Processing Method of Shooting Free Fall Motion in Physics Experiment of Senior High School with Digital SLR Camera and Frequency Flash Source

Skillful use of digital camera in physics demonstration experiment

Ye (Zhejiang Ruian Longshan Middle School 325200)

Taking the teaching of the course "Motion of Flat Throw Objects" as an example, this paper introduces two ingenious applications of digital cameras in physics demonstration experiments: First, taking photos of the demonstration experiments with digital cameras and video projection systems, and shooting and playing them. Second, use the "multi-segment" magic shooting function of digital camera to shoot, analyze and study high-speed moving objects.

Keywords: digital camera flat projection camera

With the deepening of China's educational system reform, the reform of physics education has also developed rapidly, which not only involves the innovation of teaching content, teaching form and teaching art, but also the modernization and diversification of teaching methods has gradually become an important aspect of physics education reform. At present, the wide application of digital cameras has become an indispensable assistant in people's life and work. How to introduce the application of digital camera into physics classroom teaching has become a new teaching method and one of the important topics of physics education reform. According to my own attempts in recent years, I found that the introduction of digital camera (with "multi-segment" shooting function, camera function and video output) will produce unexpected teaching effects in classroom teaching, especially in demonstration experiment teaching. Let's take the teaching of the course "Motion of Flat Throwing Objects" as an example to talk about the specific application.

In the teaching of "Motion of Flat Throwing Objects", how is the flat throwing motion decomposed into horizontal uniform linear motion and vertical free falling motion? This is the focus and difficulty of teaching. To overcome this important and difficult point, it is necessary to do a good job in relevant demonstration experiments, set up the demonstration experiments shown in figure 1 in the textbook and provide stroboscopic photos as shown in figure 2.

Figure 1 Figure 2

Figure 1 In the demonstration experiment, the object moves too fast, so students can't clearly observe the moving process in the middle. They can only roughly judge that the vertical direction is a free fall by a sound when landing, and it is even more impossible to judge that the horizontal direction is a uniform linear motion. The disadvantages of stroboscopic photos as shown in Figure 2 are high requirements for experimental instruments (high-precision, high-frequency stroboscopic flash is needed), and the operation process is complicated (strictly speaking, it must be taken in a dark room, and there are some specific professional problems in photography). If it is traditional photography, it needs a process of washing and waiting, and the production process is complicated, which is not conducive to classroom demonstration. Moreover, what students see is only a photo prepared in advance, which lacks realism and directness. Under this teaching method, most of this course can only be theoretical preaching, and students can't draw conclusions through observation and analysis experiments. They can only imagine the process and results of the experiment by their own ability level, which leads students to accept the decomposition and law of the flat throwing action mechanically, and the teaching effect of the whole class will not be ideal. If the digital camera can be skillfully introduced into the demonstration, it will have an unexpected effect on breaking through this teaching difficulty. Here are two methods of demonstrating flat throwing action with digital camera in detail.

1. Use digital camera and video projection system to shoot the flat throwing motion experiment on the spot, and shoot and play it.

According to the content of the textbook, the author improved the demonstration experiment, as shown in Figure 3: Two identical and smooth chutes, one high and one low, are fixed on the left side of the vertical demonstration board, and an electromagnet is fixed on the upper right corner.

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Figure 3

In the process of demonstration, the digital camera (with camera function and video output) is used to shoot, shoot and play the demonstration experiment on the spot, and the students are guided to observe and analyze the video clips of "slow motion", from which the law of flat throwing motion is obtained, and then the classroom teaching of this course is completed, and good results are achieved. The specific operation is divided into the following steps:

1. Prepare the selection and connection of experimental instruments and camera system equipment: the demonstration experimental device is laid flat on a horizontal desktop, as shown in Figure 3. Prepare two identical iron balls. Choose high-end and mid-range (with camera function and video output) digital cameras, with their own monitoring color screens and larger 500,000-3 million pixel CCD sensors, which can achieve higher definition. . High and medium-sized machines are equipped with 3X-30X optical zoom lenses, which have complete automatic and manual functions. Install the digital camera directly on the snake tube or adjustable lighting bracket. Connect the signal output end of the digital camera to the computer or directly to the video projector with shielded wires.

2. Demonstrate, shoot and play the experiment of "the vertical direction of flat throwing motion is free falling motion": Take two identical balls, one of which is placed at a certain height of the chute (smooth) in the upper left corner of the device and attracted by a magnet, and the other is placed at the same height in the upper right corner of the device and attracted by a magnet. Automatic switch control: When the first ball leaves the end of the chute, the other ball falls at the same time, one doing flat throwing and the other doing free falling. It can be seen from the background grid paper that they are almost at the same level in the whole falling process and finally land at the same time. The whole experiment was filmed by a digital camera. After observing the physical demonstration, the students directly observe the video, and then analyze the "slow play" of the video to inspire students to analyze and let them sum up that the horizontal throwing movement is a free falling movement in the vertical direction. Below (Figure 4-6) are several shots when the video is played slowly.

Figure 4 Figure 5 Figure 6

3. Demonstration, shooting and playing: The vertical direction of the flat throwing motion is horizontal uniform linear motion: take two identical balls, one in the upper left corner of the device at a certain height from the end of the horizontal end, and the other in the lower left corner of the device at the same height from the horizontal end with a magnet. The switch is used to control two balls to fall at the same time, one for flat throwing and the other for myopia with uniform linear motion. As can be seen from the background grid paper, they are almost on the same vertical line during the whole movement, and finally reach the small net bag on the right at the same time. The whole experiment was filmed by a digital camera. After students observe the physical demonstration, they directly observe the video, and then analyze the "slow play" of the video, which inspires students to analyze and let them sum up that the flat throwing motion is a uniform linear motion in the horizontal direction. The following (Figure 7- 10) are several shots when the video is played slowly.

Figure 7 Figure 8

Figure 9 Figure 10

Please pay attention to the following points when shooting with a digital camera:

1. When shooting, the light source should not only be used for camera light distribution, but also be beneficial for students to observe the experiment. High-frequency electronic ballast of daylight color can use high-efficiency energy-saving lamps or incandescent lamps with high power, and sometimes low-power incandescent lamps are needed for lighting special parts. All light sources should be equipped with suitable lampshades to prevent light from directly illuminating the lens of digital cameras or the eyes of students.

2. When shooting the ball with a digital camera, it is necessary to pay attention to the large contrast between the background and the ball. When shooting, it can be directly installed on the snake tube or adjustable lighting bracket or tripod to ensure the camera is still.

Second, using the "multi-segment" magic shooting function of digital camera, shoot and analyze the flat throwing motion experiment.

The so-called "multi-segment" magic shooting mode, specifically, can continuously take 16 photos within 0.5 seconds, that is, take one photo every thirtieth of a second, which is an extremely rare function (Sony -T 1 camera and other high-end digital cameras have it). Only in this shooting mode, the picture quality will be reduced. For a 5-megapixel camera, the pixel of each photo is only about 65438+ million (which is why professional digital cameras don't have this function), but it is enough for our middle school physics experiment. As shown in figure 1 1, this is the flat throwing motion photo taken by this shooting mode. Figure 12 is the result of combining multiple images into the same photo by Photoshop software, which guides students to prove that the horizontal direction is uniform motion and the vertical direction is uniform motion, and the measured acceleration is about 9.7m/s2.

Pay attention to the following points in the specific shooting process.

1, shooting interval is optional. For example, Sony -T 1 has three choices: 30 films per second, 15 films and 7.5 films, but no matter which one, only continuous shooting 16 films can be used at a time, so the continuous shooting time of each film is different, and 30 films per second can be 0.5 films in continuous shooting. 15 can be shot continuously every second 1 second, and 7.5 can be shot continuously for 2 seconds every second. Because the time in continuous shooting is very short, we must pay attention to the timing of shooting. Fortunately, digital cameras don't need film, so they can shoot again. Some cameras can keep shooting for a long time, so much the better.

3. Because the subject is a moving object, if the shutter speed is too slow, the photographed image will be blurred, so it is necessary to increase the shutter speed. Of course, many digital cameras are stupid cameras and can't control the aperture shutter. At this time, the main solution is to improve the brightness of the shooting environment and improve the ISO setting. If the problem cannot be solved, exposure compensation can be used to improve the shutter speed, at the expense of underexposure of the photo.

4. This "multi-stage" magical shooting function can also cooperate with the video projection system for real shooting and playback, which is more realistic.

Judging from the actual effect of the teaching process of the course "Motion of Flat Throwing Objects", the introduction of the camera function of digital camera and the demonstration and analysis experiment of "multi-stage" magic shooting function has created a more vivid and intuitive teaching effect for the classroom teaching of this course, which can attract students' strong interest in cognitive physics knowledge and leave a deep impression on what they have learned.

The magical shooting function of digital camera "multi-segment" can not only study flat throwing motion, but also study high-speed moving objects such as free falling motion, simple harmonic motion and momentum conservation. In the experiment of conservation of momentum, stroboscopic photography is difficult to complete, because the reciprocating motion makes multiple images superimposed on the same negative, which is difficult to identify, and the photos taken by this "multi-segment" magical shooting function mode do not exist.

In addition to shooting high-speed moving objects, digital cameras are also used to shoot interference, diffraction fringes, oscilloscope waveforms and even microscope fields of view in dark environments. For example, in the demonstration experiment of Brownian motion, after debugging, the digital camera with camera function is docked with the optical channel and inserted into the camera interface of the stereo microscope, or the observation eyepiece is pulled out and inserted into the eyepiece tube, and the signal output end of the digital camera is connected with a large-screen TV or video projector by a shielded wire. In this way, you can see the big picture of "Brownian motion" on the big screen TV or projection screen-a large number of suspended particles with yellow advertising color have been moving irregularly. In class, under the guidance of the teacher, the students carefully observed and analyzed these pictures, recorded the trajectory of a particle at different times, and had a heated discussion, which deepened their understanding of the essence of Brownian motion. All this was done in just a few minutes, which not only saved time, but also achieved good teaching results.

The shooting function of digital camera and the magical shooting function of "multi-segment" can not only shoot and play, but also capture the content of shooting into a computer through a video capture card, edit it into a video file by using the nonlinear editing software Premeiere 6.0, and then put it on the screen through a projector, which is suitable for classroom demonstration and has a very good effect.

In a word, the introduction of digital camera into classroom teaching demonstration experiment makes the classroom demonstration experiment more colorful, effectively makes up for the defects of some experiments in physics classroom teaching, and has more real and intuitive characteristics than the direct simulation method with computer. Therefore, the introduction of digital camera is an innovation of teaching means in physics classroom teaching reform, which will play a positive role in promoting educational reform and development.

References:

1. Qian Rulan's "Physics Teaching Demonstration" No.8 video shooting physics teaching in 2004

2. Application of Jason and Mou Qishan's Multimedia Technology in Stroboscopic Photography Experiment 2004, 12.