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Lesson_2.3_Printable

# Lesson_2.3_Printable - Force of Friction f m F A Friction...

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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 The friction force f between 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. surfaces opposes this motion. 1

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m A F f What 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. 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. The force of limiting friction f is 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 proportional to the normal force F n 3

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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 f 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 ( 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 The component of mg pressing 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 plane at right angles is mg cos θ 5

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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, f s = μ s mg cos θ opposes it.
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