MEASUREMENT OF ACCELERATION Pre-Lab Name: Roster# Date: 1. A tree is 15.0 m high and cast a shadow along the ground that is 30.0 m long. Draw a triangle that represents this situation. What angle does the shadow make with the ground? 2. List three types of force.,, 3. Define the following terms: a. frictional force b. weight c. acceleration due to gravity 4. (a) What would the slope of a velocity versus time graph indicate? (b) List a possible unit for the slope in this case. 5. Convert the following to meters. (a) 500 cm = m (b) 500 mm = m 1
MEASUREMENT OF ACCELERATION OBJECTIVE: To measure the acceleration of a glider on an air track and experimental determine accelerations due to gravity. APPARATUS: Air track with glider PC computer with interface and Photogate Logger Pro software Plastic "Picket Fence" on glider Elevating blocks Vernier Caliper 2-meterstick Photogate Picket Fence Adjustable leg Block Macintosh INTRODUCTION: ULI v Acceleration is defined as Lim, t 0 t Figure 1 where v is the change in speed that occurs during the elapsed time t. A glider released from rest on a sloping air track accelerates at a nearly uniform rate. In this experiment, the average speed of the glider is found for the segments of the plastic fence as each segment passes through the single photogate. The distance between successive black bands has been carefully measured by the manufacturer to be 5.000 ± 0.001 cm. The computer, with the LOGGER PRO software, is used to measure either the time required for each segment from the beginning of one black band to the beginning of the next black band or the time for each individual black band to pass through the photogate switch. The data collected is printed and graphs of speed vs. time are plotted. The slope of this speed vs. time graph is the average acceleration of the glider on the air track. From the acceleration of the glider and the slope of the air track, the acceleration due to gravity, g, can be determined from: a g = sinθ (1) PROCEDURE: 2
1. Be sure that the air supply to the air track is turned on. If you do not detect air flowing in the track, check with the lab instructor. 2. Measure and record the distance, L, along the air track between the single leg and the double legs of the track. Use the scale on the edge of the air track. 3. (Check out the Caliper from the instructor. Return it when you are finished.) Use the vernier caliper to measure (and record) the height, h, of the elevating block, then, by lifting one end of the track, place one block under the single leg to slope the air track. Compute the sin θ from step 2 and 3 information, using the laws of trigonometry. 4. If the ULI board is not turned on, contact the lab assistant. When the ULI interface is on there are two lights glowing. 5. If the computer is not on, turn it on by pressing the spacebar several times on the keyboard. Double click on the Physics Lab folder and select the acceleration program. 6. Click the COLLECT button on the screen. 7. Place the glider with the attached "Picket Fence" (without moving it through the photogate) about 20 cm above the photogate and release it from rest. 8. After the picket fence passes completely through the gate, click on the Stop button. A table and graph of your data should now appear on the screen. If this does not happen ask the lab assistant for help. 9. Go to the View menu and select Auto Scale Once. 10. The computer can find the best straight line fit for a set of data points but it calls it a linear fit line. Do this by clicking and holding on the first data point and then drag to the last data points. Let go and the box will remain there. 11. Second, go up to the Analyze menu and select Linear Fit. 12. Go to the View menu, select Graph options and make sure the following are selected Point Protector Every, Graph Title and Grid. At the bottom of the page in the box below graph title type Velocity vs. Time. Then click ok *13. If your data is satisfactory, save this data on your diskette and record the name you assigned to the saved file on your data sheet. To do this select Save As from the File menu. Save the file as a MBL file. For example, One block down.mbl 14. Repeat the lab procedure but this time place the glider at the bottom of the air track and push it hard enough to go through the photogate but not hard enough to crash into the far end of the air track. 3
15. Repeat procedures 6 through 14 but this time place two blocks under the air track leg. 15. After saving your data go to the main lab room and insert your computer disk into one of the free computers and print out the graphs for each situation. 16. Have your coversheet initialed by an instructor, and if possible begin the analysis of this data now. Computer Analysis of data: (Don t rewrite) 1. Using a free computer in SM252, click on the Physics Lab Folder icon, then select Logger Pro icon. 2. From the File menu select Open. Open your first saved file for this experiment. 3. Print the graph and data table. To do this go to the File menu and select Printing Options and type your name and roster #. Under the comment section type either One Block or Two Blocks, whichever pertains to your graph. Select the date option and then select Page Setup. Click on Landscape and hit the ok button two times. Go to the File menu and select Print screen and hit ok. 4. Repeat for the other graphs that you saved. 5. From the File menu, select Quit. CALCULATIONS: 1. From your graphs determine the magnitude and direction (either upward or downward along the air track) of the acceleration of the glider for each run. 2. From the acceleration of the glider and equation 1, calculate g for each run. Hint think slope and the laws of trigonometric functions. 3. Compute your average value of the magnitude of g for the experimental values using one block and calculate the %error. 4. Compute your average value of the magnitude of g for the experimental values using two blocks and calculate the %error. PRACTICE QUESTIONS: (Use complete sentences, no one word answers) 4
1. What is causing the glider to accelerate? 2. Why is the slope positive when the glider is going down the ramp and negative when going up? What is the direction of the force of gravity in these two cases? 3. What does the negative sign indicate on the slope of the graph? 4. What does the y intercept of your data represent? If you had released the glider from a greater distance above the photogate, would that have affected your measured acceleration. Would it have affected your y intercept. 5. Consider the difference between your measured value of g and the accepted (true) value of 9.80 m/s 2. How would friction affect your experimental value of g? 5