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• Friction is a double edge sword. It is the same force that allows a wheeled mechanism to start, and stop. Good designers maximise or minimize frictional forces where needed. This requires a thorough understanding and experiential data base of frictional information. Information that comes through reading, watching and doing.
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1. 1. Friction Hard to Live With It, Can’t Live Without It Mu Coefficient of Friction F n What’s Stopping You?
2. 2. Consider 2 Types of Friction Force of Kinetic Friction This value represents the relative force necessary to keep an object moving at a constant rate Force of Static Friction This value represents the relative force necessary to make an object move
3. 3. Frictional Force Resisting Motion Force Causing the Object to Move Kinetic Region Static Region Max
4. 4. Frictional Forces Occur When Materials are in Contact W f s F N Surfaces in Contact M 1 Spring Scale F = Force Causing Motion (Pull on Scale) F s = Force of Static Friction (Resists Motion) N = Force Normal Holds Surfaces in Contact W = Weight of Object ( Mass x Gravity)
5. 5. Friction is a Force That Resists Motion W f s F N Surfaces in Contact The Pink Block M 1 Will not Move Until the Force F (Pull on the scale ) Exceeds the Force of Static Friction f s . M 1 Spring Scale
6. 6. The Relative Force of Static Friction Between 2 Objects is Expressed as the Quotient of the Force (F) Required to Move the Object W f s F N Surfaces in Contact M 1 Spring Scale Divided by the Weight W of the Object This is Called the Coefficient of Friction
7. 7. W f s F N Surfaces in Contact M 1 Spring Scale = Force Required to Cause Motion = Weight of Object = Coefficient of Friction Coefficient of Static Friction
8. 8. Using the Gears-IDS Battery to Calculate The Static Coefficient of Friction
9. 9. Record the Maximum Force (F) (Before the Battery Begins to Move) Maximum Force F = 110 g
10. 10. Record the Weight ( W ) of the Battery 580 g
11. 11. The Coefficient of Static Friction Between the Wood Desktop and the Plastic Battery is Described Algebraically: = 580 g = 110 g = .190
12. 12. The Coefficient of Kinetic Friction Can be Found Using the Same Technique Record the Force Required to Move the Battery at a Constant Rate
13. 13. Coefficients of Friction Between Various Surfaces Have Significant Impact on the Design and Construction of Mechanisms. The Design of Competitive Mechanisms with Wheels Requires Extensive Testing in Order to Optimize the Torque, Speed and Traction (Friction) of the Drive Systems.