The force acting between two bodies in contact which opposed the reletive moiton is called frictional force.
It comes because of interlocking of molecules at the surface of contact of two bodies. It is a microscopic phenomenon. It is because of all surfaces are irregular and when two bodies are in contact the irregular surfaces get interlocked with each other and further opposes the relative motion. This opposition is called friction.
We can polish the surfaces and reduce the friction and it can not be reduced to zero. It is because when you over polish a surface the molecules comes into contact and again there is a attractive force between them which also opposes relative motion.
The frictional force between two bodies in the state of rest is called static and when they are in motion it is called kinetic friction.
Let us consider a example of a body of mass m at rest on a horizontal table. The force of gravity (mg) is cancelled by the normal reaction force (N) of the table. Now suppose a force F is applied horizontally to the body. We know from experience that a small applied force may not be enough to move the body. But if the applied force F were the only external force on the body, it must move with acceleration F/m, however small. Clearly, the body remains at rest because some other force comes into play in the horizontal direction and opposes the applied force F, resulting in zero net force on the body.
This force fs parallel to the surface of the body in contact with the table is known as frictional force, or simply friction (Fig.a below). The subscript stands for static friction to distinguish it from kinetic friction fk that we consider later (Fig. b given below). Note that static friction does not exist by itself. When there is no applied force, there is no static friction. It comes into play the moment there is an applied force.
As the applied force F increases, fs also increases, remaining equal and opposite to the applied force (up to a certain limit), keeping the body at rest. Hence, it is called static friction. Static friction opposes impending motion. The term impending motion means motion that would take place (but does not actually take place) under the applied force, if friction were absent.
When the applied force exceeds a certain limit, the body begins to move. It is found experimentally that the limiting value of static friction ( )max s f is independent of the area of contact and aries with the normal force(N) approximately as
static frictional force maximum = us N .
where μs is a constant of proportionality depending only on the nature of the surfaces in contact. The constant μs is called the coefficient of static friction. The law of static friction thus be written as fs ≤ μs N .
If the applied force F exceeds (fs ) the body begins to slide on the surface. It is found experimentally that when relative motion has started, the frictional force decreases from the static maximum value (fs ) . Frictional force that opposes relative motion between surfaces in contact is called kinetic or sliding friction and is denoted by fk . Kinetic friction, like static friction, is found to be independent of the area of contact. Further, it is nearly independent of the velocity. It satisfies a law similar to that for static friction fk = μs N.
where μk the coefficient of kinetic friction, depends only on the surfaces in contact. μk is less than μs. When relative motion has begun, the acceleration of the body according to the Second Law is ( F – fk )/m. For a body moving with constant velocity, F = fk. If the applied force on the body is removed, its acceleration is – fk /m and it eventually comes to a stop.
When two bodies are in contact, each experiences a contact force by the other. Friction, by definition, is the component of the contact force parallel to the surfaces in contact, which opposes impending or actual relative motion between the two surfaces. Note that it is not motion, but relative motion that the frictional force opposes .
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It comes because of interlocking of molecules at the surface of contact of two bodies. It is a microscopic phenomenon. It is because of all surfaces are irregular and when two bodies are in contact the irregular surfaces get interlocked with each other and further opposes the relative motion. This opposition is called friction.
We can polish the surfaces and reduce the friction and it can not be reduced to zero. It is because when you over polish a surface the molecules comes into contact and again there is a attractive force between them which also opposes relative motion.
The frictional force between two bodies in the state of rest is called static and when they are in motion it is called kinetic friction.
Let us consider a example of a body of mass m at rest on a horizontal table. The force of gravity (mg) is cancelled by the normal reaction force (N) of the table. Now suppose a force F is applied horizontally to the body. We know from experience that a small applied force may not be enough to move the body. But if the applied force F were the only external force on the body, it must move with acceleration F/m, however small. Clearly, the body remains at rest because some other force comes into play in the horizontal direction and opposes the applied force F, resulting in zero net force on the body.
This force fs parallel to the surface of the body in contact with the table is known as frictional force, or simply friction (Fig.a below). The subscript stands for static friction to distinguish it from kinetic friction fk that we consider later (Fig. b given below). Note that static friction does not exist by itself. When there is no applied force, there is no static friction. It comes into play the moment there is an applied force.
As the applied force F increases, fs also increases, remaining equal and opposite to the applied force (up to a certain limit), keeping the body at rest. Hence, it is called static friction. Static friction opposes impending motion. The term impending motion means motion that would take place (but does not actually take place) under the applied force, if friction were absent.
When the applied force exceeds a certain limit, the body begins to move. It is found experimentally that the limiting value of static friction ( )max s f is independent of the area of contact and aries with the normal force(N) approximately asstatic frictional force maximum = us N .
where μs is a constant of proportionality depending only on the nature of the surfaces in contact. The constant μs is called the coefficient of static friction. The law of static friction thus be written as fs ≤ μs N .
If the applied force F exceeds (fs ) the body begins to slide on the surface. It is found experimentally that when relative motion has started, the frictional force decreases from the static maximum value (fs ) . Frictional force that opposes relative motion between surfaces in contact is called kinetic or sliding friction and is denoted by fk . Kinetic friction, like static friction, is found to be independent of the area of contact. Further, it is nearly independent of the velocity. It satisfies a law similar to that for static friction fk = μs N.
where μk the coefficient of kinetic friction, depends only on the surfaces in contact. μk is less than μs. When relative motion has begun, the acceleration of the body according to the Second Law is ( F – fk )/m. For a body moving with constant velocity, F = fk. If the applied force on the body is removed, its acceleration is – fk /m and it eventually comes to a stop.
When two bodies are in contact, each experiences a contact force by the other. Friction, by definition, is the component of the contact force parallel to the surfaces in contact, which opposes impending or actual relative motion between the two surfaces. Note that it is not motion, but relative motion that the frictional force opposes .
Related posts :
Newton's First law of motion
Newton's second law of motion
Newton's third law of motionDetermination of Specific Charge of Electron J J Thomson Experiment
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