Tell me about your knowledge of compasses.

Teaching content:

Example 1, example 2, example 3 and "practice for a while" exercise questions 1 and 17 in the fifth grade experimental textbook of compulsory education curriculum standard published by Jiangsu Education Publishing House.

Teaching objectives:

1, knowledge goal: make students know the circle, master its characteristics and understand the relationship between the inner diameter and radius of the same circle. Let the students know the tools for drawing a circle and draw a circle with a compass of a specified size. Can apply the knowledge of circle to explain some phenomena in daily life.

2. Skill goal: Through intuitive teaching and hands-on operation, students can understand and form the concept of circle on the basis of full perception. Cultivate students' observation ability, abstract generalization ability and spatial imagination.

3. Emotional goal: let students feel that mathematics is closely related to life, feel the value of mathematical knowledge, and stimulate students' interest in learning.

Teaching focus:

Through intuitive demonstration and hands-on operation, let students perceive and master the characteristics of the circle.

Teaching difficulties:

Understand the characteristics of the circle through activities; Solve practical problems about circles in life.

Teaching aid preparation:

Multimedia courseware, compass, triangle, round paper.

Learning aid preparation:

Ruler, triangular ruler, compass, rope, thumbtack, pencil, coin, round piece of paper.

Design concept:

Follow the concept of "student-oriented, lifelong development", let students actively construct new knowledge on the basis of original knowledge, cultivate students' awareness of exploration and innovation, and promote the coordinated development of students' knowledge and ability through independent exploration, cooperation and exchange.

Teaching steps: teacher activities, student activities.

First, doubt arouses interest.

Introduce a new course

Teacher: Students, the animals in the animal kingdom invited us to watch the bicycle race. Are you happy? Please see (media presentation): the dog, the white rabbit and Xiaohua Mall have each prepared a bicycle, [the wheels are (1) oval, (2) round but the axle is not in the center, (3) round and the axle is in the center], and the race has not yet started. Let's guess first; Who can win the first prize in the end?

Is XX's guess right? After learning the lesson of "understanding circle", we can answer this question with evidence. The students answer (the questions on the blackboard). At this time, students can tell who won the first prize, but they can't tell the truth clearly, and teachers are not in a hurry to draw conclusions.

Second, practice.

Explore new knowledge

(1) Teaching examples 1

1, find the circle around you

2, abstract the circle from the real thing.

3. Draw a circle by hand, and initially perceive the characteristics of the circle.

4. Contrast: The prominent circle is a curve. Draughtsman: We have got a preliminary understanding of this circle before. Tell me, where is the circle on the objects around us?

The courseware shows some round objects. Such as five rings, buildings and circular signs. ...

Teacher: So what exactly is the circle we want to know today? Please see (show the courseware). This is a round object-a clock. If you draw along its outer edge, it will become a circle.

The circle is a beautiful geometric figure. Do you want to draw a circle yourself? Listen to the requirements: according to the nails, lines, pencils, coins, rulers and other materials on the table, work in groups and draw a circle on the paper

By comparison, which group has more ideas? Hands-on operation, patrol between divisions.

Communication after painting: Tell me how you painted it.

Teacher: What do you think is the difference between the circle and the plane figure you learned before?

Guide the students to find that the rectangle, square, triangle, parallelogram and trapezoid they have learned before are all straight lines on the plane, while the circle is a curved figure on the plane. Students give examples

The students began to draw circles.

Exchange the methods of drawing circles. If four methods are listed in the textbook, students can introduce them appropriately according to the situation if they don't think of them; If a student wants to draw a circle with a compass, don't rush to let the students tell the detailed operation method.

(B) Teaching Example 2

1, introducing compasses

Students try to draw a circle with compasses.

Step 3 organize exchanges

4. Let the students draw a circle with compasses.

5, self-study textbooks, first know the names of the parts in the circle.

6. Understand the names of the parts of the circle

(1) Fold and find the center of the circle.

(2) Understanding radius

(3) Understand the function of center and radius.

(4) Know the diameter

(3) Teaching Example 3

1, explore the characteristics of the circle.

2. Reporting and communication

3. Verify the conclusion

Teacher: We drew a circle in different ways. In mathematics, there is also a special tool for drawing circles-compasses, which can be used to draw circles conveniently and accurately.

This paper introduces the names and functions of each part of compasses.

Teacher: Can you try to draw a circle with a compass?

Think while drawing: What are the general steps of drawing a circle with compasses?

Communicate after painting, so that students can clearly understand the basic methods of drawing a circle with compasses:

1, separate the two feet of the compass and set the distance between the two feet;

2. Fix the foot on a point with a needle tip;

3. Turn one foot once with the tip of a pencil and draw a circle.

At the same time, the teacher demonstrates drawing circles on the blackboard and writes on the blackboard in time:

Fixed point, fixed distance, rotation.

According to your experience of drawing a circle just now, what do you think should be paid attention to when drawing a circle? Can I give you some friendly reminders?

Draw a circle: see who can draw quickly and well.

(1) The distance between the two legs of a compass is 2 cm.

(2) The distance between the two legs of the compass is 1. 5 cm

Projection shows students' paintings.

Teacher: When drawing a circle just now, the distance between a fixed point and a compass of two feet was also defined by mathematicians as different names and expressed by different letters. Do you want to know? Please teach yourself 94 pages of textbooks. Communicate after self-study: What's the name of the round part? Write on the blackboard according to the students' answers: center o radius r diameter d

Teacher: Take out the circular paper you cut in advance and we will fold it together: fold it in half first and open it; Fold it in the other direction and then open it, so fold it several times. Can you imagine how many creases you can fold in this way? Look carefully: can you find the center of the circle?

Point, say at the same table and mark the center of the circle o.

Point: the center of the circle is the center of the circle. It is the point where the needle tip is fixed when drawing a circle.

Teacher: Can you find the radius from these folds? Draw a radius at will and indicate it with the letter R.

Observation and discussion: What is radius? What are the characteristics of radius?

Communicate after discussion. Show the courseware and flash two endpoints at the same time, so that students can make it clear that the radius is a line segment; One of its endpoints is at the center of the circle and the other endpoint is on the circle; Its length is the distance between two feet of the compass when drawing a circle.

Judgment: (Show Courseware) 0

Which line segment on the right is the radius?

Take out the two circles you drew in front of you and tell me what their radii are. Look at the position and size of these two circles. What did you find?

Blackboard: The center of the circle determines the position of the circle.

The radius determines the size of the circle.

Teacher: Then take out the folded round paper and observe it carefully: What kind of crease is the diameter? Draw a diameter on the circular paper, which is indicated by the letter D.

Discussion: What is the diameter? What are the characteristics of diameter?

Communicate after discussion and show the courseware at the same time:

(1) shows the diameter of the circle. (2) The center of the blink circle and the two endpoints on the circle.

Let the students know the characteristics of the diameter: (1) passing through the center of the circle. (2) Both ends are on a circle.

Judge: (Show the courseware) Are the two line segments in the circle diameters? Why?

After writing the book, the question of "practice" 1.

Teacher: If we know the center, radius and diameter of the circle, we can further study the characteristics of the circle. First operate as required, and then discuss in the group to see what secrets are hidden in the circle.

Take out a round piece of paper, draw a picture, measure, compare, fold and discuss:

(1) How many radii can a circle draw? What diameter?

(2) Are the radii in the same circle equal in length? What about the diameter?

(3) What is the relationship between the radius and diameter of the same circle?

(4) Is the circle an axisymmetric figure? How many axes of symmetry does it have?

By drawing, measuring, comparing and folding, you are sure to make new discoveries. Don't forget to record the conclusion of your group, even if it is a small discovery, and fill in "My Discovery" to prepare for communication. See which group finds more.

Reporting and communication:

Tell me what you found in the operation. How did you find out?

The (1) question can be explained by the "folding" in front, that is, the folded round paper; You can also draw a picture in the form of a group competition to see who draws more in the specified time. I can imagine: how many lines can I draw if I keep drawing like this?

Problems (2) and (3) can be explained by measurement or reasoning.

You can ask: Why is it called "radius"?

Problem (4) can be explained by folding the circle in half in different directions.

According to the report, the teacher wrote on the blackboard in time:

Countless bars are of equal length. d=2r r = d/2。

Teacher: Is the above conclusion correct? Let's go and see,

Show courseware: verify the above conclusions.

The teacher took out two circles of different sizes and asked, will the diameter of this small circle be twice the radius of the big circle? Under what circumstances can the conclusions of the above three questions be established?

Emphasis: In the same circle (or equivalent circle) (blackboard writing)

Question: What can you think of when you see the relation d=2r r = d/2?

Let the students know: (1) the meaning of "half" radius, and the relationship between diameter and radius is very close. (2) The known radius can be used to calculate the diameter and the known diameter can also be used to calculate the radius. (3) Both radius and diameter can determine the size of a circle.

Ask: What are the characteristics of diameter and radius?

Finish the second exercise after the book.

Students try to draw a circle with compasses.

Communicate the situation of drawing circles and reflect on the matters needing attention in drawing circles from the problems that arise.

Students draw a circle of a specified size. Compare who draws better in the group.

Students teach themselves textbooks first, and then communicate.

The students began to fold.

Students may find that each crease intersects a point of the center of the circle. This is the center of the circle.

Student drawing radius

Observe, discuss, communicate and judge.

Students observe and answer.

Student answers

Students draw, measure, compare and fold in groups as required, and record the research results.

Organize patrols.

Students communicate after operation.

Students summarize and explain.

Third, discrimination and comparison.

Strengthen understanding and judgment: (1) The line segment with two ends on the circle is called diameter ().

(2) A circle with a radius of 3 cm is larger than a circle with a diameter of 5 cm. ()

(3) All radii are equal and all diameters are equal. ()

(4) When drawing a circle, the distance between two feet of the compass is the length of the circle radius. ()

(5) A circle has numerous symmetry axes. () Students answer orally.

Fourth, apply what you have learned.

Solve the problem 1 and finish the exercises after the book 17 1.

2. Finish the exercises after the book 17, question 2.

3. What do you see in the picture below? What else can you think of?

The radius is 4m 10 decimeter.

4. Use the knowledge you have learned in this lesson to solve the problem.

How to draw a big circle on the playground?

(2) In an ancient military battle, the army battalions were surrounded by secrets. In order to facilitate direct communication between the headquarters and the battalions, may I ask: Where is the best headquarters? (Show courseware)

(3) What other phenomena in daily life are related to what we have learned in this lesson? (For example, when someone performs art in the street, the audience will unconsciously form a circle ...)

5. Discuss the topic of introducing new courses:

Now, who can tell the truth about these three little animals and who can win the first prize? (The teacher takes out a round medal) Who is likely to get this round medal? Why?

Let the students understand that the wheel is round, the axle is installed in the middle, and the distance between the axle and the ground is always equal. Only in this way can we run fast and steadily. Students finish first, then communicate.

Students talk in groups first, and then communicate in classes.

Verb (verb's abbreviation) What did you learn from today's class? What other questions are there? Summarize and ask questions

Reflection after teaching