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Unformatted text preview: Force of Friction Friction is a force that opposes motion. m A A force F applied to the right on the box tends to move it to the right. F f A box of mass m is placed on a surface A he friction force etween these As the applied force increases, the frictional force will increase until it reaches the maximum possible value for the two surfaces. If the applied force F is not sufficient to overcome the friction, the box will not move. The friction force f between these surfaces opposes this motion. 1 m A F f hat are the factors that affect the force of The maximum force of friction between two surfaces is called the force of limiting friction ( f ). The only factor that affects the force of limiting friction between two surfaces is the normal force that presses the two surfaces together. What are the factors that affect the force of limiting friction between two given surfaces? 2 Coefficient of of Friction m A The normal force F n acts vertically up on the box. F f What is the normal force that is pressing the two surfaces together? mg F n The weight of the box mg acts vertically down. Since F n = mg, F n is the force that presses the surfaces together. f = μ mg But F n = mg when the two surfaces are horizontal. μ is a constant called coefficient of friction f ∝ F n f = μ F n The force of limiting friction f is proportional to the normal force F n 3 Kinetic and Static Friction m A Once the box is in motion, the two surfaces are not pressed together as tight as before, so the force of friction decreases. F f If the applied force F is greater than the force of limiting friction, the box moves. The force of friction between two surfaces at rest relative When the box is in motion, the force of friction between the two surfaces is less than the force of limiting friction. μ k is called the coefficient of kinetic friction. μ k < μ s μ s is called the coefficient of static friction. f s = μ s F n f k = μ k F n f k < f s The force friction between two surfaces that are in relative motion is called force of kinetic friction ( f k ). to each other is called the force of static friction ( f s ) 4 m θ The weight of the box mg acts vertically down. If the box is now placed on an inclined plane of inclination θ , what will be the normal force on the box? mg mg sin θ mg cos θ f s = μ s mg cos θ F n he component of g ressing the The component of mg pulling the box down the plane is mg sin θ f s = μ s F n = μ s mg cos θ f k = μ k mg cos θ The normal force on the box is: F n = mg cos θ The component of mg pressing the plane at right angles is mg cos θ 5 m θ mg mg sin θ mg cos θ f s = μ s mg cos θ F n mg sin θ pulls the box down the plane,...
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This note was uploaded on 05/01/2011 for the course PHY 2048 taught by Professor George during the Fall '10 term at Edison State College.
 Fall '10
 George
 Physics, Force, Friction, Mass

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