Prelab for Friction and Tension Lab 1. Predict what the graph of force vs. time will look like for Part 1 of the lab. Ignore the numbers and just sketch a qualitative graph 12-1
Dual-Range Force Sensor Friction and Tension OBJECTIVES Use a Dual-Range Force Sensor to measure the force of static friction. Determine the relationship between force of static friction and the weight of an object. Measure the coefficients of static and kinetic friction for a particular block and track. Use a Motion Detector to independently measure the coefficient of kinetic friction and compare it to the previously measured value. Determine if the coefficient of kinetic friction depends on weight. MATERIALS computer Vernier computer interface Logger Pro Vernier Motion Detector Vernier Force Sensor string block of wood with hook (wood side down) balance or scale mass set NOTE: YOU DO NOT NEED A TRACK FOR PART 1 AND 2 Wooden block Mass Pull PROCEDURE Part I Static Friction 1. Measure the mass of the block and record it in the data table. 2. Connect the Dual-Range Force Sensor to Channel 1 of the interface. Set the range switch on the Force Sensor to 50 N. 3. Open the file 12a Static Kinetic Frict from the Physics with Computers folder. 4. Tie one end of a string to the hook on the Force Sensor and the other end to the hook on the wooden block. Place a total of 1 kg mass on the top of the block, where the felt is, Practice pulling the block and masses with the Force Sensor using this straight-line motion: Slowly and gently pull horizontally with a small force. Very gradually, taking one full second, increase the force until the block starts to slide, and then keep the block moving at a constant speed for another second. 5. Hold the Force Sensor in position, ready to pull the block, but with no tension in the string. Click to set the Force Sensor to zero. 6. Click to begin collecting data. Pull the block as before, taking care to increase the force gradually. Repeat the process as needed until you have a graph that reflects the desired 12-2
motion, including pulling the block at constant speed once it begins moving. PRINT the graph for use in the Analysis portion of this activity. Label your graph Static and Kinetic Friction and put something in the footer to identify the graph as yours. Part II Kinetic Friction In this section, you will measure the coefficient of kinetic friction for the wood block and lab table top surfaces. Using the Motion Detector, you can measure the acceleration of the block as it slides to a stop. This acceleration can be determined from the velocity vs. time graph. While sliding, the only force acting on the block in the horizontal direction is that of friction. From the mass of the block and its acceleration, you can find the frictional force and finally, the coefficient of kinetic friction. Wooden block Push 1. Connect the Motion Detector to DIG/SONIC 1 of the Vernier computer interface. Open the 12b Static Kinetic Frict in the Physics with Computers folder. 2. Place the Motion Detector on the lab table 2 3 m from a block of wood, as shown in Figure 2. Position the Motion Detector so that it will detect the motion of the block as it slides toward the detector. 3. Practice sliding the block toward the Motion Detector so that the block leaves your hand and slides to a stop. Minimize the rotation of the block. After it leaves your hand, the block should slide about 1 m before it stops and it must not come any closer to the Motion Detector than 0.2 m. 4. Click to start collecting data and give the block a push so that it slides toward the Motion Detector. The velocity graph should have a portion with a linearly decreasing section corresponding to the freely sliding motion of the block. Repeat if needed. 5. Select a region of the velocity vs. time graph that shows the decreasing speed of the block. Choose the linear section. The slope of this section of the velocity graph is the acceleration. Use the analysis function to find the average acceleration of the block. Record the value of the magnitude of acceleration in your data table. 6. Repeat the procedure 4 more times. If one of your acceleration values is very different than the others, you should repeat the trial. PRINT ONLY ONE representative graph of these trials. Label this graph velocity vs time no mass The analysis should be visible and not hide the data. 7. Repeat the procedure with 500 kg securely fastened to the top of the block. USE TAPE! PRINT ONLY ONE representative graph of these trials. Label this graph velocity vs time 500 g Record acceleration values in your data table. Physics with Computers 12-3
8. Draw a free-body diagram for the sliding block, in the space below. The kinetic friction force can be determined from Newton s second law find the friction force for each trial, and enter it in the data table. DATA TABLES Part I Starting Friction Mass of block kg Part II Kinetic Friction Trial 1 2 3 4 5 Acceleration (m/s 2 ) Data: Block with no additional mass Kinetic friction force (N) Average coefficient of kinetic friction: k Trial 1 2 3 4 5 Data: Block with 500 g additional mass Acceleration (m/s 2 ) Kinetic friction force (N) Average coefficient of kinetic friction: k 12-4
ANALYSIS 1. Inspect the print or sketch of force vs. time graph from Part I. Label the portion of the graph corresponding to the block at rest, the time when the block just started to move, and the time when the block was moving at constant speed. How does your graph compare to the shape of your predictions? If it is not the same, what is different and why? 2. Still using the force vs. time graph you created in Part I, compare the force necessary to keep the block sliding compared to the force necessary to start the slide. To what type of frictional force does each correspond? 3. The coefficient of friction is a constant that relates the normal force between two objects (blocks and table) and the force of friction. Based on your graph (Run 1) from Part I, would you expect the coefficient of static friction to be greater than, less than, or the same as the coefficient of kinetic friction? 4. Your data from Part II also allow you to determine k. Does the coefficient of kinetic friction depend on speed? Explain, using your experimental data. 5. Does your data show that the force of kinetic friction depend on the weight of the block? Explain: 6. Does your data show that the coefficient of kinetic friction depend on the weight of the block? Explain: Physics with Computers 12-5
Part III Tension and acceleration in a system of interacting objects OBJECTIVE: Determine acceleration and tension for a system of interacting objects. APPARATUS: Wireless force probe DLink bluetooth key, motion detector, computer, scale, cart, track, lab jacks, pulley, string, masses. PROCEDURE : 1. Obtain a value for the mass your team will use from instructor: m b = 2. Record the mass of your cart: m c = 3. Calculate a theoretical value for a) the acceleration of the system, and b) the tension in the string, both to 3 significant figures. Assume no friction in the system, massless string, and massless, frictionless pulley. Work on back or attach separate sheet. These values are considered the true values. a = T = 4. Hand your values, above, to instructor, with name of team members. 5. Set up cart and ramp as shown above. Use the lab jacks to give you > 1m above the floor. Make sure track is level. 6. Turn on computer again and insert DLink. Open Logger Pro and use the file Speeding Up Again(L03-2A2). Scan for wireless device when prompted to do so or go to Experiment menu to connect wireless force probe. 7. Confirm that your force probe is the one detected (name on side of probe). 8. Zero force probe with nothing hanging from it. 9. Cart should start at least 0.2 m from motion detector. 10. CATCH THE CART BEFORE IT CRASHES INTO THE PULLEY. 11. Print the graph and attach to lab. 12. Calculate the % difference between experimental and theoretical ( true ) values. % difference = true value experimental value x 100 true value a exp = T exp = % error for a: % % error for T % Completed lab with no other errors: Less than 5%: 20pts Less than 10% 18pts than 15% : 15 pts. Less 12-6