The Physics
An encyclopedia of scientific essays

Coefficients of Friction for Granite

An educational, fair use website

search icon

The coefficient of friction is defined as the ratio of frictional force (ƒ) versus normal force (N). It is represented by the equation

μ=ƒ /N

There are two kinds of coefficients of friction: static and kinetic. When the object is in motion, the frictional force is kinetic. The coefficient of kinetic friction is generally lower than the coefficient of static friction on the same surface. The coefficients of friction are independent of the surface area in contact.


  1. First you tape the low-G accelerometer on the end of the surface (in this case, a slab of granite).
  2. Next, place the object whose coefficient of friction is to be measured on the surface.
  3. Slowly lift one end of the surface until the material starts sliding down the incline, away from the accelerometer.
  4. The accelerometer will measure the component of the acceleration due to gravity parallel to the surface. LoggerPro will record the acceleration on a graph. Take the acceleration value at the point where the object begins sliding down the incline. On the graph it should be where the value jumps.
  5. Repeat steps 2-4 with different materials.


The coefficient of static friction for each material is found by this formula, where ƒ is the friction force and N is the normal force:


In the case of an incline however, to find μ, first you have to find the angle of the incline. The acceleration due to gravity (g) vector is directed downwards while the measured acceleration is directed parallel to the incline.

where a is the acceleration you found from the graphs.

On an incline, weight is directed downwards. The weight is made up of two components: the parallel weight (W//) and the perpendicular weight (W). The angle of the incline is the same as the angle between the weight directed downwards (W = mg) and the perpendicular weight. The forces acting on the object are the W// and the force of static friction (when it is not moving; a = 0). Therefore:

Net Force = ma = 0
W//–ƒ = 0
mg sin θ–μmg cos θ = 0
g sin θ = μg cos θ
μ = tan θ

Material a (m/s2) θ (°) μs
copper penny 3.47 20.74 0.38
paper 4.59 27.93 0.53
granite 3.87 23.26 0.43
aluminum 3.75 22.50 0.41


Matt Grabczynski, Michelle Fung, William "The Doctor" Feng, Brittany Mejia -- 2005

Students Choice
  1. Electricity & Magnetism
    1. The (dysfunctional) crystal radio
    2. The Kelvin water-drop electrostatic generator
    3. The two-way telegraph
    4. Magnetic field of a MetroCard
  2. Acceleration
    1. Acceleration of an elevator, hydraulic
    2. Acceleration perturbations of daily living
  3. Index of refraction of various household liquids
    1. glycerin, floor cleaner, degreaser, shower gel, baby wash
    2. vinegar, honey, floor cleaner, throat medicine, foot wash
  4. Coefficients of Friction
    1. Coefficients of friction for aluminum
    2. Coefficients of friction for glass
    3. Coefficients of friction for granite
    4. Coefficients of friction for paper
    5. Coefficients of friction for rubber
    6. Coefficients of friction for human skin
    7. Coefficients of friction for steel
    8. Coefficients of friction for wood