The teaching goal of the teaching plan "measurement of length and time" in the first volume of eighth grade physics published by People's Education Press.
1. 1 knowledge and skills:
Can estimate the length according to daily experience, and can correctly use the scale to measure the length. Can estimate time according to common periodic phenomena, and can measure time with stopwatch and watch.
Know that there is an error in the measurement. Understand the difference between mistakes and mistakes.
Understand the tools for measuring length and time and the process of their development and change.
1.2 process and method:
Through specific measurement activities, we have a general understanding of the scale and time period of common objects, and form specific concepts of length and time unit size.
Through practical measurement activities, students can correctly measure the length with a ruler and the time with a timing tool.
1.3 Emotional attitudes and values:
Combined with the measurement of length and time, cultivate students' interest and habit of observation and experiment, and develop a serious and meticulous scientific attitude of seeking truth from facts.
By estimating the length and time according to daily experience, we can understand the relationship between physics and life and further stimulate our interest in learning physics.
Emphasis and difficulty in teaching
2. 1 teaching focus
The unit of length and the measurement of length.
2.2 Teaching difficulties
The concretization of the concept of measuring length unit and the reading in measurement.
teaching tool
Multimedia equipment
teaching process
6. 1 Introducing new courses
Look at the picture below. Do the students think the two circles in the middle are the same size?
Life is different.
According to the feeling that they are not the same size, then we can take out the ruler at hand or choose a rope as a reference to measure the size of two circles.
Two circles are the same size.
6.2 Introduction of new knowledge
I. Unit of length
In the previous study, we learned the unit meter of length and the related propulsion rate of meters.
Let's sum up:
In the international system of units, the unit of length is meter, and the symbol is M. The commonly used units of length are kilometer (symbol is km), decimeter (symbol is dm), centimeter (symbol is cm), millimeter (symbol is mm) and micron (symbol is? M), nanometer (symbol is nm). Their conversion relationship with rice is as follows:
1k m = 1000m = 103m；
1dm = 0. 1m = 10- 1m；
25px = 0.0 1m = 10-2m；
1mm = 0.00 1m = 10-3m；
1? m = 0.00000 1m = 10-6m；
1nm = 0.00000000 1m = 10-9m
The following are some conversion formulas of length units proposed by Xiao Liang. Please point out his mistakes in the conversion process and write the correct formula.
5.2m=5.2m？ 100= 13000px
5.2m=5.2？ 2500px= 13000px
When converting units, the correct format is that the number is unchanged and the corresponding units are replaced by the same quantity.
Second, the length measurement
Common length measuring tools:
From this picture, we can see that the zero scale line of the familiar ruler is at the left end (a short distance from the leftmost edge) and is not worn. Its measuring range is 0 ~ 200px, that is, it can measure the length of 200px at most, and its dividing value (minimum scale) is 0.25px.
teacher
How to use the scale:
Teacher: First of all, please measure the length and width of the physics textbook, and summarize the use of the ruler in the measurement process.
A: Before use:
Observe the zero scale line (where it is, whether it is worn), measuring range (measuring range) and dividing value (the distance between two adjacent minimum scale lines determines the accuracy of the scale).
B: in use:
The scale should be straightened so that the scale line is close to the measured object.
To estimate the next bit of the read index value.
When recording, the results should include readings and units.
For a thick ruler, the ruler line should be close to the measured object (as shown in Figure C). When the zero scale line is worn, it should be measured from other clear integer scale lines as the initial scale, and the measurement result should be equal to the read scale value minus the initial scale value (as shown in the figure below).
Reading: the line of sight should be right on the scale line (as shown in the figure below); In addition to reading the accurate value above the scale value, the next digit value of the scale value (estimated reading value) should be estimated. The length of the pencil in the picture is 69.5mm, of which 69mm is the exact value and 0.5mm is the estimated value.
4. Remember, it refers to the record. The recorded measurement results shall include values and units.
What is the length of the block in the picture below?
Answer: 2. 1925px.
measurement error
During the measurement just now, we found that the measurement results were slightly different, but our measurement methods were all correct. There are differences because we finally need to evaluate reading, which is inevitably big and small. This will produce an error.
Error: (the difference between the measured value and the real value)
We can't eliminate mistakes, but we should try our best to reduce them.
Ways to reduce errors:
Average through multiple measurements; Choose accurate measuring tools; Improve the measurement method.
A mistake is not a mistake. The measurement error is caused by not observing the usage rules of measuring instruments and not reading carefully, which should not happen and can be avoided.
Example as shown in the figure, measure the length of a piece of wood with a scale, and the measurement result is ().
46.25 pixels
96.25 lbs
C.19mm
47.5 pixels
Before analyzing and measuring the length of an object with a scale, it is necessary to make clear its dividing value; When measuring the length of an object, it is necessary to observe whether the measurement starts from the scale line of 0, and the starting end does not start from 0. Should scales be understood as? 0? Scale, read the ending scale value, subtract the previous scale to get the length of the object, and pay attention to the next bit of the scale estimation scale value.
As can be seen from the figure, there are 10 cells between 25px on the scale, so the length represented by a cell is 2.5px= 1mm, that is, the scale value of the scale is1mm; The left side of the square is aligned with 50px, and the right side is aligned with 96.25px, so the length of the square is l = 96.25px-50px = 46.25px ..
So choose one
For example, a classmate measured the length of the same object four times with the same ruler, and the results are as follows: 308.5px, 309px, 308.75px, 323.75px, so the length of the object should be recorded as ().
308.75p
308.5 pixels
C. 3 12.5 pixels
D. 309 pixels
If the reading with large deviation is wrong, remove the wrong reading and take the average of the other three correct measured values as the measurement result;
When measuring the length, we usually use the method of averaging several measurements to reduce the error, so we only need to calculate the average of four measurements, but when determining the final result, we should also pay attention to the retention rules of decimal places are the same as the measured values;
According to the data provided in the question, 323.75px is quite different from other data, so 323.75px is wrong and should be removed.
In order to reduce the error of length measurement, the usual method is to take the average of many measurements; Therefore, the length of the object should be recorded as: l =12.34cm+12.36cm+12.35cm.
= 308.75 pixels
So choose one
Third, the measurement of time.
Many students have a watch in their hands. As we all know, a watch is used to tell time, so it is a tool to measure time. So besides watches, you know, what else is used to measure time? How can you tell the time without a watch? Don't tell me I can tell the time with my mobile phone. )
teacher
Here is a brief introduction to the development and change process of timing tools. In ancient times, people used sundials, hourglasses and other instruments to time.
/kloc-After the 6th century, pendulum clocks with heavy hammers appeared in Europe, and modern mechanical clocks were developed on the basis of them. Modern electronic clocks are made of timely crystals, which are stable, light and convenient, with a daily deviation of less than 0.5 seconds. At present, the most accurate timing device is the atomic clock used by the Observatory, with a 300,000-year deviation of only 1 second.
Learn from the tools and methods of ancient and modern people to measure time?
Life can be timed by sundials and hourglasses.
Sundial: The rod perpendicular to the sundial disk forms a shadow on the disk under the irradiation of the sun and moves up and down with the sun.
Move, the position of the shadow on the sundial is different at different times. By marking the corresponding position on the sundial, you can use the corresponding time to time;
Hourglass: The sand in the container above the hourglass leaks the same amount of sand into the container below at the same time interval, and it is timed according to the amount of sand leaked. )
Yes, everyone summed it up well. Sometimes in the wild, we can also determine the approximate time according to the position of the sun. Next, we will learn more about the time.
unit of time
In the international system of units, the unit of time is seconds, and the symbol is s, except? Seconds In addition, the commonly used units are hours (symbol H) and minutes (symbol min). The transformation relationship between them is:
1 h = 60min,1min = 60s,1h = 3600s.
Timing tools: various quartz clocks and electronic watches used in life; Mechanical stopwatches and electronic stopwatches commonly used in sports fields and laboratories.
Use of stopwatch for experimental machinery
(1) Practice the use of stopwatches: how to start, stop and return to zero; How to read?
(2) Measure the time of pulse beating 10 times and the pulse beating times in 1 minute with a stopwatch.
The shape of the summary stopwatch is shown in the above picture. A is the zero button, and B is the control button of the chronograph second hand. When you use it, you should adjust it first, that is, press the zero key to make the second hand point? 0? Location. Then press the second hand control button to start timing. At the end of the timer, press the second hand control button, the second hand will stop at the last timer, and the time value can be read.
Teacher: So, can all sticks become levers?
Think and answer.
A teacher must have the following two conditions to become a lever with a hard stick:
(1) should play a powerful role;
(2) can rotate around a fixed point
Error handling:
There will also be errors in the measurement of splitting time. If the error is reduced, such as length measurement, the situation is similar:
Average through multiple measurements; Choose accurate measuring tools; Improve the measurement method.
Teaching objectives of teaching design of measuring length and time
I. Knowledge objectives
1. Appropriate tools will be used to measure time and length.
2. Know that there are errors in measurement, and there are differences between errors and errors.
Second, the ability goal
1. Experience the method of roughly estimating the length of time through daily experience or natural phenomena, and have a preliminary solution to simple life problems.
The ability to ask questions.
2. Be able to use simple measuring tools through simple measurement.
Third, the goal of moral education
1. Understand the tools for measuring time and length and their development process, and cultivate the love for science and technology.
2. By learning simple measurement knowledge, we realize the importance of measurement in physics and develop meticulous and rigorous study habits.
Emphasis and difficulty in teaching
Teaching focus
Correct use of measuring tools to measure time and length.
Teaching difficulties
Record the measurement results correctly.
teaching tool
mixed-media
teaching process
First, the introduction of new courses.
Students, close your eyes. It is estimated that in ten seconds, raise your hand and open your eyes. When I do accounting, let's see whose estimate is the most accurate. Let's go
Follow the instructions
Who can estimate the length and width of this table?
[Student] Answer.
[Teacher] Point out the correct figures. In order to obtain accurate data, we should learn to use instruments to measure length and time. In this lesson, we will continue to learn on the basis of the use of clocks and balances in primary school and daily life and a preliminary understanding of some units of time and length.
Second, the new curriculum teaching
1. Length measurement
Everyone knows that the length can be measured with a ruler. Can you tell me what kind of ruler you have?
A ruler, a triangle and a tape measure.
A tailor's tape measure, a carpenter's folding tape measure.
Vernier caliper and micrometer (also called spiral micrometer) are commonly used for precision measurement or special measurement.
In the process of listing rulers, teachers can show them to students one by one to enhance their perceptual knowledge.
[Teacher] Students, who else knows which parts of the human body can be used as a ruler to estimate the length? What parts of the human body have a special relationship in length?
The length of a person's outstretched arm is about equal to his height; Open your palm, and the distance from your thumb to the tip of your middle finger is called? A big oak tree? What is the distance from thumb to fingertip? A small tussah farm? If you can know yourself? Tussah? The length and height of an object can be estimated by the arm and palm.
A man's head is as long as his feet; The height is equal to 9.5 times the length of the head; A man's fist is as big as his heart.
[Health] Many parts of the human body are under pressure? Golden section? For example, the eyes are the golden section of the head and the navel is the golden section of the human body.
[Teacher] It's amazing that students know so much about human body size. You must remember what is the unit of length?
(In the process of listing units, the teacher projected the units and conversion of length to consolidate memory. )
[projection]
Unit and conversion of length:
Units: kilometer (km), meter (m), decimeter (dm), centimeter (cm), millimeter (mm), micron (? Meter), nanometer (nm).
Conversion relationship:1km =103m1DM =10-1m.
1 cm = 1 0-2m1mm =10-3m
1? m = 10-6m 1nm = 10-9m
In the international system of units, the unit of length is meter and the symbol is m.
[Teacher] Do you use rulers and tape measures that are common in life to measure the length? Please use the ruler in your pencil box to measure the length, width and thickness of the physics textbook, and exchange measurement methods and questions with your classmates.
Students operate and teachers patrol. Pay attention to the problems existing in the measurement process and discuss with students to solve them.
When measuring the length of a book, choose a longer ruler; Place the scale in parallel on the measured length, with the zero scale line on the scale aligned with one end of the measured length and the line of sight aligned with the scale at the other end of the measured length (student demonstration); Read the value on the scale and write it down. That's it.
My ruler has been used for a long time, and I can't see the score line clearly. I will aim the line I can see clearly at one end of the measured length, read the value on the scale at the other end, and subtract the previous value, so that I can also measure the length of the object.
I find it easy to read clearly and accurately when the scribed lines on the scale are closely attached to the measured object (student demonstration).
When I measure the width of the book with the scale, the zero score line is aligned with one end and the other end is not aligned with the score line on the scale. How do I calculate the length?
[T] Do other students have the same problem? In fact, we often encounter similar problems in measurement.
The students get together to discuss how to solve this problem.
A small square on the scale is 1 mm, which can be estimated as a few tenths of a millimeter according to the position of the edge of the measured object.
Please look at the values on the scale corresponding to the edge of the measured object.
I think it's 6.35 centimeters.
I think it's 6.38 centimeters.
The corresponding position of the edge of the measured object exceeds the middle of the cell, to the right, I think it is 6.36 cm.
[T] Three students read three numbers. The first two numbers are exactly the same, but only the third number is different. Why?
The first two digits are read from a ruler, of course, they are all the same, and the last digit is estimated according to personal feelings, so it may be different.
[Teacher] The answer is very good. The value read directly from the ruler is called an accurate value. The estimated value is called the estimated value. Therefore, the measured value includes two parts.
Part is the exact value, and part is the estimated value.
The measurement result must also indicate the unit, so the measurement result should include three parts: accurate value, estimated value and unit.
[T] If the edge of the measured object is aligned with a line of the ruler, do you still need to estimate it?
When the edge of the measured object is aligned with the scribing line, no estimation is needed, or the estimated value is zero.
[T] That's good. When writing the measurement results, what is your estimate? Zero? The exact value should be written in? 0? .
When measured with a ruler with a minimum scale of millimeters, it can be estimated to be several tenths of millimeters; If you use a ruler with the smallest scale of cm, you can only estimate a few tenths of a centimeter, and the result measured with mm ruler is definitely more accurate than that measured with cm ruler.
This means that we can only estimate the next digit of the minimum scale value (also called the dividing value) when reading. When measuring, choose a suitable ruler according to the measurement requirements.
When measuring the length of football field, use a ruler with the minimum scale of 1 m, and when measuring glass, use a ruler with the minimum scale of 1 mm. 。
2. Measurement of time
[think about it]
What watches do the students know? Do you think ancient people and modern people measure time in the same way?
There are many kinds of watches now, such as clocks, watches, pocket watches and small alarm clocks.
Watches can also be divided into quartz watches, mechanical watches and electronic watches.
Does everyone agree with them?
Their classification standards are different, but they are all acceptable.
Watches, clocks and alarm clocks are all used to measure time in daily life. There is also a kind of watch, which is only used when running in sports meeting. It is called a stopwatch or stopwatch.
[T] According to different users, we can also divide watches into cartoon watches for children, blind watches for the blind and various watches for adults. According to different installation positions, watches can also be divided into wall clocks, seat clocks, floor clocks and so on. Do you think so?
There are too many kinds of watches in our life now. No matter how many kinds and categories it is divided into, it is a modern time tool. Have you ever wondered how people kept track of time when there were no watches in ancient times?
I think time is measured by the sun or the shadow, because I have seen grandma in my hometown and countryside decide not to look at her watch but to look at it when she is working or cooking? Sun? Or look at the shadow of the wall in the yard. Ancient people didn't have watches, so they may be like them.
[Life] Used by ancient people? Burning incense? Time, I heard the old people say? A wick of incense? It means it's time to burn incense Have you ever seen it used in costume TV series? Fire? Calculate the time.
I think ancient people used time to measure time.
I read that there were water clocks in ancient times. You can use running water to time.
According to online introduction, the earliest timekeeping tool was sundial, which was first used by Egyptians.
From what they heard, saw, thought and guessed, the students told us so many possible punctuality methods of the ancients. Right or not, you are the best. Please see the information below.
[projection]
The development history of human timing instruments
[Teacher] How do the students feel after reading the above contents?
The development of timer has gone through a long time and many people's efforts.
The watch we use today is also the result of the concerted efforts of many people since ancient times, and it is the crystallization of their wisdom.
The students speak very well. Today's timer? Watch? Can everyone use it? Do you want to have a try?
The teacher will distribute the alarm clock and stopwatch prepared before class to the students, or let the students use their watches themselves. )
Please measure how long it takes you to write 10 with the watch in your hand, and then introduce your measurement method.
I measured it with my watch. Look at the time before writing, and then look at the time after writing. The time difference between the two times is the time when I wrote the word 10.
I use a small alarm clock to time, in the same way as my former classmates.
[Teacher] Which students use stopwatches to measure, can they use them? Who can tell you about it?
I know how to use it. There are two kinds of stopwatches, just like the picture in the book 1 1.3-2. One is a mechanical stopwatch and the other is an electronic stopwatch. There are two buttons on the mechanical stopwatch. Press the big button in the middle to start the watch, then press the watch to stop, and then press the small button next to it. There is also a small dial on the big dial. The pointer on the large dial rotates once, and the pointer on the small dial rotates once, indicating 1 point. Before use, both hands should point to zero. Press the middle key at the beginning of writing, and then press it again after writing, so that the time spent writing the word 10 can be read directly from the position indicated in the table.
The use of electronic stopwatch is basically the same as that of mechanical stopwatch, but I think the reading of electronic stopwatch is simpler. When using a mechanical stopwatch, if the measuring time exceeds 1 min, how many minutes should be read from the small dial and how many seconds should be read from the large dial. When using an electronic stopwatch, just read the displayed time directly.
Do you think they made it clear? The teacher thinks they are great. Now, please exchange watches with alarm clocks, rewrite the word 10 just written at the same speed, measure the elapsed time and compare it with the time measured for the first time.
I find it takes less time to write the same words at the same speed with a stopwatch than with a watch.
I think it is more accurate to measure time with a stopwatch. When measuring with a stopwatch, you can operate the stopwatch with your left hand, start writing at the same time, and end at the same time. When measuring with a watch, you should look at the watch first, then write, and then look at the watch after writing, so the measurement time is long, and the watch keeps going, so it is not easy to remember the correct time.
Through their own comparison, students must understand why time is usually measured by stopping the watch in orbit and in experiments.
It is convenient and accurate to measure the time interval with a stopwatch.
[Teacher] Please use the watch in your hand to measure the number of times your pulse beats in one minute. Use the pulse to estimate the time needed after class or on the way to school, and compare it with the time measured by the watch to see how much the difference is.
The students carefully feel their own pulse, and the classroom is very quiet.
[Teacher] Just now, when the students were taking their pulse, students with clocks or mechanical stopwatches must have heard the sound of clocks? Tick tock? Sound, what makes the clock tick? Please look below.
[projection]
A ticking clock
[Teacher] After reading the above, the teacher believes that the students will make a pendulum themselves. How?
You can make a pendulum with a thin thread and a small weight tied at the lower end.
(The teacher used a plastic binding rope about 1 m long, and tied a small lock at the lower end to show the students. The teacher tied the other end of the thin thread to the iron frame, and it would swing when the iron lock was opened at a small angle. )
A device like this is called a simple pendulum. Please use the watch in your hand to measure the time required for this pendulum to swing back and forth, and see how to measure it more accurately.
Students take measurements, and teachers can participate in students' discussions. )
If only one round trip is measured, sometimes it is very big and sometimes it is very small, which is definitely inaccurate. You can measure a few more times and then find out the time it takes to swing back and forth, which will be more accurate.
I think several people can measure a round trip at the same time, and averaging the results of several people can also make the measurement results more accurate.
It is a very good habit for students to think seriously. After class, we can continue to measure and discuss and find out the best scheme.
Students must remember the unit of time they study in primary schools. What is this?
Hours, minutes and seconds.
In the international system of units, the unit of time is seconds and the symbol is S.
3. Errors and errors
Now, please measure the width of the physics textbook.
[Student] Operate, read and ask questions from the teacher.
There is only one real length of the measured object, that is, the real value. Three students measure the same object and get three measured values. Explain that there is a difference between the measured value and the real value, which is the error. Can you say that the error can be completely eliminated?
People always have to use tools to measure, and there will always be some errors when estimating when measuring. So I think mistakes can't be eliminated and can only be minimized.
You can test more times, test more people and then average them to reduce the error.
Is this what we often use in physics experiments? Average multiple measurements? .
[Teacher] What do you think will happen if you accidentally tilt the ruler during the measurement, or if there is no scale line aligned with the measured object, or if you squint when reading a book?
If you measure it in this way, the error between the measured value and the real value must be great.
Students must be clear that the result obtained without the correct method is not an error, but an error. Mistakes are inevitable, and mistakes should not and should not happen.
In the international system of units, the unit of length is meters and the unit of time is seconds. What is the international system of units? Please read it.
[scientific world]
Students have learned how to measure the length of an object correctly with a ruler. Can you measure the diameter of a coin, the thickness of a page, the diameter of a thin thread, etc? With a scale? You can continue to think and discuss after class. Now let's sum up the contents of this section.
Summary after class
1. length measurement: the unit and conversion relationship of length, and the correct use of scale to measure length.
2. Time measurement: the unit of time, the tools for measuring time, and the correct use of stopwatches and watches.
3. Error and error, how to reduce the error.