问题 选择题

小车重200N,人用30N的水平力推小车沿水平路面匀速前进50m的过程中,下列判断正确的是(  )

A.重力做功为10000J

B.人对车做功10000J

C.人对车做功1500J

D.小车受到的阻力是230N

答案

根据题意得人对车做的功为:

W=Fs=30N×50m=1500J

在重力的作用下物体没有移动距离所以重力没有做功,故A选项不对.

由于物体匀速前进,所以小车受到的阻力与人的推力相等,D选项也不对.

通过计算可知人对车做功1500J,故选C.

问答题

Ragtime is a musical form that synthesizes folk melodies and musical techniques into a brief quadrille-like structure, designed to be played--exactly as written--on the piano. (46) A p analogy exists between European composers like Ralph Vaughan Williams and Edward Grieg, who combined folk tunes and their own original materials in larger compositions and the pioneer composers in the United States. Composers like Scott Joplin and James Scott were in a sense collectors or musicologists, collecting dance and folk music in Black communities and consciously shaping it into brief suites or anthologies called piano rags.

It has sometimes been charged that ragtime is mechanical. For instance, Wilfred Mellers comments, "rags were transferred to the pianola roll and, even if not played by a machine, should be played like a machine, with meticulous precision. " (47) However, there is no reason to assume that ragtime is inherently mechanical simply because commercial manufacturers applied a mechanical recording method to it, the only way to record pianos at that date. Ragtime’s is not a mechanical precision, and it is not precision limited to the style of performance. It arises from ragtime’s following a well-defined form and obeying simple rules within that form.

The classic formula for the piano rag disposes three to five themes in sixteen-bar strains, often organized with repeats. (48) The rag opens with a bright, memorable strain or theme, followed by a similar theme, leading to a melody of marked lyrical character, with the structure concluded by a lyrical strain that parallels the rhythmic developments of the earlier themes. The aim of the structure is to rise from one theme to another in a stair-step manner, ending on a note of triumph or exhilaration. Typically, each strain is divided into two 8-bar segments that are essentially alike, so the rhythmic-melodic unit of ragtime is only eight bars of 2/4 measure. (49) Therefore, not concerned with development of musical themes, the ragtime composer instead sets a theme down intact, in finished form, and links it to various related themes that are brief with clear melodic figures. Tension in ragtime compositions arises from a polarity between two basic ingredients: a continuous bass--called by jazz musicians a boom-chick bass--in the pianist’s left hand, and its melodic, syncopated counterpart in the right hand. Ragtime remains distinct from jazz both as an instrumental style and as a genre. Ragtime style stresses a pattern of repeated rhythms, not the constant inventions and variations of jazz. (50) As a genre, ragtime requires strict attention to structure, not inventiveness or exceptional skills, existing as a tradition, a set of conventions, a body of written scores, separate from the individual players associated with it. In this sense ragtime is more akin to folk music of the nineteenth century than to jazz.

(49) Therefore, not concerned with development of musical themes, the ragtime composer instead sets a theme down intact, in finished form, and links it to various related themes that are brief with clear melodic figures.

单项选择题 B1型题