## Overview

- An equal and opposite force created when an object is slid across a surface
- Dependant on
- the normal force (usually the mass of the object)
- the co-efficient of friction (μ,"
*mu*") between the surface and the object - μ is an experimental value, when comparing the μ of different materials, it is best to test yourself
- The published μ for 2009 wheels was no found to be the actual value
- Two different co-efficients of friction depending if the object is already moving (kinetic) or not (static)
- You may have noticed when trying to slide a heavy object, it's really hard to get started, but once it starts moving, it's easy to continue moving

## Forces

*F*= applied force- The amount of force used to try and move the object across the surface
*F*_{g}= gravitational force of object (*F*_{g}=*m*·*g*)- The mass of the object multiplied by gravitational acceleration
*F*_{g}=*m*·*g**g*= 9.81*m**s*^{2} = 32.2*F*_{F}= friction force- The amount of force the object and surface are resisting due to friction
*F*_{N}= normal force- the amount of force the surface is providing to support the object

*ft*

*s*

^{2}

## Example

- General friction force equation
*F*_{F}= μ ·*F*_{N}- In this example,
*F*_{g}=*F*_{N} *F*_{F}= μ ·*m*·*g*- Static co-efficient of friction between wet concrete and rubber
- μ
_{static}≅ 0.3 - Assume rubber object weights 50 lbs
*F*_{F}=*μ*_{static}·*m*·*g**F*_{F}= 0.3 · 50*lbs*= 15*lbs**F*_{F}= 0.3 · 22.7*kg*· 9.81*m**s*^{2}= 66.8*N*- In order to move this object, you must apply a force greater than 15
*lbs*or 66.8*N*

## Testing Friction

- Testing static co-efficient of friction
- Place object on surface of interest material
- ie. place wheels on FRC carpet (or terrian for specific game)
- Increase the angle of the surface until the object begins to slide
- Ensure object cannot roll on surface
- Record the angle (θ, "theta") at which the object begins to slide
- μ
_{static}= tanθ - When the object begins to slide
*F*_{N}=*F*_{g}· sinθ- Recall the general friction equation
*F*_{F}= μ ·*F*_{N}- In this example,
*F*_{N}=*F*_{g}· cosθ - μ
_{static}=*F*_{g}*F*_{g}· sinθcosθ - μ
_{static}= tanθ

## Review of important equations

- Relationship between linear and angular velocity
*v*= ω · π ·*D*- Relationship between torque and force
- τ =
*F*·*d* - Relating angular velocity, torque, and tooth count
- ω
_{1}ω_{2}= N_{2}N_{1} - τ
_{1}τ_{2}= N_{1}N_{2} - τ
_{1}τ_{2}= ω_{2}ω_{1} - Friction force equation
- F
_{F}= μ ·*F*_{N} - Measuring static co-efficient of friction
- μ
_{static}= tanθ