Rice PHYS 111 - Friction (4 pages)

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Friction



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Friction

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Pages:
4
School:
Rice University
Course:
Phys 111 - Mechanics (With Lab)
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Friction Science at bottom is really anti intellectual It always distrusts pure reason and demands the production of the objective fact H L Mencken OBJECTIVES To characterize the actual frictional forces in some situations THEORY Concerned with their rising legal costs the management of Everslip Soles Inc has decided to hire a consulting physicist you to help them improve the traction of their product Being a conscientious consultant you first review the friction literature and then set out to measure the frictional forces between shoe soles and typical flooring materials Physics textbooks provide a simple model of friction The frictional force is assumed to act between any two surfaces in contact in such a way as to resist relative motion between the surfaces The force is directed along a tangent to the plane of contact and is proportional to the force pressing the surfaces together The constant of proportionality called the coefficient of friction is therefore defined by the relation Ff FN 1 where Ff is the frictional force and FN is the force normal to the surface More specifically the maximum force that the surfaces can exert before relative motion starts defines a coefficient of static friction and the force that resists relative motion once sliding is underway defines a coefficient of kinetic friction Fs s FN Fk k FN 2 This is a very simple description of friction The coefficients are claimed to be independent of the normal force and the contact area and for the kinetic case independent of sliding velocity The physical world may or may not be so simple EXPERIMENTAL PROCEDURE To estimate the coefficients of friction you need to measure the frictional force as the sole starts sliding from rest and then moves at approximately constant speed Both the frictional force and the speed will be measured and graphed with the computer data acquisition system 1 Physical arrangement In order to explore the various combinations customers might encounter you have obtained several types of floor material and some samples of Everslip soles The flooring can be attached to a force table to measure the friction The shoe soles are attached to plywood with a tow rope at one end and a target for the motion detector at the other end You can stack metal weights on the slider to vary the normal force Figure 1 is a general sketch of the arrangement To measure the force the piece of flooring to be tested is clamped to a plywood platform sitting on rollers One end of the platform is connected to a force sensor which measures the force required to keep the platform from accelerating horizontally If there are no other horizontal forces on the platform then by Newton s second law the restraining force must be exactly opposite to the force of the slider on the platform In turn the force exerted by the slider on the platform is equal in magnitude but opposite in direction to the force of the platform on the slider which is the frictional force Although somewhat indirect this means that the force probe measures the frictional force If you are unsure of this argument draw the free body diagrams 2 Data acquisition and analysis Start the LoggerPro data acquisition program by double clicking an icon labeled Friction cmbl or load the file after the program is running This will give you a set of reasonable starting parameters and graphs for this exercise This file may also help you get you back to useful settings if you make changes later in the experiment You are welcome to save your own data and set up parameters on the hard disk but please use the Save As option to put the file under a different name so that others have the original set up available To become familiar with the data acquisition position one of the shoe sliders on the platform about half a meter from the motion detector Click the Collect button in the top toolbar Reflector Sonic ranger Weight Flooring material Force probe Shoe sole Fig 1 Overall view of the friction measuring apparatus PHYS 111 Friction 2 Fs Force Fk Time Fig 2 Typical plot of force vs time for a slider The initial peak not always present is the maximum force of static friction followed by the steady force of kinetic friction on the moving slider After a pause of a few seconds you will hear a clicking from the motion detector indicating that it is taking data Use the cord to tow the slider along the platform watching the graphs of distance velocity and force Be particularly careful that the plotted force does not exceed 50N to avoid damaging the probe The force graph should look similar to Fig 2 and the velocity graph should be consistent with the motion you produced If there seem to be problems make sure that the ranger is not being confused by fixed obstacles or consult the instructor Click again on the Collect button to clear the data when you want to acquire a new set Note that if the force changes quickly subsequent readings may scatter due to the springiness of the force probe Try to move the slider as smoothly as possible avoiding all abrupt motions For a typical analysis you would use Examine to read the peak force as the slider starts to move After the initial peak force if any you can select a region where the velocity is approximately constant and use Statistics to get the average force while sliding Repeating the measurement as closely as you can will give you a measure of the uncertainty in the force Before taking serious data you must calibrate the force probe on the 50N range Disconnect the probe from the rolling platform and connect a long string to the force probe Place the string over the pulley at the opposite end of the bench and adjust the positions of the pulley and probe so the string is level Set the zero in the calibration procedure with no weight on the string Use a weight hanger to suspend a known mass from the string to get the other calibration point Be sure that the platform and rollers are aligned properly and move freely when you reconnect the force probe PHYS 111 Friction 3 3 Measurement Program When you are satisfied that you understand the methods and can make reasonable measurements of the friction you should determine coefficients of friction for several of the available pairings of sole and floor material Your efforts should be focused on resolving the following issues a Is there any systematic variation with normal force That is can we even define the coefficient of friction as a constant for a given sole floor system b Is the coefficient


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