John Zwart, Kayt Frisch, and Tim Martin, Dordt College, Sioux Center, IA
Video analysis experiments have strong potential to reinforce student learning and build intuition; however, in the intro physics lab, students often find experimental values that are substantially different from the expected results (e.g., a curve-fit derived value for g of 11.59 ±0.02 m/s2 for a tossed golf ball). Despite giving students specific instructions for setting up video equipment, we frequently see poor experimental results. This suggests that small variations in the experimental setup produce significant systematic errors. We investigated what happens when students deviate slightly from two specific tips: “place a ruler, meter stick, or other scale item in the same plane as the motion being recorded” and “position the camera so the line of sight is normal to the plane of motion.” We also investigated the effect of different focal length choices, specifically a wide-angle setting (shortest focal length, widest field of view, objects appear smaller) and a telephoto setting (longest focal length, narrow field of view, objects appear larger).
To investigate the “place a ruler, meter stick, or other scale item in the same plane as the motion being recorded” tip, we used an array of meter sticks and measured distance relative to the center meter stick (Fig 1). We found that when using the wide-angle setting, an object placed 40 cm in front of the reference meter stick would appear 40% larger than its actual size, while an object placed 40 cm behind the reference meter stick would appear 20% smaller than the true size. The effect diminished to 10–15% for the normal and telephoto settings. For a student’s experimental setup, this means that the apparent distance traveled by a tossed ball would appear longer or shorter than the actual distance.
We investigated the second tip “position the camera so the line of sight is normal to the plane of motion” by drawing line segments on a poster board (Fig 2A) and placing the camera at a distance that filled the field of view for each lens setting. We rotated the camera so that the right side of the board was closer to the camera (Fig 2B) and measured the apparent length of line segments at the corners of the board compared to the reference length in the center of board. We observed that this error results in size discrepancies of nearly 20% when using a wide-angle setting, though it was less of a problem at the telephoto setting (Fig 2C). For a student’s experimental setup, this means that the apparent distance traveled by a tossed ball would change as it moved across the image.
Our results show that significant systematic errors can occur for relatively small deviations from ideal camera/reference placement, particularly when using a wide-angle lens. If you use a zoom lens, standing farther away and using the telephoto setting will reduce the likelihood of these types of systematic errors. When using a fixed lens that is wide angle (such as a typical cell phone), you need to be particularly careful when setting up the video collection equipment.
Are you looking for ways to expand your use of Logger Pro? Here are tips to get you started with some of the advanced features of our powerful data-collection and analysis software:
Add multiple pages within a single Logger Pro file to organize graphs, data tables, pictures, and other information. All pages share the same underlying data and data-collection settings. See the Page menu for options.
Combine data from several files into one by choosing Import from ▶ Logger Pro File from the File menu. You can use this feature to easily compare data from multiple groups.
Choose Strike Through Data Cells from the Edit menu to temporarily ignore some data points. This is a great way to remove selected data from graphs or curve fits without actually deleting the values. The values are shown with a line through them in the data table. The feature can be used to ignore preliminary motion detector data, for example.
When the graph is selected, you can use the spacebar to start or stop data collection. No more hunting for the Collect button.
Draw predictions before doing experiments. To do this, choose Draw Prediction from the Analyze menu.
Add a second vertical axis to display data of incommensurate units on the same graph. For example, graph pH and the derivative of pH on a single graph. Choose Graph Options from the Options menu to enable and configure a second y-axis.
Choose Model from the Analyze menu to plot functions on a graph; then, adjust the parameters in the function helper object by selecting the parameter and using the arrow keys or by selecting the value of the parameter and entering values by typing on the keyboard or using cursor keys. Use this feature to have students find their own best-fit line and then compare it to the least-squares fit.
Use calculated columns to display inferred quantities, even during data collection. For example, graph kinetic energy using a calculated column of 1/2 mv2 from the Motion Detector velocity data. Use a user parameter for m while you’re at it, and then add a parameter control for m using the Insert menu.
Show error bars on a graph; error bars can be a fixed fraction, fixed value, or independently entered values. Enable error bars by choosing Column Options from the Data menu.
Create a semi-log or log-log graph by choosing Graph Options from the Options menu.
One of the great, hidden features of Logger Pro is the ability to add vector displays to motion you are studying. Two sample files that use this feature are included with Logger Pro. One of the files, “Basketball Shot vector analysis,” can be found in the “Sample Movies” folder. If you open the file and play the movie, the vectors appear on the movie as the ball moves.
For a tutorial on how to set up animated displays in your Logger Pro files, choose Open from the File menu and navigate to the “Exploring Animated Displays” file in Sample Data > Physics > Animated Display Vectors.
Students doing video analysis of everyday motion can be a great way to bring physics into the real world. Due to the quality of some videos, they are sometimes difficult to analyze. Here are some tips for recording good videos for analysis:
Support the phone or camera to keep it still. Use a tripod, tape the phone to a box, or find another way to avoid the shakiness of hand-held video. This will also prevent the tendency to follow the motion with the camera.
Record the event in a well-lit area. Outdoors in daylight is the ideal environment, as even an overcast sky can provide more light than can be found indoors. However, going outside is not always practical. Old overhead projectors can provide a bright light that has no flicker, even when filming in high speed. The brighter the lighting, the less motion blur you will see in the video.
Position the camera so the line of sight is normal to the plane of motion. Ideally, all the motion would take place at a constant distance from the camera, but that is nearly impossible to achieve. Therefore, arrange the scene to have as little distance variation as possible. This is to make sure the scale you choose applies to as much of the motion as possible.
Place a ruler, meter stick, or other scale item in the same plane as the motion being recorded. Having the scale object at that same distance eliminates parallax error in scaling.