Hard to Live With It, Can’t Live Without It Coefficient of Friction What’s Stopping You? Fn Mu Friction Hard to Live With It, Can’t Live Without It 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. Coefficient of Friction

Consider 2 Types of Friction Force of Static Friction This value represents the relative force necessary to make an object move Force of Kinetic Friction There are other types of friction including fluid (air and liquids) and deformation friction. These are the only This value represents the relative force necessary to keep an object moving at a constant rate

Frictional Force Resisting Motion Max Frictional Force Resisting Motion Static Region Kinetic Region The Force of Static Friction is greater than the Force of Kinetic friction. The implication is clear: It is harder to start an object moving, than it is to keep it moving. Put another way, it takes more force to start an object moving than to keep that same object moving given that all other variables (materials in contact) remain the same. Force Causing the Object to Move

Frictional Forces Occur When Materials are in Contact fs M1 Surfaces in Contact Spring Scale W Contact between substances produce frictional forces that work in the opposite direction of the forces causing an object to move. In this example f s (Force of static friction) acts in the opposite direction of the pulling force F applied to the scale and string attached to the pink block. The force required to start the block moving is dependent on the smoothness of the surface, the weight or normal force acting on the block and the coefficient of friction between the surface of the block and the table it is resting on. F = Force Causing Motion (Pull on Scale) Fs = Force of Static Friction (Resists Motion) N = Force Normal Holds Surfaces in Contact W = Weight of Object ( Mass x Gravity)

Friction is a Force That Resists Motion fs M1 Surfaces in Contact Spring Scale W Which Way is the pink block being pulled? What direction is the force of Static Friction acting in? The Pink Block M1 Will not Move Until the Force F (Pull on the scale ) Exceeds the Force of Static Friction fs.

This is Called the Coefficient of Friction The Relative Force of Static Friction Between 2 Objects is Expressed as the Quotient of the Force (F) Required to Move the Object N F fs M1 Surfaces in Contact Spring Scale W The coefficient of static friction is found by dividing the weight of the block into the maximum force recorded on the scale before it begins moving. Divided by the Weight W of the Object This is Called the Coefficient of Friction

Coefficient of Static Friction M1 Surfaces in Contact Spring Scale W Experimenting with different materials and contact forces (weights) will yield different values for the coefficient of friction. The implication of this is clear, different materials grip or slide better or worse with respect to each other, and to increase traction on hard surfaces it is best to add weight directly over the driving wheels of a vehicle. = Coefficient of Friction = Force Required to Cause Motion = Weight of Object

Using the Gears-IDS Battery to Calculate The Static This is an experiment that is easy to do with the GEARS-IDS battery and a 1 kg spring scale. Remember the static coefficient of friction is found using the maximum force recorded before the battery begins to move. Coefficient of Friction

Record the Maximum Force (F) (Before the Battery Begins to Move) Maximum Force F = 110 g

Record the Weight (W ) of the Battery

The Coefficient of Static Friction Between the Wood Desktop and the Plastic Battery is Described Algebraically: = 110 g = 580 g This experiment yields a value for the coefficient of friction between the hard plastic of the battery and wooden desktop. Try this experiment with a variety of other materials. = .190

Record the Force Required to Move the Battery at a Constant Rate The Coefficient of Kinetic Friction Can be Found Using the Same Technique Record the Force Required to Move the Battery at a Constant Rate

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.

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