The Physics
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Coefficients of Friction for Human Skin

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Friction is a resistive force that opposes an object's motion or intended motion. The formula to compute the frictional force (f) is

f = µN

The coefficient ( µ) is unique for different materials. The normal force (N) is the countering force that the surface exerts on the object to oppose its weight.

We proceded as follows:

  1. Firmly attach the accelerometer to your wrist.
  2. Place an object on your wrist.
  3. Hit "Collect" in LoggerPro and gently raise your forearm up.
  4. Hit "Stop" when the object starts sliding.
  5. Determine and record the component of the acceleration due to gravity (a) parallel to your wrist at the instant that the object slips.
  6. Repeat steps 2–5 for four different materials.

equipment set up

After performing this experiment, we needed to compute the angle (θ) at which the object starts slipping. Since we know the acceleration components:

sin θ = a/g

After calculating the different critical angles, the coefficients µ can be calculated as follows:

N = mg cos θ
f = mg sin θ = µN = µmg cos θ
µ = tan θ

The data table below contains all our recordings and calculations.

Material Acceleration (m/s2) Critical Angle (°) Static Coefficient
Penny 6.27 39.77 0.83
Paper 6.41 40.85 0.87
Nickel 5.79 36.22 0.73
Plastic 6.86 44.43 0.98

Harvey Lei, Genna Ableman, Bridget Ritter, Africa Belgrave -- 2005

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