PHYS172_S21-Lab05-butomo

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PHYS 172 LAB 05 Friction Forces: Static and Kinetic SPRING 2021 LAST NAME (ALL CAPS): _UTOMO_________ FIRST NAME (ALL CAPS) __BRAMANTIO_________ PUID:__0032539685______ LAB Section DAY __W___ LAB Section TIME _3.30-5.20___ LAB GROUP#_6__ (Asynchronous Students: for DAY and TIME, please type DIST) Learning Goals After completing this activity, you should be able to: • Distinguish between the static friction force and the kinetic friction force acting on the same object; • Calculate the coefficients of static and kinetic friction; • Use while loops in VPython to simulate the motion on an inclined plain; • Connect the physical experiment and the computational model for the uniform motion with friction. Equipment Needed The iOLab kit (with the hook), a string, a computer with the USB port and the internet connection. Part 1: iOLab Experiment • Take the two-sticky felts (red rectangles) which come with your iOLab kit. Peel off the paper cover and stick them on the back of the iOLab cart (see the figure below): • Screw the hook into the iOLab cart for measuring force and tie a string to it. • Set up the iOLab cart on a flat surface such as the top of a table. The surface must be in contact with the iOLab cart felts. • Interface the iOLab cart with the computer. • Select the ‘Force’ sensor in the iOLab application (iOLab cart must be ON) • Click the ‘Record’ button and START TO PULL VERY VERY SLOWLY the string tied to the iOLab cart, so that the cart starts to move and continues moving with (approximately) a constant speed. • YOU SHOULD PULL EXTREMELY SLOWLY . Suggestion: Close your eyes, so that your human senses are most finely tuned to the force you are applying with your hand. Slowly and very gently keep increasing the force until it starts to move with constant velocity. Then try to keep that constant motion for a while.

Page 2 of 9 You should get a graph similar to the one shown on the figure below: If your graph is inverted with respect to the x-axis, use the ‘ Reverse y-axis ’ tool of the iOLab application. That tool appears when you click on the left bottom button of the plot and expand the toolbar (see the figure below). Remember that you can zoom in/out your graphs in the iOLab application. TAKE A SCREENSHOT OF THE GRAPH. PASTE IT BELOW.

Page 3 of 9 SHARE YOUR DATA WITH YOUR GROUP MEMBERS. Pleae note that the TAG NAME for your shared data is in this format: 'LAB GROUP' (as given in the "L-000_00" format) + ' LAB # ' (e.g. "LAB04" for this week) E.g. if you are in Lab section L-052 and Group 7, the TAG NAME for ALL data that you share for this lab will be L-052_07+LAB04 [For those students who do not have the iOLab kit because you are outside the U.S./Canada, your Lab Grader GTA will share data with you. But you do NOT need to share it with your Group.] SEE LAB Data Sharing Instructions provided on Brightspace . Go to Content (top menu) > LAB Data Sharing (side bar) Q01: Looking at your Force vs Time graph, which part of the graph (i.e. time interval) corresponds to static friction? Which part corresponds to kinetic (a.k.a. sliding) friction? Why? Explain. Hint: recall the definitions of the static and kinetic friction forces. The static friction is located at the interval of time from 0 to 2 seconds, because this happened before the object moved and static friction is always bigger than kinetic friction. The kinetic friction is on time from 2 to 10 seconds, because the object already moved. Q02: What is the maximum value of the static friction force in your graph? Explain how you obtained that number and comment on its precision. The maximum value of my static friction force is 1.183 N, at t = 2s is when the peak of the static friction froce and it can be seen from the graph as I toggle my mouse over the time. Q3: From your graph, estimate the approximately constant value of the kinetic friction force. To get the average value of a portion of a graph along the y-axis, you can use the tool shown below. With that tool activated, you can select the graph segment and it will show you the statistical data about that segment. Among that data, the first quantity denoted ‘ 𝛍 ’ is your average value (along the axis Y). My constant value of kinetic friction force is 1.121 N. Q04: In this experiment you were required to keep the speed of the cart constant after it starts moving. If that speed indeed was constant, then what is the reason that the force on your graph was not zero? Because if the net force is not zero, then the acceleration is not zero, so the speed should not stay constant. Is this a contradiction? Explain your reasoning. The net force is not zero because the Fnet is equal to the force I use to pull the cart minus the static friction force, that is why force is not zero.

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Page 4 of 9 Q05: Use the iOLab cart to measure its own weight. What is the mass (in kg) of the cart? F = m * g m = 1.947 N / 9.81 m/s m = 0.19847 kg TAKE A SCREENSHOT OF THE GRAPH YOU USED TO MEASURE THE WEIGHT OF THE iOLab CART. PASTE IT BELOW. Q06: Use the results in Q2, Q3, and Q5 to calculate the values of the coefficients of static and kinetic friction. Show your calculations in detail. Ffs = N * static coefficient Ffs(max) = Maximum static friction force N = normal force Static friction coefficient = Ffs(max) / (m * g) = 1.183 N / (0.19847 kg * 9.81 m/s) = 0.607604 Kinetic friction coefficient = Ffk / (m * g) Ffk = Average static friction force Kinetic friction coefficient = 1.121 N / (0.19847 kg * 9.81 m/s) = 0.57576

Page 5 of 9 Part 2: GlowScript Simulation Go to www.glowscript.org . In your Public folder, create a new program called “Friction” Copy and paste the code from th e following link in “Friction”: https://www.glowscript.org/#/user/phys172vpython/folder/LAB6/ This program simulates the motion of a cart sliding down the inclined plane (ramp). It takes into account the force of friction between the cart and the surface of the ramp when performing the update equations. Q07 : Run the program “Friction” and explain how it is possible that the speed stays constant while the cart is sliding down the ramp? The cart speed stays constant because the coefficient of the kinetic friction is just enough to stop the cart from accelarating and just enough to prevent the cart from deccelerating. Q08: What will happen if you change the magnitude of the initial velocity of the cart (“ CartInitVelMag ”) to zero? What does happen when you actually change it and run the program? The cart will not move from its original position. Q09: Change the static and kinetic coefficients of friction in the program with the values that you obtained experimentally in Part 1. They are called as “ CoeffStatMax ” and “ CoeffKinet ” in the “ Main Parameters ” section of the program. RUN THE PROGRAM, TAKE A SCREENSHOT OF THE GRAPHS AND PASTE IT HERE.

Page 6 of 9 Q10: Discuss with your team how to adjust the parameters of height and base of the ramp, such that you can get again a constant speed when you run the program using your experimental values of the friction coefficients. To get a constant speed we should adjust the ramp height to be the same as the kinetic friction coefficient. Q11: What are those new adjusted parameters (the height and base of the ramp)? Show clearly your calculations. Ramp Base * coefficient of kinetic friction = ramp height By keeping the base as 1 we can conclude Ramp height = coefficient of friction RUN THE PROGRAM AND TAKE A SCREENSHOT OF THE GRAPHS YOU GOT. PASTE IT BELOW. Q12: Copy below from your browser, the URL of your program edited. Remember to keep the folder PUBLIC. This will allow your TA to see your code. https://glowscript.org/#/user/utomo.bar/folder/Lab5/program/Friction

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Page 7 of 9 Part 3: Mini-Challenge Use the iOLab cart and what you learned above about how to find the coefficients of friction and test these hypotheses: • (How) Does the coefficient of friction depend upon the mass of the object? • (How) Does the coefficient of friction depend upon the surface area of contact ? Q13: Discuss with your team the design an investigation to find the answer the two questions above. Describe the experiment(s) you will perform. NOTE : You must clearly identify in your experiment the following: • independent variables (IVs) [i.e. measurable variables you will change] • dependent variables (DVs) [i.e. measurable variables that change as a consequence of changes to IVs] • controls [i.e. measurable physical quantities that you will ensure you do NOT change between trials.] Screw the hook into the iOLab cart and tie a string to it for measuring force. Set up the iOLab on a flat surface. The surface must be in contact with the iOLabcart felts. [Experiment 1] To create our experiment the independent variable, mass can be changed by taping different object with different masses to the iOLab cart. The dependent variable, forces due to friction, can be measured using the Force sensor on iOLab and by pulling the cart (using the string) with constant speed,and the control variable can be put in place by making sure to pull the cart with the same force over the same distance and time. When doing this the effects of mass changes can be can be calculated by finding the coefficient of friction where Friction force = coefficient * Fn Where Fn = m * g To create our experiment the independent variable, surface area can be changed by moving the felt pads to different sides of the iOLab cart. The dependent variable, forces due to friction, can be measured using the Force sensor on iOLab and by pulling the cart (using the string) with constant speed,and the control variable can be put in place by making sure to pull the cart with the same force over the same distance and time. When doing this the effects of mass changes can be can be calculated by finding the coefficient of friction where Friction force = coefficient * Fn Conduct the experiment(s) that you planned above and collect the data necessary to answer the two questions above. Q14: Describe the results of the experiment(s) you conducted including the data collected and analyzed results. [Experiment 1] The more mass the higher the friction Force for static and kinetic, but the coefficient of friction remains the same. In the three trials, as mass increased the frictional force increased, but when calculating for the coefficients there was no change. [Experiment 2] Changing the surface area plays no role in changing the frictional force and therefore does not change the coefficient fo friction in both of the trials.

Page 8 of 9 TAKE A SCREENSHOT OF THE EVIDENCE (GRAPHS, CALCULATIONS, DRAWINGS, ETC.) YOU WILL USE AS SUPPORT OF THE CLAIMS YOU WILL MAKE TO ANSWER THE QUESTIONS ABOVE. SHARE YOUR DATA WITH YOUR GROUP MEMBERS. Pleae note that the TAG NAME for your shared data is in this format: 'LAB GROUP' (as given in the "L-000_00" format) + ' LAB # ' (e.g. "LAB04" for this week) E.g. if you are in Lab section L-052 and Group 7, the TAG NAME for ALL data that you share for this lab will be L-052_07+LAB04 [For those students who do not have the iOLab kit because you are outside the U.S./Canada, your Lab Grader GTA will share data with you. But you do NOT need to share it with your Group.] SEE LAB Data Sharing Instructions provided on Brightspace . Go to Content (top menu) > LAB Data Sharing (side bar) Q15: What is your conclusions about each of the two questions above? Remember that any claim you make must be accompanied by a reasoning supported with evidence. Mass does not change the coefficient of friction, because as calculated they remained the same despite the change in fricitonal force. Surface area does not change the coefficient of friction because in the two trials the frictional force did not change. Q16: Discuss with your team the conclusions you got and evaluate the strength of the evidence you presented to support your arguments.

Page 9 of 9 My team came to the same conclusion as me, despite using different objects. Which strengthen the argument that mass does not change the friction coefficient. We also got the same conclusion for the surface area, which again strengthen the argument that surface area does not affect the friction coefficient. LAB Group Members’ Signatures /Typed Names (Make sure all agree to “sign off” on work) Bram Utomo Angelica Jovceski Jake Holliday Caden Cunningham Jaime Rueda BEFORE LEAVING, EACH PERSON IN GROUP MUST • Get signatures/typed names of every LAB Group member in the boxes on the left OR type in their name. • If you have printed it off, take photos of each page, and combine them into a single PDF. If you have been working in the Word file as is, then simply save it as PDF. Each person MUST upload THE SINGLE PDF File your LAB worksheet on Brightspace by 11:59 PM SATURDAY See Syllabus for LATE POLICY ASYNCHRONOUS STUDENTS: Please paste your entire Discussion Board here (or a link to Discussion video here)

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