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Video Analysis of 2D Motion
Our goal in this lab is to get a better understanding of the relationships between the position, displacement, velocity, and acceleration vectors in two dimensional motion. You will take home videos of a thrown object and an object moving in a circle and analyze the motion graphically.
You will need the following equipment to complete the lab. If you do not have any of the equipment listed below, please let your instructor know.
- Smartphone or computer with video recording capabilities. Cameras with slow motion capabilities are ideal.
- A way to prop up your phone so that you can take videos of yourself doing things (or another person to help you take videos).
- Computer that runs Vernier Video Analysis (Links to an external site.).
- An object that can be thrown without damaging anything.
- An object that spins and keeps spinning without external interference. The front wheel on a bicycle that is turned upside down is perfect. Other options might be a record on a record player, a salad spinner, or even a basketball spun on a finger, if you are good at it and have someone who can film it. As a last resort you could tie an object to a string and try to spin it around at constant speed, but this is not ideal since it’s not really possible to keep the speed constant with this method.
- Some tape to mark a point on your spinning object.
Taking the Videos
Your goal is to take videos of a number two-dimensional motion. For effective analysis these videos need to be taken carefully. For each video make sure you do the following:
- Prior to each recording, measure a distance between two well-separated points that will be in your video that lie in or close to the plane of motion. For teh video of circular motion this might be the radius or diameter of the spinning object. Record the distance between these two points and mark the two points so that they will be visible in the video.
- The motion of the object should be across the screen, not towards or away from the camera.
- Do not move the camera while the video is being recorded. This is crucial! There is a tendency to try and “track” moving objects with the camera: try to avoid doing this! Propping up the camera rather than holding it is ideal, but you can take good videos by carefully holding the camera steady.
- If possible, use slow-motion filming.
- Use a background that is of a different color than moving object. If the object is dark, use a light background, and vice-versa.
- Caution: with many phones there is a delay after you press the “record” button before filming begins. Hit the “record” button, wait until you see at least one second has elapsed in the recording, and then do the motion.
- Note for iphone 11+ users: the software can’t deal with the most recent movie formatting. Before taking you video go to settings/camera and switch your video format to “most compatable”.
Videos to record:
- Record a video of an object tossed upwards and sideways flying through the air. (The motion should look like a parabola in the video). Be sure that you capture a good amount of upwards and downwards motion. You do not need to record falling all the way to the ground.
- Record a video of an object moving in a circle at a constant speed; the motion should look circular in the video. We want to track a single point on the object, so mark the point you want to track with a piece of tape. With a bike wheel, salad spinner, or basketball you can just give it a spin and let it spin by itself and if there isn’t too much friction the motion should be close to constant speed. A record player should maintain a constant speed once it is running. If you are trying to use an object tied to a string, try and spin the object in a horizontal plane and have someone film from below or above; if you spin the object vertically the speed will likely vary because of gravity.
- Once you have taken your videos transfer them to your computer. (You can also use Video Analysis on your phone, but using a computer is much easier).
- Find and record the frame rate each video was taken at. This can be tricky; on some phones (like my old iphone 6), if you play a slow motion video file on your computer it will appear to be normal speed. In this case, locate the file and view its “Properties” and then click the “Details tab. The frame rate (in fps, or frames per second) should be displayed in the “Video” section. On other phones, like my Samsung Galaxy s10, a slow motion video taken at 240 fps will appear slowed down when played on a computer video player, and the file properties will say the video was taken at 30 fps. In this case, you can try and look up the frame rate your phone uses for slow motion. 240 fps is a good guess if you are not sure.
Video Analysis with Vernier Video Analysis
You can access Vernier Video Analysis using this link (Links to an external site.). More documentation (Links to an external site.) is available as well. Start by doing this tutorial (Links to an external site.) starting at the 1:26 mark. Follow along on your own device so you learn how to use the software. When analyzing your videos, consider the following things:
- Track the motion of the center of the object if the object is non-spherical; points on the outside of objects will likely rotate around the center, which complicates the motion in ways we don’t want (yet).
- Only analyze motion where the object is moving by itself; omit any motion where the helper’s hand is touching the object, and after the object has come to rest.
- Experiment with the “advance frames” feature. You want about 15-20 data points for the entire motion.
- You can place the origin wherever makes sense: it does not necessarily need to be at the starting location of the object.
- Use meters for distance: if you measured your known distance in another system of units, convert it to meters.
- Make the movie as large as possible on your screen so that you can place points most accurately.
- We are focusing only on the x-motion here: for the first video, there should be no motion in the y direction and in the other videos we will ignore the y direction.
Below, you will perform a series of actions and answer a series of questions. When needed, you can write equations using Canvas’ equation editor. Here are instructions for using the equation editor (Links to an external site.).
Doing The Lab
Once you have read through the instructions above, start the lab by clicking “Start the Quiz” below. The questions will contain further instructions. Enter your answers but do not hit the “submit quiz” button until you are done with the entire lab. In theory Canvas should save anything you enter into the quiz even before you submit the quiz, though I recommend keeping a backup copy of your answers on your own computer. You should be able to enter and leave the quiz as much as you like until the due date.