如图所示，在倾角为37°的固定斜面上静置一个质量为5 kg的物体，物体与斜面间的动摩擦因数为0.8。求：
(1)物体所受的摩擦力；(sin 37°＝0.6，cos 37°＝0.8)
(2)若用原长为10 cm，劲度系数为3.1×103 N/m的弹簧沿斜面向上拉物体，使之向上匀速运动，则弹簧的最终长度是多少？(取g＝10 m/s2)
解：(1)物体静止在斜面上受力分析如图所示
则物体受到的静摩擦力Ff＝mgsin 37°
代入数据得Ff＝5×10×sin 37°N＝30 N，摩擦力方向为沿斜面向上
(2)当物体沿斜面向上被匀速拉动时，如图所示，弹簧拉力设为F，伸长量为x，则
F＝kx
F＝mgsin 37°＋F滑
F滑＝μmgcos 37°
弹簧最终长度l＝l0＋x
由以上方程解得l＝12 cm
| 阅读理解。 |
| One day Mrs Wilson took Trudy and Ben to go shopping. They went to the supermarket in the new shopping mall (购物中心). "Why do you buy things here, Mum?" Trudy asked. "Because they are cheaper than those at the corner store," Mrs Wilson said. "Help me check (检查) the prices." The Wilsons were not wealthy (富有的) and Mrs Wilson was always careful with her money. She looked carefully at the prices of things. She bought some groceries (杂货) in the supermarket. When they got home, the children said, "We don't think you saved money by going to the supermarket there." "Of course I did," said Mrs Wilson, "everything was cheap there." "We know," said the children, "but we came home by taxi because we had too much to carry. The cost (花费) of the taxi was more than the money you saved!" Mrs Wilson added everything up and found her children were right. "Well done," she said, "next time we will do shopping near our home." |
| 1. Mrs Wilson took Trudy and Ben ______. |
[ ] |
| A. to the supermarket B. to the zoo C. to the park D. to the museum |
| 2. Mrs Wilson bought things in the supermarket in the new shopping mall because ______. |
[ ] |
| A. goods there had good quality B. there were many goods C. goods there were cheap D. goods there were expensive |
| 3. Which of the following is correct? |
[ ] |
| A. The Wilsons were rich and they bought a lot of things in the supermarket. B. The Wilsons were not rich and they bought things in a cheaper supermarket. C. The Wilsons enjoyed shopping in the shopping mall because it was large. D. The Wilsons were poor and they seldom went shopping. |
| 4. Mrs Wilson went home ______. |
[ ] |
| A. by bus B. by minibus C. by taxi D. on foot |
| 5. Mrs Wilson decided to go shopping ______ next time. |
[ ] |
| A. in the shopping mall far away B. in the cheapest shop C. in the most expensive shop D. near her home |