问题 选择题

如图所示,一只小猫趴在自动扶梯的水平踏板上,随扶梯一起向上做匀加速运动.关于小猫在上述过程中的受力情况,下列说法正确的是(  )

A.小猫只受到重力和踏板的支持力作用

B.小猫对踏板的压力一定小于小猫受到的重力

C.小猫所受合力的方向一定和扶梯的加速度方向相同

D.小猫所受踏板作用力的方向一定和扶梯的加速度方向相同

答案

A、小猫具有沿扶梯向上的加速度,知小猫受重力、支持力和静摩擦力大小.故A错误.

B、将加速度沿水平方向和竖直方向分解,在竖直方向上有向上的加速度,则支持力的大小大于重力,压力等于支持力,所以小猫对踏板的压力大于小猫受到的重力.故B错误.

C、根据牛顿第二定律知,加速度的方向与合力的方向相同.故C正确.

D、小猫受到踏板的支持力和摩擦力,两个力的合力方向与最终三个力的合力方向不同,所以与扶梯加速度的方向不同.故D错误.

故选C.

单项选择题

(二) Mozart was born in what is now called Austri, but, at that time, it was part of the Holy Roman Empire. He was baptized the day after his birth at St. Rupert’s Cathedral. His father Leopold was from Augsburg. He was a minor composer, and an experienced teacher. In the year of Mozart’s birth, his father published a violin textbook, which achieved a great success. In the fourth year of his age his father, for a game as it were, began to teach him a few minutes and pieces at the clavier. He could play it faultlessly and with the greatest delicacy, and keeping exactly in time. At the age of five, he was already composing little pieces, which he played to his father who wrote them down. While Leopold was a devoted teacher to his children, there is evidence that Mozart was keen to progress beyond what he was taught. His first ink spattered composition and his efforts with the violin were of his own initiative and came as a surprise to Leopold. Leopold eventually gave up composing when his son’s outstanding musical talents became evident. He was Mozart’s only teacher in his earliest years and taught his children languages and academic subjects as well as music. During Mozart’s youth, his family made several European journeys in which he played at the court in Munich, and at the Imperial Court in Vienna. A long concert tour spanning three and a half years followed, taking the family to the courts of Munich, Paris and London. During this trip, Mozart met a great number of musicians and acquainted himseff with the works of other composers. These trips were often arduous. Travel conditions were primitive; the family had to wait for invitations and reimbursement from the nobility. They endured long, near -fatal illnesses far from home. After one year father and son set off for Italy, leaving his mother and his sister at home. This travel lasted from December 1769 to March 1771. As with earlier journeys, Leopold wanted to display his son’s abilities as a performer and a rapidly maturing composer. After finally returning with his father from Italy on 13 March 1773, Mozart was employed as a court musician by the reler of Salzburg, Prince Archbishop Hieronymus Colloredo. The composer had a great number of friends and admirers in Salzburg. Despite these artistic successes, Mozart grew increasingly discontented with Salzburg and redoubled his efforts to find a position elsewhere. One reason was his low salary. In August 1777, Mozart resigned his Salzburg position and, on 23 September, ventured out once more in search of employment, with visits to Augsburg, Mannheim, Paris, and Munich. Since Archbishop Colloredo would not give Leopold leave to travel, Mozart’s mother Anna Maria accompanied him. Mozart became acquainted with members of the famous orchestra in Mannheim, the best in Europe at the time. He also fell in love with Aloysia Weber, one of four daughters in a musical family. There were prospects of employment in Mannheim, but they came to nothing, and Mozart left for Paris on 14 March 1778 to continue his search. One of his letters from Paris hints at a possible post as an organist at Versailles, but Mozart was not interested in such an appointment. Mozart’s new career in Vienna began well. He performed often as a pianist, notably in a competition before the Emperor on 24 December 1781, and he soon "had established himself as the finest keyboard player in Vienna".

Mozart grew discontented with Salzburg and tried to find another position because______

A.he was not interested in the position of the court musician

B.he was not satisfied with his low salary

C.he was not getting along well with the ruler of Salzburg

D.he fell in love with Aloysia Weberhe in Mannheim

问答题 简答题