问题 计算题

(8分)气球质量为200,载有质量为50的人,静止在空中距地面20 m高的地方,气球下悬一根质量可忽略不计的绳子,此人想从气球上沿绳慢慢下滑至地面,为了安全到达地面,则这根绳长至少应为多少米?(不计人的高度)

答案

25 m

以人和气球为研究对象(系统),这个系统所受重力与空气浮力平衡,故系统动量守恒,此系统类似于人船模型。

设人的质量为,气球的质量为M,气球静止时离地高为H,当人到达地面时,气球又上升了h,则有:

, (4分)

其中, , m ,即可求得:m , (2分)

故此人要想从气球上沿绳慢慢安全到达地面,这根绳长至少应为:

H + h =" 25" m。(2分)

本题考查动量守恒定理,以人和气球为研究对象(系统),这个系统所受重力与空气浮力平衡,故系统动量守恒,此系统类似于人船模型,找到初末状态,由平均动量守恒可求得高度h

填空题

Motivation is "the driving force within individuals that impels them to action." And goals are the sought-after results_____(1) motivated behavior.Motivation can be either positive or negative_____(2) direction. We may feel a driving force toward some object or condition, _____(3) a driving force away from some object or condition. For example, a person may be impelled toward a restaurant to fulfill a need, hunger, and away_____(4) an airplane to fulfill a need of safety. Some psychologists refer to positive drives_____(5) needs, wants or desires,_____(6) negative drives as fears or aversions._____ (7), though negative and positive motivational forces seem to differ dramatically_____(8) terms of physical and sometimes emotional activity, they are basically similar in_____(9) they both serve to initiate and sustain human behavior.____(10) this reason, researchers often refer _____(11) both kinds of drives or motives as needs, wants and desires.Goals, _____(12), can be either positive or negative. A positive goal is one toward_____ (13) behavior is directed and it is often referred to as an approach object. A negative goal is _____(14) from which behavior is directed away and it is sometimes referred to as an avoidance object. Since both approach and avoidance goals can be considered objectives of motivated behavior, most researchers refer to_____(15) types simply as goals. Consider this example. A middle-aged woman may wish to remain_____(16) attractive as possible. Her positive goal is to appear desirable, and_____(17) she may use a perfume advertised to make her irresistible. A negative goal may be to prevent her skin_____(18) aging, and therefore she may buy and use face creams._____(19) the former case, she uses perfume to help her achieve her positive goal — attractiveness; in the____(20) case, she uses face creams to help avoid a negative goal — wrinkled skin.

单项选择题