上、沿斜面向上、垂直斜面向下.物体还受竖直向下的重力，可能受到斜面对导体棒的支持力，由共点力的平衡条件容易知道选项A、C中导体棒可以处于静止状态.对于选项B容易漏选，忽略了当
时，此时不受斜面的支持力，物体恰好处于平衡状态.所以答案应为ABC.
Passage Three
| For centuries, explorers have risked their lives venturing into the unknown for reasons that were to varying degrees economic and nationalistic (国家主义的). Columbus went west to look for better trade routes to the Orient and to promote the greater glory of Spain. Lewis and Clark journeyed into the American wilderness to find out what the U. S. had acquired when it purchased Louisiana, and the Appolo astronauts rocketed to the moon in a dramatic show of technological muscle during the cold war. Although their missions blended commercial and political military imperatives, the explorers involved all accomplished some significant science simply by going where no scientists had gone before. Today Mars looms (隐约出现) as humanity’s next great terra incognita (未探明之地). And with growing emphasis on international cooperation in large space ventures, it is clear that imperatives (需要，必要) other than profits or nationalism will have to compel human beings to leave their tracks on the planet’s reddish surface. Could it be that science, which has long played a minor role in exploration, is at last destined to take a leading role The question naturally invites a couple of others: Are there experiments that only humans could do on Mars Could those experiments provide insights profound enough to justify the expense of sending people across inter- planetary space With Mars the scientific stakes are arguably higher than they have ever been. The issue of whether life ever existed on the planet, and whether it persists to this day, has been highlighted by mounting evidence that the Red Planet once had abundant stable, liquid water and by the continuing controversy over suggestions that bacterial fossils rode to Earth on a meteorite (陨石) from Mars. A more conclusive answer about life on Mars, past or present, would give researchers invaluable data about the range of conditions under which a planet can generate the complex chemistry that leads to life. If it could be established that life arose independently on Mars and Earth, the finding would provide the first concrete clues in one of the deepest mysteries in all of science: the prevalence of life in the universe. |
At present, a probable inducement (诱因) for countries to initiate large scale space ventures is______.
A．international cooperation
B．nationalistic reasons
C．scientific research
D．long-term profits