Lesson_Text_2.3

# Lesson_Text_2.3 - 1 Lesson 2.3 Force of Friction 1 Force of...

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1 Lesson 2.3 Force of Friction 1. Force of Friction : When one surface moves over another surface, always a force exists that opposes the motion. This force is called the force of friction. The force of friction between two surfaces depends on two factors: The force of friction depends on the normal force that presses the two surfaces together. When the surfaces are pressed harder, the force of friction is larger and vice versa. The other factor that affects friction is the nature of the surfaces involved. Some surfaces offer more friction than others. mg F n F f Fig. 1. Friction opposes motion . A B In fig. 1 Above, an object A of mass m kg sits on a surface B. The weight mg of the object presses the surface downward and the surface applies an equal and opposite force upward. This is called the normal force and is represented by F n . F is an external force trying to move the object in the positive x direction. f is the frictional force that opposes motion. If F is not large enough, the friction will balance it, and there will be no motion. If you increase the force F, the frictional force f also will increase, but it cannot increase beyond a certain maximum. The maximum frictional force between two surfaces is called the force of limiting friction . The force of limiting friction between two surfaces depends on the normal force pressing the two surfaces together. This statement is called the law of friction. The force of limiting friction ( f ) between two surfaces is directly proportional to the normal force (F n ) pressing the two surfaces together.

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2 f F n . or , f = μ F n ……(i) Here μ is a constant which depends on the nature of the two surfaces and is called the coefficient of limiting friction. When the surface B is horizontal, F n is the weight of object A which is mg. Therefore, in this case equation (iii) above can be written as: f = μ m g …..(ii) A θ W=mg F n mg sin f Fig. 2. F n = mg cos θ f = mg cos μ μμ θ θ mg cos θ A B On the other hand, if the surface B is inclined at an angle θ with the horizontal, as shown in fig. 2 above, the weight mg of object A is vertically down. The weight W = mg is performing two functions: (i) A component of W at right angles to B presses object A to surface B. This component is mg cos θ and this is the normal force that presses the two surfaces together. Therefore, in this case F n = m g cos θ . (ii) Another component of W is parallel to surface B and pulls the object A down the surface B. This component is m g sin θ . Since m g sin θ pulls the object down the plane, the frictional force f acts up the plane. When the object does not slide down, m g sin θ is balanced by the frictional force f . In this case equations (i) and (ii) can be written as: f = μ F n But F n = m g cos θ …….(iii) f = μ m g cos θ ….. (iv)
3 Kinetic and Static friction : The friction between two surfaces which are at rest relative to each other is greater than when there is relative motion between the two surfaces. For

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Lesson_Text_2.3 - 1 Lesson 2.3 Force of Friction 1 Force of...

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