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Access to Engineering
Coulomb Friction
Date 00-0-06
Objective of the experiment is to verify the Laws of Coulomb friction through practical investigations, to obtain a set of results from different range of materials from which we would be able to understand FRICTION.
Materials wooden flat runaway, specimen blocks, various masses, hangers and pulley.
Diagram of set-up
The force that occurs when things slide against each other, that stops movement ---its scientific name is friction.
In 1781, the French scientist Coulomb (176-1806) announced his friction law clarifying the scientific characteristics of friction.
With regard to the friction that operates between two sliding solids (dry surfaces), the following Laws are in effect.
1) Friction is proportional to the load pressing the solids together and almost independent of the area of contact.
) Kinetic friction is independent of the speed at which the solids are sliding.
) The force require to overcome static friction is always greater than the force to overcome kinetic friction. (to sustain motion)
Coulomb not only verified the inverse-square force between electric charges and measured them absolutely, but also established the Laws of Friction. Coulomb's main result was that the force preventing two solids with plane faces in contact from sliding was independent of area of contact, but proportional to the normal force pressing the two solids together. The ratio of the frictional force to the normal force is called the Coefficient of friction, m = F/N
Frictional forces that obey these rules are called Coulomb friction. The normal force is the force applied that is perpendicular to the surface of contact. The Coefficient is a unitless quantity that must be measured by experimenting with different materials and different normal forces. There are different coefficients for every pair of materials.
Investigation 1 Effects of varying the load on contacting surface.
Steel block has been placed on the level surface, hanger is attached and masses added to move the block at constant speed. Masses on both the block and the hanger are noted.
Mass is increased on the steel block and to the hanger until the specimen moves at constant speed.
Result The greater the mass of the specimen block, the greater the force required moving the specimen block. This is because increasing the mass
Will increase the friction between the steel block and the runway. This is because the surfaces are being pushed together. Refer to the graph and the result table of different range of materials used the masses applied and its coefficients.
Investigation Effect of materials on contacting surfaces.
Steel specimen is placed on the level surface, hanger is attached and masses added to move the specimen at constant speed. Masses were recorded on both the specimen and the hanger.
Experiment was carried out for a range of different materials, which included Aluminium, Brass and Ferabestos.
The Coefficient of Friction for each material was determined
Investigation Varying the area of contacting surfaces
The Area Block is placed on the slideway loaded with mass. Different amounts of weight were used to carry out this experiment.
The coefficient of friction doesn't effect the area of contact because returning to the equations for friction, F = m N, that friction only depends on weight of the object being moved and the coefficient of friction that exists between the two surfaces. The coefficients of friction do not depend upon the area of the surfaces in contact.
Conclusion
Friction is a force like any other and affects motion. Friction acts on materials that are in contact with each other, and it always acts in a direction to oppose motion. When two solids come into contact, the friction is mainly due to irregularities in the two surfaces. When one object slides against another, it must either rise over the irregular bumps or else scrap them off. In both cases a force is required.
In our experiments we have attempt to quantify the amount of friction present between the specimen block and the surface it was sliding across.
The laws of dry friction, also known as laws of Coulomb friction were verified through experiments " The greater the mass of the specimen block, the greater the force required to move the specimen". This is because the mass will increase the friction between the specimen and the surface.
The friction force between two objects is not constant, but increases until it reaches a maximum value. When the frictional force is at its maximum, the body in question will either be moving or will be on the verge of moving.
There are three factors, which affect the size and direction of frictional forces
§ The size of the frictional force depends on the type of surface.
§ The size of frictional force depends on the size of the force acting at right angles to the surfaces in contact, called the normal force. Thus, if the weight of a specimen block is doubled, the frictional force is doubled when it is sliding on the same surface.
§ The direction of the friction force is always opposite to the direction of motion. Thus the frictional force opposes motion.
The coefficient of friction m, is a measure of the amount of friction existing between two surfaces. It is the ratio of a force, and it has no units.
There are disadvantages of frictional forces
§ Energy is wasted when motion through air occurs.
§ Energy is wasted in the bearings associated with shaft, axles and gears due to heat being generated.
§ Wear is caused by friction.
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