# 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:

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

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/s^{2}) |
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

Students Choice- Electricity & Magnetism
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