问题 计算题

火车站上由于工作人员操作失误致使一节车厢以4m/s的速度匀速滑出了车站,此时在同一轨道上一列火车正在以72km/h的速度匀速驶向车站,技术娴熟的火车司机突然发现这种紧急情况后,立即以大小为0.8m/s2的加速度紧急刹车,之后又立即以此加速度使火车反向加速运动,若车厢与火车相遇恰好不相撞。求:

(1)司机发现车厢向自己驶来开始制动到刚好相遇用的时间;

(2)司机发现车厢向自己驶来开始制动时离车厢的距离。

答案

解:以货车原行驶方向为正方向,设火车原行驶的速度为v1,车厢行驶的速度为v2,司机发现车厢向自己驶来开始制动到刚好接住车厢用时用的时间为t,这段时间内火车行驶的位移为x1,车厢行驶的位移为x2;司机发现车厢向自己驶来开始制动时离车厢的距离为x0;则由匀变速运动的规律可知:

      

      

            

                

代入数值解得

多项选择题
填空题

[A] "Customers don’t buy products," Mary says. "They buy results." Mary explains that it is her job to help customers get results. Results are what the products do for the customers. Customers buy products for what the products do.

[B] Mary enjoys her job. Her job is customer service. She helps customers use her company’s products. When people ask her what she does, Mary says. "Salespeople sell our products. My job is to make sure they stay sold."

[C] She finds someone who is doing exactly what the customer who called is interested in seeing. Mary calls customers who are already using the product. She asks if the customer who wants to see how the products are used can visit these companies. Most customers cooperate. They let other customers visit to see how products are used. They help others because they know they may need help themselves some day. They know they will be able to ask for help later if they give help to others now.

[D] Mary gets copies of sales orders from Steve, John, Helen, and other salespeople. When she gets the orders, Mary talks to the salespeople about the customers. It is part of Mary’s job to find out how customers are using her company’s products. Mary tells customers, "I’m in the information business. My job is to give you any information you need about our products and how they’re used."

[E] Mary makes appointments for her company’s customers to visit each other. Mary’s company is very happy to have cust6mers compare how they use its products. The company feels that this is how customers help sell its products to other customers.

[F] Sometimes Mary helps customers to help each other. Customers call to ask about using her company’s products. Mary gives them product literature. She also gives them information from her files on how products can be used. Sometimes customers want to see products being used. These customers are interested in seeing exactly what is done with the products. When this happens, Mary looks in her files. ’She looks to see which customers are using the products. Then she looks to see how the products are used. [G] Mary keeps files of information about products and customers. Files are groups of documents set up to make it easy to find any document or record which is needed. In her files, Mary has printed sheets about her company’s products. These printed sheets about the things the company makes are called product literature. Product literature tells about the things the company makes. The literature also tells what the products do. The idea is to show how products help customers who use them.

[H] Mary has a list of all the company’s customers. When she receives copies of orders, Mary checks them to see if there are any new customers. Mary also checks to see if any old customers are buying product that has not been made before. Mary checks orders to see which customers buy it. Any time customers buy products they have not used before, Mary calls to see if she can help. Mary asks if the customer needs information. She also asks how the customer is using the product. Mary puts information in her files on how customers use her company’s products.

Order:

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