Visual Physics Lab Project 1

Transcription

1 Page 1 Visual Physics Lab Project 1 Objectives: The purpose of this Project is to identify sources of error that arise when using a camera to capture data and classify them as either systematic or random errors. Your TA will help you to understand how to find these sources of error. This experiment is designed to magnify these sources of error so that you will be able to see them more clearly. Expectations: You are expected to have read Chapter 2 from Young & Freedman s University Physics before coming to this lab. You must understand 1 dimensional motion and know how to identify displacement, constant velocity, and acceleration on a graph. You will also need to have read the handout on error analysis that is available on the visual physics website. EXPLORATION As you observe yourself on camera, think about how what we perceive visually could translates to what exists in a Cartesian world. Preflight Questions: How could our perceptions change when we look at things from different angles? If you cover up one eye, how could you estimate distances? How can the panoramic setting on your camera and IMAX cameras project images onto a rounded surface? Think about why they do this. Experiment 1 In this experiment we will study what errors may occur in our estimation of displacement when we arrange the camera poorly. Since we will not be examining motion, you will not need to record more than 1 second of video, because you will only be looking at the first frame. Set Up Begin by taking some masking tape and a pen and making several equidistant marks on the air track that can easily be identified in the video. Be sure to record what the separation distances are in your Lab Journal so that you can compare later. Next you arrange the camera into 3 different orientations and record a short video in each arrangement.

2 Page 2 Arrangement 1: Align the camera directly above the center of the track in such a way that the camera is pointing at the center of the track. Arrangement 2: Align the camera above one end of the track in such a way so that the camera is pointing at the center of the track. Arrangement 3: Align the camera directly above the center of the track in such a way that the camera is pointing at the center of the track on the table behind (in front of) you. After you have captured video from all the different arrangements, use the LabApp to select all of the points you marked earlier.

3 Page 3 Note: Do not use LabApp s calibration feature for this experiment. After you have selected the points and saved them open the document in EXCEL and evaluate the separation between the points (in pixels) QUESTIONS Are they equidistant apart in all of the listed arrangements? If not, why not? What does this tell you about how the camera must be arranged in order to evaluate 1 dimensional displacement? Prepare a technical memo that addresses the above questions. EXPLORATION Think about a straight road with a car approaching from the distance. If it seems to speed up as it passes you and then slow down again as it disappears into the distance, can it be moving at constant speed? Preflight Questions Can distortion in an image effect the appearance of velocity in the analysis? Do you think the scenario listed above might cause distortion in the velocity estimates? If so, which ones? Experiment 2 In this experiment we will look at constant velocity motion in 1 dimension. You will need to level the track before doing this part. Your TA will explain how this is done. After your track is leveled make sure the cart glides smoothly along the track s surface without slowing down. 1. You will need to arrange the camera into each of the 3 positions that were described in experiment In each arrangement you need to set the cart into motion and capture enough video so that you have the cart traveling the full length of the track. 3. After you have captured all the videos you will need to run LabApp. 4. In LabApp you need to select a point on the moving cart to click on in each frame of the video. In this way you will track the motion of the cart as it glides along the track. 5. After you have clicked on the cart in each frame of the video calibrate and save the data using a. 2 Points near the center of the track b. 2 Points near the end of the track. c. 2 Points near the other end of the track.

4 Page 4 QUESTIONS In your analysis you will need to compare these different results and explain why you got them. Try to figure out what the source of this error is. Were any of the results surprising? Can you think of any other types of error that may not have been as obvious? If so, Could another camera arrangement magnify these errors? Write a technical memo to your TA reporting your findings. EXPLORATION If you throw a ball into the air you can make it go higher by throwing it harder. If you and a friend throw a ball into the air at the same time it may appear that your ball is accelerating faster and goes higher than theirs. How does your perspective create this illusion? Preflight Questions If the projectile is closer (further away) from the camera than you think, what effect will that have on your calculation of acceleration? What is the difference between instantaneous and average velocity? Acceleration? Write a definition of each. Can you have constant velocity with constant acceleration? Give an example? Experiment 3 In this experiment we will be looking at constant acceleration in 1 dimension. You will need to relocate your camera to the vertical post at your workstation. It will need to be aimed at the other end of the table, as shown in the picture.

5 Page 5 Next you need to get the Marble Launcher from the cabinet and place it at the other end of the table aimed straight up (90 degrees). Then you will suspend your meter stick from the scaffolding, using some string, such that the meter stick hangs vertically next to the marble launcher. 1. Adjust your video camera to show the entire length of the meter stick. Then figure out the best setting for the marble launcher. Try a couple of different settings to see which one gives the best trajectory for your video. (Ideally the marble will travel from one edge of the screen to the opposite edge without going off the screen.) 2. Now you will record a short video of the marble as it is released from the launcher and falls back down. 3. After you have recorded your video open Lab App. Now calibrate the data using the ends of the meter stick and then capture the position of the marble while it is in flight (Only click on the frames where the marble is in flight) 4. Have the other members of your group repeat step After this you should have 3 sets of data. You will need to do a comparison of these 3 data sets to estimate human error. 6. Next you should move the meter stick a few centimeters closer to the camera and repeat steps 1-3. Be sure to record in your notebook how far from the plane of motion you placed the meter stick. 7. Now move the meter stick a few centimeters behind the plane of motion and repeat steps 1-3 again. After you have captured all your data clean up your station and you can begin your analysis. Since the marble is undergoing constant acceleration (g ~ 9.81 m/s 2 at sea level) the formula for its motion should be a 2 nd order polynomial in time (1/2 at 2 + bt + c). Make sure you understand why. Graph your data and try to fit it with a 2 nd order polynomial Trend line in Excel. QUESTIONS Does the trend line fit the data very well? What values do you get for acceleration?

6 Page 6 Does it make a difference if the meter stick is in front of (behind) the plane of motion? In doing future experiments how can you use this information? Write a technical memo to your TA reporting your findings. Post-Flight Extensions: Suggestions What is the role of precision and accuracy in an experiment? Can you be accurate but not precise? How? Can you be precise but not accurate? How? Keeping a careful journal of your results might help in writing up the lab later. What are some other advantages to keeping an accurate lab record?