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如图所示，在同一水平面的两导轨相互平行，并处在竖直向上的匀强磁场中，一根质量为0.9kg的金属棒垂直导轨方向放置．导轨间距为0.5m．当金属棒中的电流为5A时，金属棒做匀速运动；当金属棒中的电流增加到8A时金属棒能获得2m/s2的加速度，g取10m/s2．求：

（1）磁场的磁感应强度B．

（2）导棒与水平导轨间的滑动摩擦系数μ．

答案

金属棒匀速运动时，安培力与摩擦力，则有：f=BI1L

棒加速运动时，由牛顿第二定律可得：BI2L-f=ma

联立得：BI2L-BI1L=ma

代入数据得：B=1.2T

匀速运动时，有：f=BI1L=1.2×5×0.5N=3N

根据摩擦力公式得：f=μN=μmg

联立以上方程，代入数据得：μ=

1
3
，

答：（1）磁场的磁感应强度B为1.2T；

（2）导棒与水平导轨间的滑动摩擦系数μ为

1
3
．

填空题

在新民主主义向社会主义过渡时期，经济上处于领导地位的是（）
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填空题

Some consumer researchers distinguish (1) "rational" motives and "emotional" (or "non-rational") motives. They use the term "rationality" (2) the traditional economic sense that assumes (3) consumers behave rationally when they carefully consider all alternatives (4) choose those that give them the greatest utility (i.e., satisfaction). (5) a marketing context, the term "rationality" implies that the consumer selects goods based (6) totally objective criteria, such (7) size, weight, price, and so on. "Emotional" motives imply the selection of goods (8) to personal or subjective criteria—the desire (9) individuality, pride, fear, affection or status.
The assumption underlying this distinction is (10) subjective or emotional criteria do not maximize utility or satisfaction. (11) , it is reasonable to assume that consumers always attempt to select alternatives that, (12) their view, serve to maximize satisfaction. Obviously, the assessment of satisfaction is a very personal process, based (13) the individual’s own needs as (14) as on past behavioral, social, and learning experiences. What may appear (15) irrational to an outside observer may be perfectly rational (16) the context of the consumer’s own psychological field. For example, a product purchased to enhance one’s self-image (such as a fragrance) is a perfectly rational form of consumer behavior. (17) behavior did not appear rational to the person who undertakes it (18) the time that it is undertaken, obviously he or she would not do it. (19) the distinction between rational and emotional motives does not appear to be warranted.
Some researchers go so far (20) to suggest that emphasis (21) "needs" obscures the rational, or conscious, nature of most consumer motivation. They claim that consumers act consciously (22) maximize their gains and minimize their losses; that they act not (23) subconscious drives but from rational preferences, (24) what they perceive to be (25) their own best interests.
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