◆ Typical wrong questions:

Title: The conductor is 7 () and the Yangtze River is more than 6,400 ()

Hangzhou Bay Bridge No.36 ()

Students' misunderstanding: the car is 7 (decimeter) long and the Yangtze River is more than 6,400 (meter) long.

Hangzhou Bay Bridge 36 meters

◆ Cause analysis:

1. Teachers should pay attention to let students establish representations such as 1 cm, 1 decimeter, 1 meter, 1 km, etc. But practice estimation less, for example, show a length and ask students to estimate how long it is. Or give students a length to gesture.

2. Students are too lazy to use the established concept of length to make an accurate judgment, just write a unit casually.

3. Students lack life experience. For example, how long is the Hangzhou Bay Bridge? Many people haven't been there. No wonder the bridge at home is only tens of meters long, and there are no bridges over 100 meters. For example, the length of the Yangtze River is more than 6,400, which is already a considerable number. They think the meter is long enough, it needs several kilometers. They find it incredible. They just did the problem in their imagination.

◆ Teaching suggestions:

1. Help students to establish the corresponding concept of length. Understand the establishment of length units, not rote learning. The understanding of length should be derived through actual measurement and hands-on operation. Decimeter and kilometer should permeate students and also come from practice, so these length units should be strengthened.

2. Cultivate students' visual ability at ordinary times. Not only 1 decimeter, but also students can figure out how long 3 decimeters are and how long the table is. Where is it from home 1 km, 5 km, etc. Through evaluation, measurement, thinking, discussion, talking and doing in teaching, students can establish the appearance of these length units, and then develop a sense of space. Activities such as "walking", "running", "taking a look" and "discussing" can be set to experience "1 km", so that students can establish the concept of "km" on the basis of fully perceiving its length.

3. Excavate the materials in life to help students perceive kilometers. For the perception of kilometers, we should combine the actual perception with the development of students' estimation ability. For example, some students walk around the playground and imagine the length of five laps, or about 1500 steps from home to a place is 1 km, but it is still unclear how far it is and it is difficult to estimate. In the process of forming the concept of kilometers, teachers can first ask students to tell where it is about 1 kilometer to walk from the school gate (teachers must take a bicycle test in advance, and the answers should be varied, and students do feel walking at ordinary times), and then let students estimate where it is about 2 kilometers to walk from school on the basis of preliminarily establishing the length of 1 kilometer. How many kilometers does it take to walk from school to my home? (Teacher's random evaluation) Let students perceive the length of several kilometers. 1 km walk for about 15 minutes. What about 2 kilometers? /kloc-how long can I walk at 0/hour and where can I walk? The car 1 hour can drive about 80 kilometers, so how long does it take to drive 800 kilometers?

Ask the students to talk about their car experience in life, such as where to go and how long it will take. Perceive the length with practice. Perceive the actual distance corresponding to a large number of units from it. Help students build familiar lengths in their minds for reference.

On this basis, teachers can also inspire and guide students to describe 1 km and the length of several kilometers with things and scenes around them, and connect km with students' normal life, so that students' perception of km can go through a dynamic generation process from fuzzy to accurate. Teaching students to encounter unfamiliar situations can be solved by reasoning, comparison and exclusion.