问题 计算题

(8分)质量为2 kg的铅球从离地面2 m高处自由落下,陷入沙坑中2 cm深处,如图所示,(g取10 m/s2)求:

(1) 小球落地时的速度为v

(2) 沙子对铅球的平均阻力Ff

答案

(1)(2)

(1)小球自由下落阶段只有重力做功,设落地小球的瞬时速度为v

解得

(2)(方法一)小球从陷入沙中到停止运动这段时间里,重力对小球做正功,阻力Ff对小球做负功,由动能定理有.   解得.

(方法二)小球从开始下落到停止运动,重力在全过程中均做功,阻力则只在小球陷入沙坑后才做功.由动能定理有, 故.

本题考查的是自由落体运动中动能定理的应用问题。小球自由下落,只受重力,机械能守恒,陷入沙坑,除了重力作用外,还有沙子对小球的平均作用力,此时用动能定理解题比较方便。

选择题
单项选择题

By the early 1950s, some business people began to recognize that efficient production and extensive promotion did not guarantee that customers would buy products. These businesses, and many others, found that they must first determine what customers want and then produce it, rather than make products and try to change customers’ needs to fit what is expected. As more organizations realized the importance of knowing customers’ needs, U.S. businesses entered the marketing era, one of market or customer orientations (取向, 方向). Orientation toward customer satisfaction has resulted in increased concern about ethics and social responsibility and brought about an expansion into global markets. Management at many organizations has realized that we are in the "Total Quality Era", in which improved product quality, and customer focus are major components of successful domestic and global operations.

Because the marketing concept affects many parts of a business’s operations, and not just marketing, an organization’s top management must adopt it wholeheartedly. High-level executives must incorporate the market orientation into their management philosophies so completely that customers become the organization’s most important concern.

Management’s second major task is to restructure the organization. To satisfy customers’ objectives as well as its own, a company must coordinate all activities. To achieve this, the internal operations and overall objectives of one or more departments may need restructuring. If the head of the marketing unit is not a member of the organization’s top-level management, he or she should be. Some departments may have to be abolished and new ones created. Implementing (实施, 执行) the marketing concept demands the support not only of top management, but also of managers and staff at all levels. The transformation to an organization with a market orientation takes time. In the short run, a firm may experience the need to retain employees, financial constraints, limits on technology, capital equipment limitations, and prohibitive union work rules.

Even when an information system is established and the company reorganized, the firm’s new marketing approach may not work perfectly. First, a firm’s ability to satisfy customers’ needs for a particular product is limited. In a mass production economy, most business organizations cannot tailor products to fit the exact needs of each customer. Second, a company may be unable to learn what customers want, and when it does correctly identify customers’ needs, it often has a hard time developing a product that satisfies those needs. Third, by striving to satisfy one segment (部分, 部门) of society, a firm sometimes dissatisfies other segments. Producers of tobacco currently face this situation. Fourth, a company may have trouble maintaining employee morale during any restructuring to coordinate the activities of various departments.

A firm’s new marketing approach may not work perfectly because of().

A. lack of an information system

B. limited ability to satisfy customers’ needs

C. difficulty in developing tailored products

D. all of the above