PhysicsLAB Lab
Coefficient of Friction

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In this lab you are gong to investigate the coefficient of friction between a sliding wooden block and the surface of a piece of wooden shelving.
Each group will need the following equipment:
  • massless, frictionless pulley (with attached clamp)
  • wooden shelving
  • large C-clamp to fasten shelving to lab table
  • string
  • hanging (hooked) mass
  • stopwatch
  • meter stick
  • tape/marking method
Make sure that the length of string connecting the block to the hanging mass is sufficiently long to insure that the block still has a room to slide across the shelving after the hanging mass has reached the floor.  Carefully let the string run over the pulley and mark the position on the table where the mass no longer feels any tension from the string. Mark the slack position of the pulley/hanging mass.
Run several test trials to determine the best release position of the hanging mass so that  block stops sliding along the shelving before it reaches the pulley. Mark the release position.
Measure and record the following values:
  • the mass of the sliding block
  • the mass of the hanging mass
  • the mass of the second block
You are to run three trials and record the time between when the block passes the slack position and when it comes to a stop. For each trial record the block's stopping distance.
Repeat these time and distance measurements in three more trials in which you place a second block on top of the original block. Make sure that the two blocks are moving together across the shelving - there can be no slippage between the blocks.
For the data from the first three trials (using only one block) use kinematics and each trial's time and distance data as the block was sliding to a stop, to calculate the block's original velocity for each trial as it reached the slack position. Then calculate the block's acceleration.
Draw two freebody diagrams of the block: (1) while it was being pulled by the hanging mass, and (2) once the string goes slack and the block is sliding to a stop
Using Newton's 2nd Law, calculate the frictional force between the mass and the surface of the shelving. Using f = µkN, calculate the coefficient of kinetic friction between the block and the shelving. Average the three values of µk.
Now repeat your analysis for the data from the three trials when both blocks were sliding across the shelving. Once again average these three values for µk.
Calculate a percent difference between your two values for the coefficient of friction.
Why should the coefficient of friction have been the same regardless of whether only one block was sliding to a rest or two blocks were sliding to a rest?
Discuss what could have happened experimentally to have resulted in your percent difference or lack thereof.
If the coefficient of friction was supposed to have been the same, what differences  did using two blocks have on your data.

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