Physics Lab 7 - Virtual Sliding Friction Nancy Gonzales PDF

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Nancy Gonzales 1101.001

Sliding Friction INTRODUCTION The purpose of this experiment is to find the coefficient of static friction and the coefficient of kinetic friction for different surfaces. In this lab, you will be using the Physics Aviary simulations. Within the simulation, you will pick the surfaces, one for the tabletop and the other for the box being pulled across the tabletop. The simulation will generate a plot of the virtual force sensor data vs. time. A plot of the frictional force versus the normal force yields both static and the kinetic coefficients. THEORY When a force (F) is applied to an object resting on a surface, it will not move until the force applied to it is greater than the maximum force due to static friction. The coefficient of static friction ( μs ) is defined to be the ratio between the maximum static frictional force (Fs) and the normal force (FN): F s F max μs = = F N mg where Fmax is the maximum applied horizontal force that does not cause slipping and FN equals mg since there is no vertical motion. To keep the object moving at a constant velocity, the applied force (F) must equal the kinetic frictional force. The coefficient of kinetic friction ( μk ) is defined to be the ratio between the kinetic frictional force (Fk) and the normal force (FN): μk =

F k Fk = F N mg

Here is the link to the Physics Aviary Friction simulation: 

http://www.thephysicsaviary.com/Physics/Programs/Labs/ForceFriction/index.html

The graph below is an example of the data generated by the virtual experiment.

Written by Chuck Hunt; modified by Marc Keltner & J. Alquiza

Nancy Gonzales 1101.001

The arrow on the graph is pointing to the static friction data where you can determine the value of the maximum static frictional force. The horizontal data is the kinetic frictional data where you can estimate the value of the avg. kinetic frictional force. PROCEDURE 1 1. Pick the surfaces for the run. 2. Adjust the mass to a low value, say 100 grams. 3. Start the simulation. Use the snipping tool to snip the resulting graph which you will add into your finished lab report.

Written by Chuck Hunt; modified by Marc Keltner & J. Alquiza

Nancy Gonzales 1101.001

4. From the graph, determine the value of the maximum static friction. Then estimate the average kinetic friction from the horizontal portion of the graph. Then in a Word document, generate a table, like the one below, listing the surface as the table header. Record the mass, the max. static frictional force, and the average kinetic frictional force. Calculate the normal force and record that. 5. Repeat steps 2 through 4 an additional four times with increasing mass (increase the mass by 300 g - approximately) so that you have five pairs of data for maximum static frictional force and normal force, and also for average kinetic frictional force and normal force. 6. Change the surfaces, and repeat steps 2 through 5. 7. You will have generated five data sets. One for each pair of surfaces.

Written by Chuck Hunt; modified by Marc Keltner & J. Alquiza

Nancy Gonzales 1101.001

Example Data Table – Surfaces: Rubber on Ice Mass max. static friction 100 g .10 N 282 g .35 N 465 g .54 N 742 g .84 N 1050 g 1.3 N Aluminum on Steel Mass max. static friction 100 g .61 N 458 g 1.9 N 689 g 4.01 N 1127 g 7.1 N 1409 g 8.3 N Glass on Glass Mass max. static friction 100 g .91 N 374 g 3.54 N 794 g 7.1 N 1231 g 10.05 N 1459 g 13.21 N Graphite on Graphite Mass max. static friction 100 g .17 N 356 g .58 N 709 g 1.01 N 1229 g 2.07 N 1634 g 2.90 N Rubber on Concrete Mass max. static friction 100 g .90 N 403 g 3.80 N 808 g 7.1 N 1074 g 10.2 N 1420 g 12.8 N

Written by Chuck Hunt; modified by Marc Keltner & J. Alquiza

avg. kinetic friction .075 .022 .035 .057 .085

Normal force .98 N 2.76 4.56 7.28 10.29

avg. kinetic friction .046 2.05 3.50 5.02 6.54

Normal force .98 4.48 6.74 11.01 13.73

avg. kinetic friction 0.40 1.54 3.09 4.93 5.89

Normal force .98 3.66 7.78 12.03 14.23

avg. kinetic friction 0.17 0.60 1.20 2.30 2.60

Normal force 0.98 3.49 6.95 12.04 16.01

avg. kinetic friction .80 2.90 5.60 7.2 9.8

Normal force .98 N 16.01 7.89 10.45 13.72

Nancy Gonzales 1101.001

Written by Chuck Hunt; modified by Marc Keltner & J. Alquiza

Nancy Gonzales 1101.001

ANALYSIS: 1. For each data set, open a graphing utility such as Excel, generate a scatter plot of Frictional Force vs. Normal Force (graph titles are y vs x). 2. Determine the slope of each graph and record in a table of Surfaces and their coefficients of static and kinetic friction. Answer the questions in the Conclusion below. Your lab report will include: 1. One simulation data graph for each surface, snip and save to a Word document, title graph with surface types. Done only need one per instructor 2. The five data tables, one for each type of surfaces. done 3. The five Excel graphs. Be sure to include a title on the graph, label the vertical and horizontal axes with the variable, e.g. Frictional Force. Include the units in parentheses. 4. The Conclusion page with complete sentences for each question. 5. A Screenshot of the Simulation experiment page showing the picture of the virtual experiment setup.

Written by Chuck Hunt; modified by Marc Keltner & J. Alquiza

Nancy Gonzales 1101.001 CONCLUSIONS

B

C

Force A

D

1. The graph above is representative of the force applied to an object as it is pulled across a horizontal surface. Draw a free body diagram for each of the positions labeled in the graph above. Describe the motion of the object for the positions labeled in the graph. Answer in complete sentences For part A on theTime graph the body is at rest. For part B on the graph the body is on the brinks of motion, but just hasn’t hit it quite yet. For part C on the graph the kinetic friction is less then static friction. This means that this point is in motion and the force is decreasing as the time goes. Compared to point B where the point was not in motion just yet. For part D this is in motion. Once the body of the object starts its motion we apply Kinetic friction which is less then static friction like point C.

2. What relationship exists between the static frictional force and the normal force on an object? Are they directly proportional, indirectly proportional. Explain using complete sentence. The kinetic friction is directly proportional to normal force on an object. So F=Mk N where Mk N is the coefficient of the kinetic friction.

Written by Chuck Hunt; modified by Marc Keltner & J. Alquiza

Nancy Gonzales 1101.001 3. What is the physical meaning of the slope for the frictional force vs. normal force graphs? Answer using complete sentences. The frictional force is when the object goes in two different directions. The force of the object often is opposite of the direction of the motion of the object. The physical meaning of slope of kinetic or the static frictional force versus the normal force is the coefficient of static friction. 4. Why should the vertical intercept for the frictional force vs. normal force graphs be zero? Answer using complete sentences. This is because the vertical intercept represents the static friction. 5. What pattern do you notice between the values of the coefficients of static and kinetic friction? Why is this important when driving a car? Answer using complete sentences. This is because objects like cars require more force to actually start the car or move the car then to actually keep the car in motion. Once the car is in motion at a same speed not much force is required as it was when it was accelerating to that said speed. I know when driving a standard car we have to have it at the lowest gear when taking off because it requires more force then when it gets to a certain speed we can maintain it until we need to downshift when slowing down. 6. Why do anti-lock brakes help to avoid automobile accidents? Answer using complete sentences. Anti-lock brakes in vehicles help avoid locking (anti-lock) so that the tires wont skid in place. When tires skid the friction of the tire hitting the asphalt is greater and keeps the car in motion instead of stopping the tires from turning hence why we need anti-lock brakes. This helps cars reduce fish-tailing and losing control.

Written by Chuck Hunt; modified by Marc Keltner & J. Alquiza...