Bài 1: Trong thực tế khi sử dụng các máy cơ đơn giản bao giờ chúng ta cũng phải tốn một công để thắng lực ma sát. Do vậy công toàn phần (A_tp ) phải lớn hơn công có ích (A_i ) để nâng vật trực tiếp. Tỉ số A_i /A_tp gọi là hiệu suất của máy, kí hiệu là H. Để làm tăng hiệu suất cho Mặt phẳng nghiêng, theo em ta phải làm gì?

Bài 2: Người ta dùng một mặt phẳng nghiêng để kéo một vật có khối lượng 80kg lên cao 1,5m.

a) Nếu bỏ qua ma sát (ma sát không đáng kể) thì lực kéo là 200N. Tính chiều dài của mặt phẳng nghiêng.

b) Thực tế có ma sát đáng kể và lực kéo là 240N. Hãy tính hiệu suất của mặt phẳng nghiêng.

Bài 3: Dùng một palăng để đưa một vật nặng 20N lên cao 10cm, người ta phải dùng một lực kéo dây đi một đoạn 40cm. Tính lực kéo dây và công đã sinh ra. Giả sử ma sát ở các ròng rọc là không đáng kể.

Bài 4: Một động cơ xe lửa có công suất 12kW kéo một toa hàng bằng một lực 12000N từ vị trí A đến vị trí B trên đoạn đường nằng ngang trong thời gian 30 phút. Tính quãng đường AB.

Bài 5: Người ta dùng một cần cẩu có công suất 20kW để nâng vật nặng 2 tấn lên cao 5m. Hiệu suất của động cơ là 80%. Hãy tính thời gian nâng vật.

Bài 6: Mũi tên được bắn đi từ cái cung có cơ năng tồn tại ở dạng nào?

Bài 1: Một người công nhân kéo vật liệu xây dựng lên cao bằng ròng rọc cố định. Lực kéo của người đó là 400N, kéo lên tầng hai cao 3,5m trong khoảng thời gian là 1 phút. Hãy tính công và công suất của người đó.

Bài 2: Một thang máy chở người và hàng, có tổng khối lượng 300kg, di chuyển từ tầng 1 lên tầng 15 của một tòa nhà, với mỗi tầng cao 3,4m; thang máy di chuyển liên tục, không dừng lại ở các tầng, mất thời gian là 70s. Hãy tính công thực hiện được và công suất của thang máy.

Bài 3: Dùng mặt phẳng nghiêng có độ dài là bao nhiêu để đưa được vật có khối lượng 50kg lên độ cao 1m mà chỉ bằng lực có cường độ 125N?

Bài 4: Một đầu máy kéo 1 chiếc xe bằng lực 3000N chạy đều, biết công suất làm việc của đầu máy là 30kW. Tính vận tốc mà xe đạt được

Bài 5: Một đầu tàu hỏa chuyển động đều với vận tốc 36km/h. Đầu máy phải thắng một lực cản bằng 0,005 trọng lượng của đoàn tàu. Biết công suất của đầu máy là 750kW. Hãy xác định khối lượng của đoàn tàu.

Bài 6: Một HS rót nước từ phích vào cốc. Cơ năng của dòng nước tồn tại ở dạng nào?

Chuyển câu chủ động thành câu bị động

1. Mary’s parents sent her some money.

2. Each competitor must take a bottle on the river bank

3. Did they build this monument ten years ago?

4. They delayed the football match.

5. People put many kinds of newspapers on the table

6. Tom must write this essay.

7. My father has just bought a lot of new books

1. People will publish this novel in February.

2. They are building a new hotel.

3. Betty has just sent Mai many postcards.

4. Hoa sent her mother a gift.

5. They will build many new schools in the future.

6. Students must write the answer in ink.

7. He wrote all the songs on this new album.

8. They held rice-cooking festival in Thon Trieu last year.

9. We must solve this problem soon.

10. Nguyen Du wrote Truyen Kieu.

1 . We must do something for these poor men

2. They have to separate the rice from the husk.

3. He shouldn’t do that silly thing.

4. The boy operates that tape-recorder.

5. They gave us a post card.

6. The student cleaned the room every day.

7. I had lost some of my books.

8. We believe these boys to become better.

9. The scientists found the problem.

10. The cat pulls the pan into the sink.

1. He gives her a novel about a love story.

2. They were painting a house.

3. She is watering those flowers.

4. We had finished our reports in three months.

5. He ought to sell that car.

6. We have seen her at the station.

7. The librarian will give students the books.

8. He showed me the way to Thames River.

9. He was learning in his room at 8.30 yesterday.

10. You can buy me an envelope.

1. We have just bought a car.

2. We have just met Jim.

3. They visited the church two years ago.

4. Mrs. Green is cooking the food in the kitchen.

5. The doctor examined the patients.

6. These boys made that noise.

7. I can’t do these exercises quickly.

8. She had finished the report by noon.

9. The movie has disappointed us very much.

10. They have to pay the bill before the first of the month.

1. They will ask you several questions.

2. My parents promised me a bicycle two days ago

3. Somebody has locked the box and I can’t open it.

4. Somebody calls the president every morning.

5. Airplanes are carrying food to the flood region now.

6. Nam can repair this sewing machine.

7. People advised us not to go out alone.

8. The police ought to put the robbers in prison.

9. They had checked her handbag.

10. Susan’s father has promised her a new watch.

1. You will answer all the questions on the paper.

2. The earthquake has destroyed many buildings.

3. He has just killed a tiger.

4. Lan’s mother is making her a dress.

5. We saw Mrs. Quyen leave her house at 9 o’clock.

6. Mr. Lam must finish this work before June.

7. My parents are going to buy a new TV tomorrow.

8. We haven’t seen Peter yet.

9. Nam and Ba are painting the fence.

10. Hoa reads a story book after dinner.

1. They were building a new school here when we came.

2. He will send his parents a gift.

3. She has bought two new dresses recently.

4. You must write these music lessons in ink.

5. We saw our friends outside the church yesterday

6. Lan saw an accident on Le Loi street yesterday.

7. People can grow potatoes in this farm.

8. I have seen that exciting film many times.

9. My father drinks a cup of tea in the morning.

10. Bill called his boss yesterday.

1. Susan is phoning her boyfriend in the living room now

2. Mr. and Mrs. Long were playing chess when we came.

3. His parents have just given him a new dictionary.

4. We grow rubber trees in our country.

5. He met his father at the front gate.

6. People can’t plant rubber trees in the desert.

7. We haven’t seen Peter yet.

8. Ba and Nam are painting the fence.

9. Linh reads a story book after dinner.

10. He will send his parents a gift.

Chuyển câu chủ động sang câu bị động

1. A stranger started the fire. .......................................................................................

2. They will publish the book next year. .......................................................................................

3. A French architect designed the bridge. .......................................................................................

4. Lightning struck the house. .......................................................................................

5. Maria won the first prize. .......................................................................................

6. They will sign the contract tomorrow. .......................................................................................

1. They sent him to Los Angeles. .......................................................................................

2. Mrs. Kennedy teaches the class. .......................................................................................

3. They published the book in 1997. .......................................................................................

4. The police have caught the thief. .......................................................................................

5. They gave Linda a prize for one of her books. ........................................................................................................

6. Someone has told me about her success. .......................................................................................

7. People speak English all over the world. .......................................................................................

8. They sell English books in that shop. .......................................................................................

9. Someone is repairing my car at the moment. ...........................................................................................................

10. People always admire this picture. .......................................................................................

1. He uses milk for making butter and cheese. ............................................................................................................

2. She makes progress every day in the world of science. ...........................................................................................................................................

3. The author will write this novel in June. .......................................................................................

4. Nobody will solve this hard problem. .......................................................................................

5. Her mother will take her to hospital next week. .......................................................................................................

6. Someone will clean the room. .......................................................................................

7. They had to cut down that tree. .......................................................................................

8. Someone should tell Sally what happened. .......................................................................................

9. They’re going to build a new hospital. .......................................................................................

10. We can solve the problem. .......................................................................................

1. Someone has to finish the job. .......................................................................................

2. They may send the man to prison. .......................................................................................

3. We must do something now. .......................................................................................

4. I don’t like people starting at me. .......................................................................................

5. I dislike people making jokes about me. .......................................................................................

6. They separated the rice from the husk. .......................................................................................

7. The boys made a fire by their camp. .......................................................................................

8. They built this house more than one hundred years ago. .........................................................................................................................................

9. Every year Viet Nam exports millions of tons of rice. ..............................................................................................................................................

10. The publisher published his book in 1996. .......................................................................................

1. They repaired my desk during the weekend. ...........................................................................................................

2. Nobody saw her at the party last night. .......................................................................................

3. The government has not changed the regulation. .......................................................................................

4. She didn’t introduce me to her mother. .......................................................................................

5. Someone is cutting the grass now. .......................................................................................

6. Someone ought to repair this window. .......................................................................................

7. Peter is writing a letter to his friend. .......................................................................................

8. My friends are making some phone call in the evening. .......................................................................................................

9. Bad drivers are causing many accidents. .......................................................................................

10. People play football all over the world. .......................................................................................

1. They will sign the contract tomorrow. .......................................................................................

2. We will issue the library cards soon. .......................................................................................

3. Thuy has to finish her homework tonight. .......................................................................................

4. People must hand in their weapons. .......................................................................................

5. They are repairing my piano at the moment. ...........................................................................................................

6. A doctor has just examined John. .......................................................................................

7. The fire destroyed many houses yesterday. .............................................................................................................

8. Tom will return those books to the library tomorrow. ..........................................................................................................

9. Bad weather has delayed Flight 202 from Miami. .......................................................................................

10. You must keep your room tidy. .......................................................................................

1. He is awarding the prize to the Thon Trieu team. .......................................................................................

2. The company has ordered some new equipment. .......................................................................................

3. I decorated the Christmas Tree yesterday. .......................................................................................

4. The tornado destroyed thirty houses. .......................................................................................

5. The manager should sign these contracts today. .......................................................................................

6. Lan will clean all the glass windows. .......................................................................................

7. Lan’s mother has to buy some oranges. .......................................................................................

8. They performed Christmas songs 800 years ago. .......................................................................................

9. Somebody will call Mr. Green tonight. .......................................................................................

10. These workers are building a new hospital.............................................................................................................

1. Nobody has used this room for ages. .......................................................................................

2. Hoa sent a Christmas card to Lan last Christmas. .......................................................................................

3. Santa Claus gives children presents on Christmas Eve..............................................................................................................................................

Giới thiệu 1 tác phẩm văn học đặc sắc dành cho thiếu nhi

xy+x-2y=3

<=>x(y+1)-2y-2=3-2

<=>x(y+1)-2(y+1)=1

<=>(x-2)(y+1)=1

<=>x-2 và y+1 là ước của 1

Lập bảng tìm x;y

90 tấn = 90 000 kg