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5.1 Friction Intro and Statics SWBAT describe what factors affect the frictional force. SWBAT calculate the static friction between two surfaces. Read and outline p. 135-139 A phonebook is at rest on top of a table. If it’s mass is 2 kg, what is the normal force acting on the book? After answering this question in your notebook, answer the following: The pages of phone book A and B are interleaved (shuffled together). How much force does it take to pull them apart? Do you think you can do it??? Definition: Friction is the force exerted by a surface as an object moves across it or makes an effort to move across it. 1. Do all surfaces exert the same frictional force? 2. What other factor can increase frictional force? We measure a surfaces’ “roughness” using the coefficient of friction. μ (mu) is the symbol of the coefficient of friction. – μ does not have units. – μ is a scalar quantity. The larger the μ, the rougher the surface. (when μ=0, the surface is frictionless) Where can we find the coefficients of friction? Typical Metal Surface Polished Metal Surface Wood Surface Porcelain Surface 1. Do all surfaces exert the same frictional force? – No, we use μ to differentiate between surfaces. 2. What other factor can increase frictional force? Which will have a larger force of friction? (Which will be harder to move?) Fa= 5 N Fa= 5 N Which will have a larger force of friction? (Which will be harder to move?) Fa= 5 N Fa= 5 N Which will have a larger force of friction? (Which will be harder to move?) Fa= 5 N 1. Do all surfaces exert the same frictional force? – No, we use μ to differentiate between surfaces. 2. What factors can increase the force of friction? – Changing the normal force acting on the object. How are these factors related to the frictional force? Frictional Force = (coefficient of friction) (Normal Force) Where is the formula for friction? A force that resists the initiation of sliding motion between two surfaces that are in contact and at rest. Static Friction (Ffs) acts opposite the applied force (Fa) As Fa increases, Ffs increases until FfsMAX is reached and the block starts to move. FfsF fs FF a a Friction always acts parallel to the surface of contact A 5 kg wooden block is at rest on a wooden table. a) How much force is required to start the block moving? b) What would the frictional force be if the applied force is 10 N? Zip the monkey wants to move a stationary 20 kg crate of bananas across a horizontal floor. The material of the crate and the floor are unknown, but it takes 10 N to start the crate moving (the maximum static friction is 10 N). What is the coefficient of static friction between the crate and the floor? A 10 kg copper block is at rest on a steel table. How much force is required to start the block moving across the table?