|
|
|
AP Info | Courses | ILDs | Labs | Problems | Video |
|
|
|
|
CBL
|
Measuring Acceleration with a Picket Fence |
|
Goals: To practice the picket fence method of determining acceleration and to use the method to determine acceleration in several situations Introduction: For this experiment, a plexiglass strip is divided into alternating dark and clear strips of equal width. When the strip moves through a photogate interfaced to a CBL and calculator, time intervals are measured for the passage of successive dark/clear strips through the gate. Because the spacing of the strips is known and the times of passage are measured,the raw position and time data can then be graphed or converted to a graph of velocity vs. time or even acceleration vs. time. Equipment / Materials:
General Data Collection Information: Load the program PICKET onto your calculator. Attach your calculator to the CBL using a link cable. Using the adapter, connect the photogate in the channel 1 port of the CBL. Select and run the PICKET program on your calculator. Select the CHECK GATE option in order to test your photogate. The photogate should have an "Unblocked" status ata first. Slowly slide the picket fence through the gate to see if the screen alternately displays BLOCKED and UNBLOCKED. If the gate appears to not be working, ask for help. Otherwise, return to the main menu as the program instructs. To properly set the CBL/calculator for data collection, select the COLLECT DATA option. Follow the on-screen directions to arm the gate (timing will begin when the first dark band begins to break the beam). Once the time data has been collected, the program stores all collected and calculated data in the following lists on the calculator: L1 time bar has traveled (used for d-t plot) L2 displacement of bar L3 time bar has traveled (used for v-t plot) L4 velocity of bar L5 time bar has traveled (used for a-t plot) L6 acceleration of bar At this point, the program will allow you to view the d-t, v-t, and a-t graphs individually. After predicting the shape (and possibly even the value of the slope) of each graph, have a look at each graph. While the program does allow you to see the plots of the data, it does not do a fit for any of the graphs. Thus, you must exit the program to continue your analysis. For each of the steps below, make predictions, record data, and answer questions as you go along. Refer to the General Data Collection Information section of this instruction sheet as necessary. 1) Taking data with the cart on a horizontal track: Measure the distance in meters from the beginning of each dark band to the next on your picket fence bar. The default value used in the calculator program is 0.050m. Check that your picket fence matches the default value. Check the level of the track (devise a method). If your track is not level, ask for assistance in adjusting it. Then attach a picket fence in the on the top of the cart using small pieces of tape on both sides. Practice giving the cart a short-lived push to send it through the gate. However, make sure to stop pushing before the cart enters the gate. Be sure to position the photogate so that the picket fence can pass unobstructed through the photogate. Be sure that the photogate beam is perpendicular to the plane of the strip. Predict approximately what the acceleration of the cart should be as it passes through the gate; write down your prediction and your reasoning! Now collect the data for the glider on the horizontal track. View your x-t and v-t graphs to check their shapes. Exit the program. Either perform any further analysis on the data now or store the 6 data lists as a program in your calculator for later analysis. Analyzing the data.... Discuss the shape of the x-t and v-t graphs. Do the appropriate fit to the v-t graph to obtain the best acceleration value for the cart. Be sure to write out the fit equation for the v-t graph in both math and physics form (with units!). Also record the r value for the fit. The correlation coefficient, r, is a measure of the probability that the variables are actually correlated in a linear relationship. A value of 0 means no correlation; 1 (or -1) means perfect correlation. Expect all the fits to be very good (r nearly ±1). If they are not, assume that you made a mistake and try again (exception: if the value of the acceleration is very nearly zero, the correlation coefficient may not be near 1 because the fit has a hard time deciding whether the acceleration is positive or negative; in this case don’t worry about the value of the correlation coefficient). a) What does the slope of your v-t graph represent? Is the value what you expected? Explain. b) What does the intercept represent? Be specific. 2) Taking data with the cart traveling down a tilted track: : : Now tilt the air track with riser blocks. Record the height of the blocks you used to the nearest 0.5mm You may have to readjust the photogate to make sure the pickets pass through unobstructed. Release the cart just above the gate and catch it before bouncing. Collect the data for the glider going down the tilted track. Predict, view, store and analyze as before. a) What does the slope of your v-t graph represent? b) What does the intercept represent? Be specific. 3) Taking data with the cart traveling up a track::: In this part, you will give the glider a push from the bottom of the track so that the picket fence passes all the way through the gate (but not while you are pushing it). But once again, before taking data, predict the approximate value of the acceleration that you should obtain in this part. Collect, view, store, and analyze as before. a) Compare the slopes from steps 2 and 3. Are the results what you expected? Explain. b) Compare the intercepts from steps 2 and 3. Are the results what you expected? Explain. 4) Taking data with a different cart traveling down a tilted track: : : In this part you will mount the picket fence on a different cart, and then release this cart above the photogate as in part (2) above. Keep the height of the riser blocks the same. Record what is different about the cart. Once again, before taking data, predict the approximate value of the acceleration that you should obtain in this part. Collect, view, store, and analyze as before. Compare the slopes from steps 2 and 4. Are the results what you expected? Explain. 5) Taking data for a picket fence in free fall: : : Position a photogate on its stand sideways so that it extends over a table end. Put a foam pad or box on the floor below the gate. Drop a picket fence lengthwise through the gate while collecting data. Predict, collect, view, store and analyze the data as before. Is the acceleration within 1% of the expected value (show calculation)? If not, try again. Record all attempts.
The Picket Fence The term "picket fence" refers to a clear plastic bar with evenly spaced opaque bands. This picket fence is useful in many photogate activities. You can purchase a picket fence from Vernier OR .... You can make your own picket fence. Basically, you need a stiff clear plastic strip (1/16 - 1/8 inch thick works well) at least 2 inches wide (purchased ones are 2.5 inches wide) and 15 inches long. [Home improvement type stores will often cut sheets of plexiglass for a free or a very small fee. A clear plastic ruler might work if there are too many markings on it.] You also need a roll of opaque tape. I prefer plastic tape (electrical or similar) instead of masking tape or duct tape. Tape that is 1 inch wide is ideal, slightly thinner is okay, but wider that 1 inch can be problematic. Most software (both computer and CBL) written for use with the picket fence/photogate combination expects exactly 8 opaque bands on the fence. In fact, most software expects the distance from the beginning of one opaque band to the beginning of the next to be 5.00cm. All calculations within the software will make this assumption, so you need to construct your fence accordingly. Use your tape to create your opaque bands on your strip of plastic. Be sure to get your tape straight. Also, don’t stretch plastic tape so tightly that it "curls up" and messes up your perfect 5.00cm spacing.
copyright 2009 The North Carolina School of Science and Mathematics
|
||
