Materials Included In Kit

Additional Materials Required

Safety Precautions

The materials in this lab are considered nonhazardous. Please follow all laboratory safety guidelines.

Disposal

All the materials in this lab should be saved and stored for future use.

Teacher Tips

• Enough materials are included for one group of students. This laboratory activity can reasonably be completed in one 50-minute class period. If time is limited, students should collect data for the four activities during class and do the analysis and post-lab questions outside of class.
• Additional Friction Blocks kits (AP4605) may be purchased for each individual lab group. A classroom set of six (AP6222) is also available.
• If the spring scale measures in grams instead of Newtons, students should multiply their measurement in grams by 0.0098 to obtain the force in Newtons.
• If the tabletop is too smooth to obtain good results, have students tape a sheet of notebook paper to the tabletop and use that surface as the “tabletop.”
• If manufactured masses are not available, other objects, such as books, can be used as substitutes. Make sure that students measure the total mass with a balance for each trial in Part C and record the weight in Newtons in Data Table C.

Sample Data

Data will vary depending on the smoothness of the tabletop used.

Data Table A

Data Table B Data Table C Data Table D

Answers to Questions

Part A. Frictional Forces Versus Surface Areas

1. Does it take more force to start an object sliding over a surface or to keep it sliding at a constant speed?

   The force required to keep the object sliding is less than the force required to initially get it to slide. Sliding friction is less than static friction.

2. How do the frictional forces between the two different experiments compare? What influence does the surface area have on the frictional force? Why?

   The static frictional force and the sliding frictional force were the same for both experiments. The surface area is not a factor affecting frictional force when the other variables are constant (on most surfaces). Only the Normal force and the coefficient of friction (static or sliding) determine the frictional force acting against motion.

Part B. Frictional Forces Versus Different Surfaces

3. Which surfaces in contact with each other produced the largest frictional force?

   The sandpaper and wood side of the block produced the greatest frictional force, static and sliding. The mirror and the table top produced the least frictional force, static and sliding.

4. What influences the frictional force between the sliding objects?

   Refer to Background information.

Part C. Frictional Forces Versus Normal Force

5. On a separate sheet of graph paper, draw a graph of static frictional force versus Normal force, using the information in Data Table C. Draw a “best fit” line through the data points.
6. Does the data produce a straight “best fit” line? If yes, what does the slope of the line represent?

   Yes, the slope of the line represents the coefficient of static friction between the block of wood and the tabletop.

7. On the same sheet of graph paper, draw a graph of the sliding frictional force versus Normal force, using the information in Data Table C. Draw a second “best fit” line through this data. A different color pen or pencil can be used to distinguish between the two sets of data.
8. Do these data produce a straight “best fit” line? If yes, what does the slope of this line represent?

   Yes, the slope of the line represents the coefficient of sliding friction between the block of wood and the tabletop.

9. Determine the coefficient of static friction and sliding friction between the wood block and the tabletop from the corresponding graphs. Refer to Equation 1.

   Coefficient of Static Friction equals the slope of the “best fit” line from the first set of data. The Coefficient of Sliding Friction equals the slope of the “best fit” line from the second set of data. Find the slope by taking the “rise over the run” of the line, or from the sample data:

   {12865_Answers_Equation_2}

Part D. Additional Frictional Force Experiments and Predictions

10. For the first experiment, what is the static frictional force between the mirror and the tabletop? What is the static frictional force between the sandpaper and the wood block? Are these two values the same? Why or why not?

    The static friction between the mirror and the tabletop equals 0.25 N (from sample data). The static friction between the sandpaper and wood block equals 0.25 N (from sample data). Yes, these two values should be the same because static friction can be any value up to a certain limit. When this limit is reached, static friction is overcome and the surfaces slide past each other and now experience sliding friction. So, the maximum attainable friction occurs just before the mirror begins to slide across the tabletop. Even though the sandpaper and wood block do not begin to move, they experience the same frictional force as the mirror and tabletop. Once the mirror begins to slide across the tabletop, the sliding frictional force between the mirror and tabletop will be the same as the static friction preventing the sandpaper and wood block from sliding.

11. What is the static frictional force between the mirror and the wood block? How does this value compare to the maximum value of static friction between the sandpaper and the tabletop?

    The static friction between the mirror and wood block equals 0.20 N (from sample data). This value is less than the maximum value of static friction between the sandpaper and the tabletop.

Teacher Handouts

12865_Teacher1.pdf

References

Pedrotti, Leno S.; Principles of Technology, Unit 4, 2nd Ed.; Center for Occupational Research and Development: Waco, Texas, 1992; pp 8–11, 26–30.

Introduction

Friction might slow you down, but without it you wouldn’t go anywhere. In this experiment, some of the properties of friction will be tested and discovered.

Concepts

• Friction
• Force

Background

Friction is created when any two surfaces are in contact with each other. The factors that influence friction include the surface finish (or smoothness), the cohesive and adhesive ability of molecules and the force holding the surfaces together.

Surface finish is a major contributor to the frictional force. All surfaces, no matter how smooth, have irregularities. Even the smoothest object will still have irregularities at the atomic level because atoms and molecules never bond together to form completely flat surfaces. These irregularities cause a grinding action to occur when two surfaces move against each other. As a result, heat is generated (the moving objects lose energy) and particles are worn away from the materials. Smaller irregularities, or smoother surfaces, produce less frictional force to act against the direction of motion than larger irregularities.

Another contributor to friction is the cohesive and adhesive abilities of the atoms and molecules in the materials. Cohesion is the attraction of like atoms or molecules. When two plates of glass come in contact with each other (with no air pockets), the two pieces nearly “fasten” together. Glass is considered to have a very smooth surface, but it is very difficult to pull or slide the two pieces apart. This is an example of cohesion. Adhesion is the attraction of unlike atoms or molecules. Water droplets clinging to the side of a glass beaker is an example of adhesion. These properties affect frictional force because the attraction between two surfaces at the atomic level requires more force to break in order to move the object and maintain its motion at the macroscopic level.

The perpendicular force (or Normal force) holding two surfaces together is a very important factor influencing the frictional force. The larger the force holding two surfaces together, from gravity or some other outside force, the larger the force of friction. In the case of an object sliding on a horizontal surface, the Normal force equals the weight of the object. A mathematical equation for frictional force is as follows:

F_f = Force of friction
μ = Coefficient of friction (Greek letter mu)
N = Normal force

The coefficient of friction is a unique property used to describe the relationship between the frictional properties of two surfaces in contact with one another and the frictional force. This property is based on the first two factors that contribute to friction—the smoothness of the two surfaces, and their adhesive or cohesive properties.

There are two different types of friction (and therefore coefficients of friction). Static friction is the frictional force that initially prevents two surfaces from sliding past each other. Sliding (or kinetic) friction is the frictional force between two surfaces in contact that are moving past each other. Static friction is larger than sliding friction because when two surfaces are in contact with each other and at rest, the tiny irregularities of the two surfaces tend to interlock with each other. Also, the adhesive and/or cohesive properties can take affect. When the surfaces slide past each other, there is less of a tendency for the surface’s tiny grooves to interlock, and the cohesive or adhesive properties are less effective. Instead, the two surfaces just ride along the outer edges of “bumps” and there is less force acting against motion. Static friction is the frictional force up to a certain limit. Once that limit is exceeded (by an applied force), the static friction will be overcome and the object will begin to move and sliding friction will take over.

Materials

Prelab Questions

In this lab activity, static and sliding friction of different objects on different surfaces will be studied. In order to obtain accurate measurements, practice the following procedures until confident and reproducible results are obtained.

The maximum static frictional force is the force that is measured on the spring scale just before the block begins to slide. This is best measured by slowly and evenly pulling the block horizontally with the spring scale, while closely observing the spring scale needle. Record the maximum force measured by the spring scale immediately before the block begins to move. Once the block begins to move, the force registered by the spring scale will decrease slightly. With a spring scale, practice measuring the maximum static friction between the tabletop and the wood block until confident results are obtained.

Sliding frictional force is measured when the block is moving with constant speed. Use the spring scale to pull the wood block horizontally. As the block slides, adjust the amount of force needed to keep the block moving until the spring scale reading is balanced. When the spring scale is balanced, the block will be moving at a constant speed because the pulling force and the sliding frictional force are balanced (no net force acting on the block means no acceleration). The measurement on the spring scale at this point will be equal to the sliding frictional force. Again, practice this technique until consistent results are obtained. The speed of the object will not influence the sliding frictional force, but it is generally easier to read the moving spring scale when it is traveling slowly. Pulling the block a meter or two along the tabletop may be required in order to adjust the pulling force appropriately to obtain a constant speed (and therefore balanced forces).

During the experiments, several trials or practice runs may need to be performed in order to obtain accurate measurements for each step. So for each measurement step, perform several trials and record only the most reliable measurement. Record the results (in Newtons) in the appropriate table on the Friction Blocks Worksheet.

Safety Precautions

The materials in this lab are considered nonhazardous. Please follow all laboratory safety guidelines.

Procedure

A. Frictional Forces Versus Surface Area

1. Place the wood block, without the mirror, flat on a tabletop.
2. Attach a spring scale to the eyebolt.
3. Measure the static friction between the block and the tabletop. Record the results in Data Table A.
4. Measure the sliding friction between the block and the tabletop. Record the results in Data Table A.
5. Now, place the same wood block on its edge (thin side) and again attach a spring scale to the eyebolt.
6. Measure the static friction between the block on its edge and the tabletop. Record the results in Data Table A.
7. Measure the sliding friction between the block on its edge and the tabletop. Record the results in Data Table A.
8. Answer the questions in Part A of the Analysis Section.

B. Frictional Forces Versus Different Surfaces

1. Place the wood block, with the attached mirror, wood side down on the tabletop and attach a spring scale to the eyebolt.
2. Measure the static and sliding frictional forces between the wood side of the block and the tabletop. Record the results in Data Table B.
3. Turn the block over onto its mirror side.
4. Measure the static and sliding frictional forces between the mirrored side of the block and the tabletop. Record the results in Data Table B.
5. Tape the sandpaper strip with its gritty side up onto the tabletop.
6. Place the block wood side down on the sandpaper.
7. Measure the static and sliding friction between the wood side of the block and the sandpaper. Record the results in Data Table B.
8. Repeat step 15 with the mirror side down. Record the results in Data Table B.
9. Answer the questions in Part B of the Analysis Section.

C. Frictional Forces Versus Normal Force

18. Obtain 5 manufactured masses at 100-g increments (or equivalent).
19. Place the wood block, without the mirror, on the tabletop and attach a spring scale to the eyebolt.
20. Measure the static and sliding friction between the wood block and the tabletop. Record the results in Data Table C.
21. Place one 100-g mass on the wood block. Be sure to measure the mass with a balance, if necessary. Enter the weight of the mass in Newtons in Data Table C. (Multiply the mass in grams by 0.0098 to obtain the weight in Newtons.)
22. Again, measure the static and sliding friction between the wood block, with the additional mass and the tabletop. Record the results in Data Table C.
23. Repeat step 22 using 200-g, 300-g, 400-g and 500-g masses (or equivalent). Record the total weight used for each trial, and the corresponding frictional force results in Data Table C.
24. Answer the questions in Part C of the Analysis Section.

D. Additional frictional force experiments and predictions

25. Place the mirror with the sandpaper attached to one side on the tabletop, mirror side down.
26. Place the wood block without the mirror on top of the mirror/sandpaper piece.
27. Attach a spring scale to the eyebolt on the wood block.
28. Predict what will happen. Will the wood block slide? Will the mirror slide? Or both? Record your prediction in Data Table D.
29. Measure the static frictional force before any sliding occurs. Note which object’s surfaces slide past each other, once the static friction has been overcome and there is motion. Record your observations and measurements in Data Table D.
30. Now turn the mirror/sandpaper piece mirror side up.
31. Place the wood block on top of the mirror and sandpaper piece.
32. Again, predict what will happen. Record prediction in Data Table D.
33. Measure the static frictional force before any sliding occurs. Note which object’s surfaces slide past each other once the static friction has been overcome and there is motion. Record your observations and measurements in Data Table D.
34. Answer the questions in Part D of the Analysis Section.

Student Worksheet PDF

12865_Student1.pdf