======Surface Friction======


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**Alternative titles:** Friction Lab With Spring Scale
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====Summary====                   
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====Procedure====  
  - Set two tape marks 40 cm apart on the table to define the pulling distance.
  - Check and zero the spring scale according to your teacher’s directions.
  - Attach the scale hook to one end of the wooden block so the largest face stays in contact with the test surface.
  - Practice pulling smoothly so the scale reading stays approximately constant along the 40 cm path.
  - Ignore the brief higher reading needed to start the motion; record only the steady value once the block slides at a nearly constant speed.
  - For the assigned surface (for example, lab table, sandpaper, linen, dowels, wax paper, paper towels), perform three trials and record the constant pulling force for each.
  - Repeat the three-trial measurement for each additional surface.
  - Compute the average constant force for every surface tested.
  - Measure the block’s mass with a balance and record it for later calculations (weight and coefficient of kinetic friction).
  - Clean up any test materials and ensure the work area is dry and safe.



====Links==== 
Surface Friction Lab - C. Stephen Murray: 
{{youtube>jRyRqByjnXE?}}\\


📄 Friction Lab - www.newpaltz.k12.ny.us: [[https://www.newpaltz.k12.ny.us/cms/lib/NY01000611/Centricity/Domain/812/Friction%20Lab%202019%20Science%208.pdf]]\\


====Variations====
  * Test the effect of added water on a porous surface such as paper towel and compare to the dry condition.
  * Flip the block to change the contact area while keeping the same surface to investigate area dependence.
  * Add small masses on top of the block to test how normal force affects the required pulling force.
  * Replace steady pulls with constant-speed pulls verified by a metronome or tick marks to improve consistency.
  * Use different grit numbers of sandpaper to see how surface roughness changes the measured force.



====Safety Precautions====
  * Keep fingers clear of the spring scale hook and block edges to avoid pinches.
  * Ensure the path is clear of obstacles and liquids; wipe spills immediately to prevent slips.
  * Pull gently and in line with the surface to avoid snapping the spring scale.
  * Do not exceed the spring scale’s maximum rated force; use an appropriate range.




====Questions to Consider====
  * What is the independent variable in this investigation? (Surface type.)
  * What is the dependent variable in this investigation? (Average constant pulling force for 40 cm, a proxy for kinetic friction.)
  * Which surface produced the smallest average constant force, and by how much compared with the largest? (Use your data to name the surfaces and report the numerical difference.)
  * Why should the initial peak force be excluded from the data set? (It reflects static friction; the lab focuses on kinetic friction at steady speed.)
  * If water is added to a paper towel surface, how could friction increase instead of decrease? (Water can increase adhesion and capillary bridging between fibers and wood, raising effective shear resistance.)
  * How would you compute the block’s weight from its measured mass? (\(W=mg\), showing units in newtons.)
  * What are likely sources of random error and systematic error in this lab? (Random: reading fluctuations, hand steadiness. Systematic: scale not zeroed, angle of pull adding lift or extra normal force, inconsistent contact area.)