Lab #1: Flashlight Lab



Understanding Free Fall and Terminal Velocity

Objective

In this lab, you will use a video camera to record the motion as we drop a variety of objects. You will then use Logger Pro software to graph the motion of the objects and analyze the distance, velocity, and acceleration of objects in free fall.

Background

In this lab we will be using a standard video recorder with a USB port. This will allow us to instantly transfer the video to a computer where you can then analyze the motion over time. In a graph of the position over time, the slope of the line is the velocity at that point. Using curve fitting techniques, we will be able to approximate g, the Earth’s acceleration due to gravity.

Materials

Video camera

Meter stick

Variety of balls and other objects to drop

Computer with Logger Pro software

Procedure

1. Write down the preliminary information in your lab notebook. This should include your name and your partner’s name, title of the lab, and other header information. Continue through to the objective and materials. You may reference this handout for the procedure. Much of your work will be done on the computer, answer questions in your lab notebook. Make sure your graphs and data tables are included in your report. Each person needs to create and print their own graphs.

2. There will be an area set up to drop objects. Make sure you have a meter stick visible in your drop zone, Logger Pro need to be able to calibrate using a known distance, the meter stick will be the reference length.

3. You will use the camera to video the objects as they are dropped. Make sure you can see both the starting point and the landing zone. Hold the camera steady, do not follow the object during its decent. Record any qualitative observations in your notebook.

4. Make a recording for each object being dropped. You will drop at least two objects. Once should be a golf ball, the other can be an item of your choosing. Record one item at a time so that the rest of the class can get started on their analysis.

5. Once your video has been recorded, load the file onto your computer. Start-up Logger Pro and click INSERT/MOVIE. Select your movie file, it will open.

6. You may have to scale your image down to see the buttons on the bottom right. Advance the frames until you are one frame before the object starts to fall.

7. Click SYNC MOVIE TO GRAPH, make sure there is a check mark where it says, “Use this synchronization in video capture.” Put 0 in the GRAPH TIME and click OK.

8. On the video, click the red dot with the cross hairs. This is how you will track your image on the video. Each time you click on the location of the object, a data point is created and the frame is advanced. You can advance an extra frame if the object hasn’t moved far enough for another dot. Continue tracking the object until it just hits the ground.

9. Click ORIGIN on the right menu and then click on the first dot.

10. Click SET SCALE on the right menu, then drag the cursor with accuracy over the meter stick and click OK.

11. Get rid of your horizontal data by clicking on SHOW/HIDE DATA BROWSER. Expand VideoAnalysis, click on X(m), hit delete and close the window.

12. You have just created a graph of the distance your object fell and it is synced to your video. Click ANALYZE/REPLAY to watch the object fall and the data points appear. Save your work.

Analyzing Distance

13. On the ANALYZE menu, click EXAMINE. This will turn on a vertical beam that shows the data in the data box. Put the beam on time 0 and read the Y distance. It should be very close to 0. Now move to 0.1 second and write down the Y value in your lab notebook. Continue every 0.1 seconds until the end of your graph. Subtract successive reading from each other to find how far the object fell in each 0.1 second interval.

Observing Velocity

14. Select INSERT/METER/GAUGE. Place the gauge on the screen, scale it down to fit. Right-click on the gauge, select GAUGE OPTIONS and change the left velocity to 0 and the right velocity to -10.

15. Select ANALYZE/REPLAY. Scale the speed down to 0.2 times original and hit PLAY. If you have the video on top, you will see the ball fall, the data points appear, and the downward velocity increase.

16. When you are done, you can delete the GAUGE. You should also delete the video from your Logger Pro page at this time.

Analyzing Velocity

17. Select the ANALYZE/TANGENT function. This will create a short line, usually between three points. The slope of this line is the change in distance over the change in time. You can directly read the velocity of the object in the box.

18. Highlight three points at the right of the graph and select ANALYZE/LINEAR FIT. If a windown pops up, select Video Analysis|Y and OK. This will create a straight line through those three points. Move the information box close to the points. You can directly read the slope, which is the instantaneous velocity at that point.

19. Repeat this linear fit in three more places on the same graph. Do one very close to the starting point, put the other two in the middle of the graph, evenly spaced. Each time you do this, the data will remain on the screen. Once you have finished, print this page in landscape.

Analyzing Acceleration

20. Go to PAGE/ADD PAGE/COPY CURRENT PAGE AND GRAPH and hit OK. Delete the information boxes.

21. Go to ANALYZE/CURVE FIT. If a window pops up, select Video Analysis|Y and OK. Highlight the graph along where it appears to be accelerating. Select General Equation (At^2+Bt+C), then TRY FIT, and OK. The system will create a curve that fits your data. The value of the A term should be approximately g/2.

22. Repeat the curve fit, this time after the curve is created, select manual and change the A term to -4.905. This will create a second acceleration curve on the screen that is what would be expected if the object accelerates at exactly -9.81 m/s2. Print this graph in landscape.

Analysis

1. Examine the data from your distance measurement. Is the object continuing to accelerate or has it reached terminal velocity. Use date from your graphs to support your reasoning. Be specific.

2. Look at the graphs of the CURVE FIT and the MANUAL FIT. Come up with some possible reasons why the two lines are not on top of each other. Use date from your graphs to support your reasoning. Be specific.

3. Are the sets of graphs of the two different objects dropped similar or different? In what ways are they similar, in what ways are they different? Explain the differences and the reasons for the differences. Again, be specific, use data from your graphs to support your reasoning.

4. If you were to do this lab again, what would you do differently to improve your results?

5. What specific recommendations do you have to improve this lab for future students?

When you are done, sign and date the lab indicating that the work you have done is your own. Copied labs will earn a failing grade for all parties involved.

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