FRICTION

Friction : is a force that resists motion. It involves objects thatAre in contact with each other.

The cause of friction is not simple. As the surfaces of two objects Rub against each other, they tend to interlock and thus offer Resistance to being moved over each other.

5 PRINCIPLES OF FRICTION:

1. Friction acts parallel to the surfaces that are in contact andin the direction opposite to the motion of the object or tothe net force tending to produce such motion.

2. Friction depends on the nature of the materials in contact and the smoothness of their surfaces.

3. Sliding friction is usually less than starting friction

4. Friction is practically independent of the area of contact.

5. Starting or sliding friction is directly proportional to the force pressing the two surfaces together.

FrictionFrictional resistance to the relative motion of two solid objects is usually proportional to the force which presses the surfaces together as well as the roughness of the surfaces. Since it is the force perpendicular or "normal" to the surfaces which affects the frictional resistance, this force is typically called the "normal force" and designated by N. The frictional resistance force may then be written:

= coefficient of frictionk = coefficient of kinetic frictions = coefficient of static frictionN = normal forceFf = Frictional force

Ffriction = N

The frictional force is also presumed to be proportional to the coefficient of friction. However, the amount of force required to move an object starting from rest is usually greater than the force required to keep it moving at constant velocity once it is started. Therefore two coefficients of friction are sometimes quoted for a given pair of surfaces - a coefficient of static friction and a coefficent of kinetic friction.

Friction and Surface Roughness In general, the coefficients of friction for static and kinetic friction are different. Frictional force is equal and opposite theApplied force.

Normal Force Frictional resistance forces are typically proportional to the force which presses the surfaces together. This force which will affect frictional resistance is the component of applied force which acts perpendicular or "normal" to the surfaces which are in contact and is typically referred to as the normal force. In many common situations, the normal force is just the weight of the object which is sitting on some surface, but if an object is on an incline or has components of applied force perpendicular to the surface, then it is not equal to the weight.

Coefficients of Friction

Friction is typically characterized by a coefficient of friction which is the ratio of the frictional resistance force to the normal force which presses the surfaces together. In this case the normal force is the weight of the block. Typically there is a significant difference between the coefficients of static friction and kinetic friction.

Static Friction

Static frictional forces from the interlocking of the irregularities of two surfaces will increase to prevent any relative motion up until some limit where motion occurs. It is that threshold of motion which is characterized by the coefficient of static friction. The coefficient of static friction is typically larger than the coefficient of kinetic friction.

The difference between static and kinetic coefficients obtained in simple experiments like wooden blocks sliding on wooden inclines roughly follows the model depicted in the friction plot from which the illustration above is taken This difference may arise from irregularities, surface contaminants, etc. which defy precise description

Kinetic Friction

When two surfaces are moving with respect to one another, the frictional resistance is almost constant over a wide range of low speeds, and in the standard model of friction the frictional force is described by the relationship below. The coefficient is typically less than the coefficient of static friction, reflecting the common experience that it is easier to keep something in motion across a horizontal surface than to start it in motion from rest.

Friction Plot

Static friction resistance will match the applied force up until the threshold of motion. Then the kinetic frictional resistance stays about constant. This plot illustrates the standard model of friction.

The experimental procedure described below equates the vector component of the weight down the incline to the coefficient of friction times the normal force produced by the weight on the incline.