Online Friction Lab

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California State University, Fullerton *

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225L

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Mechanical Engineering

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Dec 6, 2023

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docx

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fd173269f8ad715f218f765fa56dbd4e96afe2eb.docx 9/21/2020 5:07 AM 1 of 8 Online Friction Lab Purpose To use a pHet simulation Forces and Motion Basics https://phet.colorado.edu/en/simulation/forces- and-motion-basics to investigate the laws of physics governing friction. Objectives 1. To understand the effect of the forces of friction on the motion of objects. 2. To explore experimentally the static and kinetic coefficients of friction. Technical Requirements This lab uses a free simulation from pHet from the University of Colorado. It is available at the following link: https://phet.colorado.edu/en/simulation/forces-and-motion-basics This is an HTML5 simulation. The technical requirements are as follows: HTML5 sims can run on iPads and Chromebooks, as well as PC, Mac, and Linux systems. iPad: iOS 12+ Safari iPad compatible sims Android: Not officially supported. If you are using the HTML5 sims on Android, we recommend using the latest version of Google Chrome. Chromebook: Latest version of Google Chrome The HTML5 and Flash PhET sims are supported on all Chromebooks. Chromebook compatible sims Windows Systems: Microsoft Edge, latest version of Firefox, latest version of Google Chrome. Macintosh Systems: macOS 10.13+, Safari 13+, latest version of Chrome. Linux Systems: Not officially supported. Please contact phethelp@colorado.edu with troubleshooting issues. More information can be found at https://phet.colorado.edu/en/help-center/running-sims#q66-header Have a Mac? Mac help can be found at: https://phet.colorado.edu/en/help-center/running-sims/mac For best results it is strongly recommended that you run the sim on a computer, not a phone. If you use a mobile device you assume all risk.

fd173269f8ad715f218f765fa56dbd4e96afe2eb.docx 9/21/2020 5:07 AM 2 of 8 Instructions 1. To access the sim click the following link: https://phet.colorado.edu/en/simulation/forces-and-motion- basics You should you will see the following screen If you wish to run the lab in a browser (online access) click the start button in the middle of the picture. You can also download the html5 file to your computer and run it off-line by clicking the Download button. If you have a fast computer or a slow internet connection or if you have trouble running it in a browser please try this option! The file will be a html5 file probably located in your downloads folder. Clicking on the file will start the sim. 2. When the sim starts you will see the following screen: Part 1: Static Friction In the following assume all numbers are accurate to 3 significant digits. 1. Click on “Acceleration”. Your screen will look like this (I have marked the yellow Forces box, the Stop button and the orange Reset button):

fd173269f8ad715f218f765fa56dbd4e96afe2eb.docx 9/21/2020 5:07 AM 3 of 8 2. Select all of the checkboxes in the yellow Forces box: Forces, Sum of Forces, Values, Masses Speed and Acceleration. Click the Stop button under the yellow Forces box to turn it into a Start button. This will freeze the sim so that you do not overshoot the maximum static friction force. Keep the slider that regulates the friction at the center of the scale (do not touch it). 3. Replace the crate with the 40 kg kid by dragging the crate to the lower left box next to the other crate and dragging the kid to replace the crate. 4. The double headed arrows under Applied Force add force in 50 N increments in that direction. The single headed arrows add 1 N per click. The opposite arrows subtract force in similar increments. 5. Add 100 N of force to one side of the kid by clicking the right side double headed arrow under Applied Force twice. At this point your screen will look like this: 6. The net force and the acceleration are still zero but the kid is just ready to move. This is the maximum static friction case for the 40 kg kid. 7. Test this statement by adding 1 N of force by clicking the single headed right arrow. The net force changes to 1 N to the right. The screen looks like this:

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fd173269f8ad715f218f765fa56dbd4e96afe2eb.docx 9/21/2020 5:07 AM 4 of 8 Note that the Sum of forces is now 1 N to the right so the box will move if you click the Start button and the maximum static friction force was 100 N (NOT 101N). 8. Record 100 N for the maximum static friction force (F s,max = 100 N) in the Static Friction Data Table row for the 40 kg mass (m). Calculate the Weight (F w ) for the 40 kg kid (g = 9.80m/s 2 ) and the Normal force (F N ). Record these in the 40 kg row of the data table. What force is the Normal Force equal to?____________ 9. Determine the Static Coefficient of Friction (  s ) where μ s = F s.max /F N . Record this in the Static Friction Data Table in the 40 kg row. 10.Click the Start button. The friction force drops to the much lower kinetic friction force as the kid starts to move. The kid immediately accelerates (acceleration is no longer zero). Your screen will look like this: 11.What is the value of the acceleration?________________m/s 2 12.Calculate the value of Sum of Forces/ mass____________. Is the acceleration equal to ∑F/m? Yes, no, maybe.

fd173269f8ad715f218f765fa56dbd4e96afe2eb.docx 9/21/2020 5:07 AM 5 of 8 13.Now we will test other masses. Reset the sim by clicking the orange reset button under the yellow Forces box. Select all of the boxes in the Yellow Forces box. Replace the 40 kg kid with a 50 kg crate. Do not change the friction slider. Click the stop button to change it to a start button, freezing the sim. 14.Add 100 N of force to one side of the crate by clicking the right side double arrowed boxes under Applied Force twice. Then add force in 1 N increments by clicking the right side single arrow box repeatedly until you have exactly 125 N applied force. Note that the applied force exactly equals the friction force and the net force and the acceleration are still zero. At this point your screen will look like this: 15.The box is just ready to move. Test this by clicking the single arrowed applied force one more time. The Applied Force is 126 N and the Sum of forces now reads 1 N instead of zero . Your screen looks like this: 16.Click the start button. The box will now start to move and the acceleration and sum of forces are not zero. 17.What is the value of the acceleration?________________m/s 2 ?

fd173269f8ad715f218f765fa56dbd4e96afe2eb.docx 9/21/2020 5:07 AM 6 of 8 18.Calculate the value of Sum of Forces/ mass____________. Is the acceleration equal to ∑F/m? Yes, no, maybe. 19.So the maximum static friction force is 125 N = F s,max (NOT 126 N). 20.Record 125 N (F s,max ) in the Static Friction Data Table in the row for a mass of 50 kg. Fill in the rest of that row by calculating the weight of the box (F w ), the Normal Force (F N ) and the coefficient of static friction (μ s ). 21.Reset the sim by clicking the orange reset button below the yellow Forces box, 22.Select all of the boxes in the yellow Forces box and click the stop button under the yellow Forces box to turn it into a Start button, freezing the sim. 23.Replace the crate with the 80 kg adult and repeat the steps to determine the maximum static friction force. Remember you want the maximum static friction force (the force where the item being pushed does NOT move but is just about to move). Fill in the data table. 24.Repeat for the other masses (100 kg, 120 kg, 140 kg) in the Static Friction Data Table. You can add masses by dragging them and piling them on top of each other. 25.Reset the sim by clicking the orange reset button. Check all of the boxes in the yellow Forces box and click the stop button under the yellow Forces box to turn it into a Start button. Add the 200 kg refrigerator to the 50 kg crate. Apply forces until you get the maximum of 500 N. 26.Does the combined mass move? Yes/ no 27.Why or why not? 28.How could you get the combined mass to move (there is only one way)? Data: Static Friction Assume all numbers are accurate to 3 significant digits and g = 9.80 m/s 2 . Mass m (kg) Weight F w (N) Max Static Friction force F s, max (N) Normal force F N (N) Static coefficient of friction μ s = F s.max /F N 40 100 50 125 80 100 120 140 Part 2: Kinetic Friction 1. Reset the sim by clicking the orange reset button. Check all of the boxes in the yellow Forces box. Replace the crate with the 40 kg kid. Do not change the friction slider. This time you will want to leave the stop button as it is (do not freeze the sim). Add Applied force by clicking the right pointing arrows under the Applied Force until the kid starts to move. 2. Allow the kid to move. Note that the acceleration is not zero and the net force is not zero. 3. Now back off the applied force by clicking the left arrows until the net force and the acceleration are both zero. The speed must be constant and nonzero. The friction force equals the applied force (Newton’s Laws). At this point you have determined the kinetic friction force (F k = 75 N) which you should record in the Kinetic Friction Data Table for

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fd173269f8ad715f218f765fa56dbd4e96afe2eb.docx 9/21/2020 5:07 AM 7 of 8 the 40 kg mass . Your screen will look like this: 4. Repeat for the other masses and fill in the Kinetic Friction Data Table. Kinetic Friction Data Table Assume all numbers are accurate to 3 significant digits and g = 9.80 m/s 2 . Mass m (kg) Weight F w (N) Kinetic Friction force F k (N) Normal force F N (N) Kinetic coefficient of friction μ k = F k /F N 40 75 50 80 100 120 140 Does Speed Affect the Kinetic Friction Force? 1. Reset the sim by clicking the orange reset button. Check all of the boxes in the yellow Forces box. Replace the crate with the 40 kg kid. Do not change the friction slider. This time you will want to leave the stop button as it is (do not freeze the sim). Add Applied force by clicking the right pointing arrows under the Applied Force until the kid starts to move. 2. Allow the kid to accelerate and build up speed while you keep an eye on the friction force. What happens to the kid’s friction force as the kid’s speed increases? Increases, stays constant, decreases. 3. Reset the sim by clicking the orange reset button. Check all of the boxes in the yellow Forces box. Replace the 40 kg kid with the 50 kg crate. Do not change the friction slider. Leave the stop button as it is (do not freeze the sim). Add Applied force and make the crate accelerate. 4. Watch the friction force as the crate builds up speed. What happens to the crate’s friction force as the crate’s speed increases? Increases, stays constant, decreases. 5. Repeat this for several other masses (your choice). Does the kinetic friction force depend on the speed (assuming it is not zero)? Yes, maybe, no.

fd173269f8ad715f218f765fa56dbd4e96afe2eb.docx 9/21/2020 5:07 AM 8 of 8 Finishing the Lab After you have completed the experiment and filled out this lab handout you should take the Lab Assessment quiz. Do not turn in the handout! It is for your use during the quiz. Go to Blackboard, Labs, Online Friction Lab and click on the Online Friction Lab Assessment Quiz. This quiz is open book open notes. You can use this handout to assist you on the quiz. Do not forget to click Submit when finished with the quiz so that you get a grade for the lab!