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FORCE DUE TO FRICTION What do you think? • How would you describe friction? ▫ How is friction related to: heat, energy, force, velocity Review: Weight and Mass • Mass is the amount of matter in an object ▫ Kilograms, slugs • Weight is a measure of the gravitational force on an object ▫ Newtons, pounds ▫ Depends on the acceleration of gravity • Weight = mass acceleration of gravity ▫ W = mag where ag = 9.81 m/s2 on Earth ▫ Depends on location ag varies slightly with location on Earth ag is different on other planets Review: Normal Force • Force on an object perpendicular to the surface (Fn) • It may equal the weight (Fg), as it does here • It does not always equal the weight (Fg), as in the second example ▫ Fn = mg cos Static Friction • Force that prevents motion • Abbreviated Fs • How does the applied force (F) compare to the frictional force (Fs)? • Would Fs change if F was reduced? If so, how? • If F is increased significantly, will Fs change? If so, how? • Are there any limits on the value for Fs? Kinetic Friction • Force between surfaces that opposes movement • Abbreviated Fk • Does not depend on the speed • Using the picture, describe the motion you would observe ▫ The jug will accelerate • How could the person push the jug at a constant speed? ▫ Reduce F so it equals Fk Calculating the Force of Friction (Ff) • Ff is directly proportional to Fn (normal force) F f Fn Ff Fn • Coefficient of friction (): ▫ ▫ ▫ ▫ Determined by the nature of the two surfaces s is for static friction k is for kinetic friction s > k Typical Coefficients of Friction • Values for have no units and are approximate Classroom Practice Problem • A 24 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. ▫ Draw a free-body diagram and use it to find: the weight the normal force (Fn) the force of friction (Ff) ▫ Find the coefficient of friction. • Answer: s = 0.32 Classroom Practice Problem • A student attaches a rope to a 20.0 kg box of books. He pulls with a force of 90.0 N at an angle of 30.0˚ with the horizontal. The coefficient of kinetic friction between the box and the sidewalk is 0.500. Find the magnitude of the acceleration of the box. – Start with a free-body diagram. – Determine the net force. – Find the acceleration. • Answer: a = 0.12 m/s2