1 Weight, 100 g, with hook Measuring tape 1 Weight, 200 g, with hook Sandpaper, carpet, or other rough surface

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1 Work and Friction That is why we labor and strive; because we have put our hope in the living God, who is the savior of all people, and especially of those of believe. 1 Timothy 4:10 Introduction In Physics, work is performed on an object when a force acting upon the object causes it to move. For example, a piano may be pushed across the floor to another location, or it may be pushed up an inclined plane (a ramp) into a truck. Mathematically, work is defined as: Work = Fd cos Θ where F = force, d = distance traveled and Θ is the angle between the force and direction of displacement. In the case of the piano being pushed horizontally across the floor, the ϴ is 0, the cos Θ is 1, and the above work equation simplifies to: Work = Fd. Work may be made considerably more difficult by friction. Friction is the resistance one surface encounters as it moves over another surface. If we push our piano over a smooth tile floor, there will be much less resistance (and a lot less work!) than if we tried to push the piano over thick carpet. When the piano contacts the carpet and they move relative to each other, the friction between the two surfaces converts kinetic energy into heat energy. In trying to push a heavy object across the floor, you may notice it takes considerable effort to start the process. But, once the object is in motion, the process is easier. Indeed, static friction the interactions between two surfaces when there is no motion is slightly greater than kinetic friction. Kinetic friction is the interactions between two surfaces that are moving. Friction is often characterized by a coefficient of friction which is a ratio of the frictional resistance force to the normal force, pressing the two surfaces together. You will obtain coefficient of frictions between metal masses and two surfaces. Learning Objectives: Determine the coefficient of frictions Compare movement along different surfaces Materials Required: From Physics Kit Student Supplied Spring Scale, 500g/5 Newton Table (choose a long object for an inclined plane) 1 Weight, 100 g, with hook Measuring tape 1 Weight, 200 g, with hook Sandpaper, carpet, or other rough surface Safety Falling masses can cause injury. Make sure that the setup is secure in a clear area Keep hair, jewelry and loose clothing away from moving parts. Experiment Determine µk In this lab, you will be pulling three different masses (100-g, 200-g, 300-g) along two types of surfaces and them comparing them. will be able to be hooked to the hooked mass or taped to the top of the hooked mass for extra weight Catholic Initiatives in Math and Science, LLC All Rights Reserved 1

2 Determine First Coefficient of Friction 1. Prepare: Find a flat working surface such as a table Mark a starting point (START) on the table with a piece of masking tape Measure a point exactly 0.5 meters away from the START Mark this point with another piece of masking tape (END) Place the 100-g mass on the Spring Scale Place leading edge of the mass at the START marked on the table 2. First Measurement: In Table A, describe the surface on which the mass will be pulled; describe the texture, what it is made of, whether it is smooth or rough, etc. Carefully pull the hooked mass at a steady speed from Start to Finish Note: It is important to keep the same speed in all trials Note: Spring Scales measure in Newtons and grams - You will record this measurement in grams. Record the measurement (in grams) in Table A You may want to repeat this several times to ensure you have a consistent reading 3. Second Measurement: Replace the 100-g mass with the 200-g mass Place the 200-g and the spring scale at the START Move the spring scale horizontally along the table at a constant velocity While pulling, read the amount of force (in grams) Record in Table A 4. Third Measurement: Add the 100-g mass to the 200-g mass; there is a bottom bar especially prepared for this use Repeat pulling and reading the force Record in Table A 5. Graph: Use the graph paper below Table A Graph Mass of the Weights (g) (y-axis) vs. Friction Force (g) vs (x-axis) 6. Find the Coefficient of Friction Find the slope of the line The slope of the line corresponds to the coefficient of friction The coefficient is specific for these two particular surfaces Record the slope of the line/coefficient of friction in Table A 2015 Catholic Initiatives in Math and Science, LLC All Rights Reserved 2

3 Determine Second Coefficient of Friction 1. Find: Find a different surface on which to perform the experiment. One option is to cover the table you used in Part 1 with a different material such as sandpaper, carpeting, or a blanket. Another option is to move the spring scale and masses to a carpeted area or to a different surface Be sure to mark a START and a STOP 0.5-meter away 2. Repeat: Repeat Steps 2-6 as in Part 1 Graph your data on the SAME graph as in Part 1 so that you can 3. Perform Data Analysis and Conclusions 2015 Catholic Initiatives in Math and Science, LLC All Rights Reserved 3

4 Lab Report for Data Organization TABLE A First Coefficient of Friction Describe Surface on Which Mass is Being Pulled Mass (g) Force (g) Slope of Line Coefficient of Friction (µ) Be sure to label the graph appropriately. Include a title, y-axis with units, x-axis with units, numbered lines and especially, labeled points and lines. Describe Surface on Which Mass is Being Pulled TABLE B Second Coefficient of Friction Mass (g) Force (g) Slope of Line Coefficient of Friction (µ) 2015 Catholic Initiatives in Math and Science, LLC All Rights Reserved 4

5 Data Analyses and Conclusions Show an example of your calculations. 1. What was the effect of adding masses on the Frictional Force as the spring scale was moving along the table? 2. Compare the textures of the three surfaces that were used in the experiment. Try to imagine the surfaces at the microscopic level. The surfaces are interacting at this level as they move past each other, even though one may appear to be smooth. 3. Which surface had the greater coefficient of friction? Explain. 4. Were your results surprising? Explain. How may your results be used to make your work life easier? 2015 Catholic Initiatives in Math and Science, LLC All Rights Reserved 5