Unit Dynamics Module The Forces of Friction page 1 of 1 Understanding the Frictional Force Between Two Surfaces Kinetic friction fk is the force between two moving objects that resists their relative motion Its direction is opposite to the direction of the motion An approximate equation for kinetic friction is fk k N where k is the dimensionless coefficient of kinetic friction Static friction fs is the force between two stationary objects that resists their relative motion Static friction is maximal when fs s N where s is the coefficient of static friction s is always greater than k If you slide a block across a table you can feel the frictional force between the table and the block resisting their relative motion A crude way to measure friction is to attach a scale to the object and pull with uniform speed Since there is no acceleration by Newton s second law the force of friction is equal to the scale reading Kinetic friction is the force between two moving objects that resists their relative motion Friction depends slightly on the relative velocity and the size and shape of the objects Most importantly friction is proportional to the normal force The dimensionless proportionality coefficient of kinetic friction is k The value of k is usually between 0 and 1 and it varies depending on the composition and texture of the surfaces Static friction is the force between two stationary objects that resists motion Static friction changes depending on the force applied to the objects As the force increases static friction increases to prevent the objects from moving Static friction is maximal when is s N where s is the coefficient of static friction s is always greater than k The graph shows the relationship between a force applied to an object and the frictional force Initially the object is stationary and static friction increases linearly with the applied force When the applied force reaches s N the object moves and the kinetic friction becomes approximately k N Notice that the value of kinetic friction varies slightly This is because the texture and composition of the surfaces is rarely completely uniform www thinkwell com info thinkwell com Copyright 2001 Thinkwell Corp All Rights Reserved 1848 doc rev 03 29 2001
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