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# Week3b - The tangential component of the contact force...

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The tangential component of the contact force exerted by one body on another is called the frictional force. For example if we sit a block on an incline it will not necessarily side down. This is because the incline exerts a force, the frictional force , equal and opposite to the component of the gravitational force acting on the body down the incline. Its direction will always be opposite to the direction in which the motion would take place if there is no friction. The The frictional force frictional force is parallel to the contact surfaces. is parallel to the contact surfaces.

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For bodies in contact there are two types of friction. A) Static Friction - the contact surfaces are at rest At some value of applied force, F , the block will be on the point of moving. At this point the maximum force of static friction , f s , is found to be proportional to the normal force , N. Hence f s,max = μ s N where μ s is the coefficient of static friction . Note: 1) f s is to a good approximation independent of the area of contact. 2) 0 " f s " μ s N While the block is at rest, the frictional force will vary to balance the applied force, F , up to a maximum value of μ s N.
B) Kinetic Friction - the contact surfaces have relative motion. If the applied force is greater than μ s N , then and it is found experimentally that f " N, so that f = μ k N, where μ k is the coefficient of kinetic friction . Properties of μ k . 1) μ k is approximately independent of speed and area of contact, but depends on the nature of the surfaces. For example μ k # 0.4 for wood on wood. 2) μ k < μ s In other words it is more difficult to start motion than it is to maintain it.

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Frictional Force F fr 45 o Any car is propelled forward by virtue of the frictional force that exists between the tyres and the road surface. In order to achieve maximum acceleration, it is necessary to maintain this frictional force as large as possible. We have seen that frictional forces depend only on the normal forces.
Why do dragracers use tyres that are much wider than normal road car tyres?

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Week3b - The tangential component of the contact force...

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