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| Kinetic Friction |
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| If the force applied on the body is greater than its limiting friction, then the body begins to slide. This is when we forget about static friction and consider kinetic or dynamic friction. |
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| Kinetic or dynamic friction comes to play when there is a relative motion between the two surfaces in contact. |
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| This type of friction is again classified into sliding friction and rolling friction. |
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 Sliding friction opposes the applied force and has a constant value. |
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 Normal reaction and force of sliding friction are proportional quantities. |
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 Keeping normal reaction constant, the frictional force is independent of the surface area in contact. |
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 For reasonable velocities, the value of sliding friction is constant. |
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| Imagine the following situation. A heavy crate is resting on the floor in a warehouse. You push it horizontally with a steady force, but it does not move. That is because the force that you apply is balanced by a frictional force exerted horizontally on the bottom of the crate by the floor and opposite to the direction of your push. Remarkably, this frictional force automatically adjusts itself, in both magnitude and direction, to cancel exactly whatever force you decide to apply. Of course, if you push hard enough you will be able to move the crate. |
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| As you increase the applied force, the static frictional force increases upto the limiting friction value. After that, if you still increase your force, the block breaks loose from its interlocked contact with the surface and begins to accelerate. |
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| The force that acts during motion between the two bodies in contact is called the kinetic frictional force. Once the block is in motion and the value of the applied force is just equal to the value of kinetic friction, the block neither accelerates not decelerates but moves with a constant velocity. |
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| As long as the body is stationary, the frictional force (static friction) is a self-adjusting force and once it begins to move, the frictional force (kinetic force) is a constant force, but has a value less than the value of limiting frictional force. |
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| Coefficient of static friction |
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| When you push a wooden block on a smooth steel sheet it is easy, in case of a wooden sheet it is more difficult, but when you push it on a road it becomes quite difficult. There must be greater interlocking on the road with the block and least interlocking on the steel sheet with the block. This is a property of the two surfaces in contact. This property is called the coefficient of friction. |
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For any two surfaces in contact, stationary with respect to each other, the ratio of limiting friction to the normal reaction between  |
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| Coefficient of kinetic friction is a similar constant which is a property of two surfaces with relative motion. |
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| Coefficient of kinetic friction can be defined as the ratio of kinetic friction to normal reaction. |
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| The coefficient of kinetic friction always has a smaller value than the coefficient of static friction. This means that the opposing force for a sliding body is less than the opposing force for a stationary body in its limiting state. |
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