Starter FRICTION LAB Consider a box sliding across a floor. What are two factors that determine how strong the frictional force is? Consider a box sliding across a floor. What are two factors that determine how strong the frictional force is?

Practice The two factors are: 1. How rough the surfaces are. A measure of the roughness is given by a dimensionless number called the coefficient of friction. The symbol is the Greek letter m, or . 2. How hard the objects are pressed together. This is given by the normal force, N. The two factors are: 1. How rough the surfaces are. A measure of the roughness is given by a dimensionless number called the coefficient of friction. The symbol is the Greek letter m, or . 2. How hard the objects are pressed together. This is given by the normal force, N. f =  N

Starter FRICTION LAB From the F vs. t graph for a box on a horizontal surface that weighs 20 Newtons, determine the coefficient of static friction. From the F vs. t graph for a box on a horizontal surface that weighs 20 Newtons, determine the coefficient of static friction.  = f/N = 4/20 =.200

Practice Static Friction – at break point: f =  s N Kinetic Friction – sliding: f =  k N static sliding break point

Horizontal Surface N = mg At break point, Pull =  s mg After it starts sliding at a constant speed, Pull =  k mg Therefore, to experimentally find the coefficients,  s = Pulling force at break point/mg  k = Pulling force while sliding/mg Therefore, to experimentally find the coefficients,  s = Pulling force at break point/mg  k = Pulling force while sliding/mg

Application Design a lab to determine the coefficients of friction between wood and cork, Teflon, and felt by recreating the friction vs. time graphs and finding the pulling forces at break point and while sliding. Available Equipment: Force sensor Friction blocks with wool, Teflon, and felt surfaces Masses Logger pro / Data studio

Lab Report Items DATA Felt TrialTotal MassNormal Force Friction Force at Break Point Friction Force while sliding ss kk Average Cork TrialTotal MassNormal Force Friction Force at Break Point Friction Force while sliding ss kk Average Plastic TrialTotal MassNormal Force Friction Force at Break Point Friction Force while sliding ss kk Average

LAB REPORT ITEMS Cover Sheet Data table Sample Calculations Sample Graph Question: As the mass of the cart increases, how does the coefficient of friction change? Summary Cover Sheet Data table Sample Calculations Sample Graph Question: As the mass of the cart increases, how does the coefficient of friction change? Summary

Connection Name some real life applications where friction is desirable, and not desirable. Name some real life applications where friction is desirable, and not desirable.

Exit If you change the mass in the friction cart, what effect would that have on the coefficient? If you change the mass in the friction cart, what effect would that have on the coefficient?