Static and Kinetic Friction

Transcription

1 Ryerson University - PCS 120 Introduction Static and Kinetic Friction In this lab we study the effect of friction on objects. We often refer to it as a frictional force yet it doesn t exactly behave as such. It is defined as a force that impedes or resists motion but cannot move the object itself. Friction is typically split into two types: Static and Kinetic friction where as the name implies, one acts while the object is static and the other while the object is kinetic (or moving). These frictional forces are said to depend on the Normal (often times referred to as the weight) of an object. This is quite observable as heavier objects (of the same shape and size) are generally harder to push than lighter ones. We define the force of friction to be: f f = µ f N (1) This lab will use several methods to determine the coefficient of static friction µ s and the coefficient of kinetic friction µ k. The first method involves using a force sensor used to monitor the force applied onto a massive object as it is travels across a horizontal surface. By adding masses, we can build a relationship between mass and frictional force. The second method allows one to measure the kinetic friction of the block by observing it come to a rest after giving it an initial velocity. Once again, we build a relationship between mass and kinetic frictional force. The last method involves applying a force to pull a massive object up a ramp. By adjusting the angle of the ramp, we build a relationship between angle and frictional force. Apparatus Wooden Block String 500 g ± 1% Mass 1 kg ± 1% Mass Vernier Motion Sensor Vernier LabPro Vernier Force Sensor LoggerPro software interface Aluminum track Retort stand with clamp Meter stick Pre-Lab Questions Please complete the following questions prior to coming to lab. At the beginning of lab, you will be given a short quiz which is heavily based on one (or more) of these questions. Page 1 of 6

2 1.) Read through the entire lab writeup before beginning. Ryerson University - PCS ) What is the specific goal of this lab? Exactly what question(s) are you trying to answer? Be as specific as possible. ( To learn about topic X... is not specific!) 3.) What specific measurements or observations will you make in order to answer these questions? 4.) Consider a block with mass M being pulled at a constant velocity along a horizontal plane. Using a coordinate system along the horizontal plane, draw the free body diagram of the box. Hint: consider the condition for constant velocity What is the condition for the (external) force such that the block remains moving? What would happen to velocity and acceleration if the block was pulled onto a frictionless surface? 5.) Consider a block with mass M being pulled at a constant velocity up an incline of angle θ. Procedure Method I Using a coordinate system along the surface of the incline, draw the free body diagram of the block. If the block is not being pulled anymore such that it is at rest on the incline, write the condition for the maximum angle theta max that this can occur. Hint: consider what forces are still acting on the block If the angle of inclination were to be increased (ie θ > θ max ), what would happen to the block? In pushing a heavy box across the floor, is the force you need to apply to start the box moving greater than, less than, or the same as the force needed to keep the box moving? On what are you basing your choice? How do you think the force of friction is related to the weight of the box? Explain. 1.) Open the LoggerPro experiment file PCS120 Static and Kinetic Friction.cmbl from the physics website (following the instructions on the website on how to do so). There are 3 pages - begin on Page 1 2.) Calibrate the force sensor by doing the following: Page 2 of 6

3 Ryerson University - PCS 120 (a) Ensure that the force sensor is connected to Channel 1 of the LabPro interface. Set the range switch on the force sensor to ± 10 N. Note: When changing ranges, LoggerPro will prompt an indication that the sensor settings have changed; Click Use Sensor Settings to continue. (b) In LoggerPro choose Experiment Calibrate CH1: Dual Range Force Sensor (c) Click Calibrate Now. (d) Holding the force sensor as vertical as possible, with no mass hanging from the sensor, enter 0 in the Reading 1 field. Click Keep. (e) Hang the 500 g mass (using a string if necessary) from the sensor (this applies 4.9 N of force). Enter 4.9 in the Reading 2 field. After the reading shown stabilizes, click Keep. (f) Click Done. 3.) Holding the force sensor as vertical as possible, hang the wooden block using the hooks and click to collect a set of data. 4.) Determine the mass of the wooden block (including uncertainty) using the average measured weight you just measured. Do this by selecting Analyze to Statistics. Record this data in the adjacent table 5.) Place the block on the horizontal track and attach a string to the hooks on the force sensor and block. 6.) Without any forces applied to the sensor, zero it by selecting Experiment Zero. 7.) Click to start collecting data. Slowly pull the block using the force sensor until it starts moving. When it does, try to maintain a constant force for 2-3 seconds. You may want to repeat this step until you are satisfied with the data you ve obtained. 8.) From the graph, identify the (maximum) peak force, and average constant force applied while moving the block. Record these values in the corresponding table. 9.) Repeat this process with additional masses of 500 g up to 1500 g. Method II 1.) Navigate to Page 2 on the LoggerPro interface. 2.) Ensure that the motion sensor switch is set to cart mode, and connected to the LabPro device. 3.) Place the motion sensor at one end of the track. Page 3 of 6

4 Ryerson University - PCS ) Place the block on top the track with the hook facing away from the motion sensor. 5.) Click to start collecting data. Once you hear the clicking of the motion detector, and give the block a push toward the detector. Note: try to avoid hitting the motion sensor as it could damage the detector as well as provide poor data. 6.) From the velocity vs. time graph, verify that the data you ve collected is acceptable. If it is, click and drag selecting a linear portion of the graph that represents deceleration. 7.) Select Analyze to Linear Fit and record the slope in the corresponding table 8.) Repeat this process with additional masses of 500 g up to 1500 g. Method III 1.) Navigate to Page 3 on the LoggerPro interface. 2.) Attach the aluminum track to the retort stand using the clamp and set the track at an angle where the block does not slide down on its own. 3.) Place the wooden block with 1 kg of additional mass near the bottom of the track. 4.) Click to start collecting data. Slowly pull the block using the force sensor up the ramp similar to Method I. 5.) along with the (maximum) peak force, and average constant force applied while moving the block. 6.) Determine the angle of incline and record it in the corresponding column on the table. 7.) Repeat this process for 2 more angles making sure that the block does not slide on its own. Analysis Method I 1.) On one of the force vs. time plots created in Method I, describe what is happening physically during each different section of plot. 2.) Using the peak forces you obtained, calculate the static friction coefficient µ s by determining the slope of the peak force vs. mass. Do so by selecting the bottom graph and selecting Analyze to Linear Fit 3.) Similarly obtain the kinetic friction coefficient µ k using the same method. 4.) What can one conclude about static friction compared to kinetic friction in general? Page 4 of 6

5 Method II Ryerson University - PCS ) Determine the kinetic friction coefficient µ k from this method. How does it compare to the value of µ k determined in Method I 2.) Does the coefficient of kinetic friction depend on the speed at which you pushed the block? Explain. 3.) Here we ve used two methods to determine the kinetic friction coefficient µ k. Comment on which method you think is better and give reasons why. Compare the two values by computing a percent error using the process you believe to be better as the expected value. Method III 1.) On one of the force vs. time plots created in Method III, descibe what is happening physically during each different section of plot. Draw a free-body diagram corresponding to each section. 2.) Here we ve used two methods to determine the static friction coefficient µ s by adjusting either mass (Method I) or angle (Method III). Comment on which method you think is better and give reasons why. Compare the two values by computing a percent error using the process you believe to be better as the expected value. Wrap Up The following questions are designed to make sure that you understand the physics implications of the experiment and also to extend your knowledge of the physical concepts covered. Each member of your group should be able to answer any/all of these questions. Your TA will check that this is the case; please check out with your TA before exiting lab. 1.) Using your results, determine the minimum angle at which the block would not be able to stay on the track. Verify this by slowly raising the track with the block on it until it begins to slide. 2.) Some of you might have noticed that the track would sometimes slide on the table before the block would when pulling it and needed to stop the track from moving. Based on this observation, what can be said about the coefficient of static friction between the block and the track compared to the track and the table? 3.) How is the force of friction or the coefficient of friction affected by the surface area of the block? Briefly outline an experiment that can test your hypothesis. 4.) Should the linear fits created for all the plots pass through the origin? If so, comment and provide reasonable explanations as to why your results do not reflect that. Page 5 of 6

6 Last Few Steps Ryerson University - PCS 120 At the end of the lab, you will submit the work you ve done. This should include a simple record of the data/analysis of the experiment, sample calculations for equations used during the lab, uncertainty calculations, and some discussion such as Wrap Up questions. 1.) Save your LoggerPro file with an easily identifiable name such as PCS120 Static and Kinetic Friction YOURNAME.cmbl. 2.) You can view your LoggerPro file at a later time using the software. You can download a copy here. Note: You can only download these while on a Ryerson network. 3.) Submit the.cmbl file to your group submission folder on D2L. 4.) Submit any other related documents such as discussion and Wrap Up questions that you have completed during the lab. 5.) Once this is complete and you are certain that the data is saved, restart the computer when all experiments are completed. 6.) Lastly, tidy up your work station for your fellow students in other sections. In particular, return the track such that it is resting on the table. Page 6 of 6