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# Friction; Conceptual questions about friction.

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Does kinetic friction depend on area? What about Normal force? or speed? And what is the difference between static and kinetic friction.

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Does kinetic friction depend on area? What about Normal force? or speed? And what is the difference between static and kinetic friction.

When two surfaces are in contact there molecules are close enough to exert intermolecular force which is electromagnetic force on each other. This force may be attractive or repulsive in nature depending on the distance between the molecules. Thus the force behaves as the force of a spring connecting two blocks. The distance between the molecules is less the force is repulsive and if the distance is more that a particular distance the will be attractive.
When a block is placed on a horizontal surface of a table, due to its weight the block have a tendency of accelerating downward. This compress the surface of the block and the table but as the molecules comes close there is a resultant repulsive force on each surface in the direction normal to the surfaces and called normal force or normal reaction. This force will be upward on the block and balances the weight of the block while it is downward on the table and added to its weight.
Now if we apply a force F on the block to move it to the right. This force will try to move the surface relative to each other and in that way try to separate the molecules which are already close to each other. This action will create a force between the molecules of the surfaces in tangential direction will not allows the surface to move relative to each other in tangential direction. This tangential force f is called friction acts to the left on the block and to the right on the table. Here the forces on the table are not shown. The friction f will be equal to the force applied F and hence the block is in equilibrium. As the force F is increased the friction will increase and will be equal and opposite to F. But the friction f cannot increase more than a particular value called limiting friction. If the force F is greater that this value of friction the block will start sliding on the surface
The friction force between the surfaces when they are not moving relative to each is called static friction. The limiting (maximum) static friction is directly proportional to the normal reaction. The ratio of the limiting friction force and normal reaction is called coefficient of static friction and this depicts the nature of the two surfaces.
When the two surfaces moves relative to each other, the friction is slightly less than the limiting (maximum) friction force and remains constant not increasing with the force applied. In this case the friction force is called kinetic friction. The kinetic friction is also directly proportional to the normal reaction and the ration of the kinetic friction and the normal reaction is called the coefficient of kinetic friction which is less than coefficient static friction.
The coefficient of kinetic friction between two surfaces is, to a large extent independent of the normal reaction, area of the surfaces and the relative velocity between them. But as the friction force is the product of the coefficient and normal reaction depends on it.
Kinetic friction force = coefficient of kinetic friction*normal reaction

The graph of friction force f verses force applied F for a given situation and given normal reaction tells the whole story.

Till the applied force F is less than the limiting friction force both remains equal hence the slope of the graph will be of 450 and the body remain in static equilibrium but when F is slightly greater than limiting friction, the friction decreases to kinetic friction and the body start sliding. The kinetic friction then remains constant whatever may be the applied force.

This content was COPIED from BrainMass.com - View the original, and get the already-completed solution here!

Â© BrainMass Inc. brainmass.com December 24, 2021, 7:41 pm ad1c9bdddf>
https://brainmass.com/physics/equilibrium/friction-conceptual-questions-friction-210199