The Physics
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Coefficients of Friction for Rubber

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Static friction results from the normal force of two objects in contact, acting opposite the intended motion of one surface against the other. The maximum force of static friction can be calculated by multiplying the normal force by the coefficient of friction specific to the pair of materials in contact. Thus, the coefficient of static friction may be calculated by dividing the maximum force of static friction by the normal force.

µ = f/N

The experiment is performed as follows:

  1. Set up the apparatus as shown in the photograph.
  2. Slowly lift one side of the track without moving the opposite end. Record gravity's parallel component as measured by the accelerometer when the object being tested begins to slide.
  3. Repeat this process for every material being tested.

Because it is attached parallel to the plane of the track, the accelerometer measures the component of gravity (a) along that plane. Since the full acceleration due gravity (g) is known, the angle ( θ) of the ramp …

θ = sin-1 (a/g)

and the coefficent of static friction …

µ = tan θ

can be determined.

Material Acceleration (m/s2) Angle (°) µs
Copper 6.53 41.8 0.89
Aluminum 4.46 27.1 0.51
Rabbit Fur 6.28 40.0 0.83
Rubber 6.34 40.3 0.84

Geeta Malieckal, Jerard Kneifati-Hayek, Erik Hageman, Michael Calamera -- 2005

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