Một lò xo có có độ cứng 25 N/m và chiều dài tự nhiên là 10 cm. a) Giữ một đầu lò xo cố định, đầu còn lại được kéo bởi một lực kế. Hỏi khi lò xo bị dãn 2 cm thì lực kế chỉ giá trị bao nhiêu? b) Treo lò xo thẳng đứng và móc vào đầu tự do của nó một vật có khối lượng m thì lò xo dãn 3 cm. Cho g = 10 m/s2 .Nếu treo thêm vật M = 1,5m thì lò xo dãn bao nhiêu? c) Đặt lò xo lên mặt phẳng nghiêng góc 300 như hình vẽ trên rồi móc vào đầu tự do của nó vật m. Tính chiều dài của lò xo khi hệ cân bằng. d) Với mặt phẳng nghiêng cố định và hệ vật - lò xo như câu c thì cần phải tác dụng vào đầu kia của lò xo một lực tối thiểu là bao nhiêu để hệ vật – lò xo chuyển động thẳng đều hướng dọc lên mặt nghiêng đó? Giải giúp em câu c với d ạ

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Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions. The Earth Day comes from the idea of a former US politician Gaylord Nelson. He was worried about the world pollution, and the health of plants and animals. In 1969, he decided that the world should have a special day to teach people about how to make environment better. His idea became popular and on 22 April 1970 the first Earth Day was celebrated. Since then, environmentalists have used the day to educate people about their roles in protecting our planet. Earth Day finally became a global celebration around the world. On this day, people come together to talk about global warming, protecting rare animals and saving energy sources. Question 20: Environmentalists have used the Earth Day to ________. A. ban people from using energy sources B. carry their research C. educate people about their roles of protecting our planet D. get people around the world to play together 3 Choose the best answer A, B, C or D to complete the sentence. Question 10: Too much use of electric lights in cities may cause ___________ pollution. A. light B. water C. visual D. thermal 4 Choose the best answer A, B, C or D to complete the sentence. Question 12: The polluted water__________ in the death of the fish. A. results B. leads C. causes D. makes 5 Choose the best answer A, B, C or D to complete the sentence. Question 7: I think the waste from the factory has ___________ the lake. A. polluted B. damage C. damaged D. pollution

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