# Coefficients of Friction for Glass

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The static frictional force is the force that opposes the intended motion of two objects in contact. This force is equal to the coefficient of friction (μ) times the normal force (N).

f = μN

The value of μ depends on whether or not the object is in motion. μ static is usually higher than μ kinetic.

1. First, we set up our station by plugging the LabPro into the laptop, and then opening LoggerPro.
2. Next, we taped an accelerometer to a piece of glass with the arrow perpendicular to the horizontal sides of the glass.
3. We placed the selected material (paper, copper, rubber, metal) on the glass below the sensor.
4. Then we kept the glass flat on the desk increasing the angle until the object began slipping.
5. Then, the material slid down the glass and one person form the group hit "stop" on the laptop and recorded the maximum component of the acceleration due to gravity parallel to the glass (a).
6. We repeated the experiment three more times with the other materials.

The angle inclination is found using…

θ = sin−1 (a/g)

Fnet = ma = 0
W//f = 0
mg sin θ–μmg cos θ = 0
mg sin θ = μmg cos θ
sin θ = μ cos θ
tan θ = μ

Object (Material) key chain (steel) penny (copper) key chain (rubber) card (paper)
a (m/s2) 1.83 1.44 6.45 2.14
θ (°) 10.75 8.44 41.11 12.60
μstatic 0.19 0.15 0.87 0.22

Manuel Caban, Whitney Daniel, Alex Grisales -- 2005

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