The relative motion or tendency toward such motion between two surfaces is resisted by friction. Friction may damage or 'wear' the surfaces in contact. However, wear can be reduced by lubrication. The science and technology of friction, lubrication, and wear is known as tribology
Sliding may occur between two objects of arbitrary shape, whereas rolling friction is the frictional force associated with the rotational movement of a somewhat disclike or other circular object along a surface. Generally the frictional force of rolling friction is less than that associated with sliding kinetic friction. Typical values for the coefficient of rolling friction are less than that of sliding friction. Correspondingly sliding friction typically produces greater sound and thermal bi-products. One of the most common examples of sliding friction is the movement of braking motor vehicle tires on a roadway, a process which generates considerable heat and sound, and is typically taken into account in assessing the magnitude of roadway noise pollution.
Static friction (also called kinetic friction) is a contact force that resists the sliding motion of two objects or an object and a surface. Sliding friction is almost always less than that of static friction; this is why it is easier to move an object once it starts moving rather than to get the object to begin moving from a rest position.
Where Fk, is the force of kinetic friction. μk is the coefficient of kinetic friction, and N is the normal force.
Examples of Sliding Friction
- Pushing an object across a surface
- Rubbing one's hands together (The friction generated by rubbing's one hands is what generates heat.)
- A car sliding on ice
- Opening and closing a sliding door
- A car turning a corner
- Rope and a pulley system
- Almost any motion where there is contact between an object and a surface
Motion of Sliding Friction
The motion of sliding friction can be modeled (in simple systems of motion) by Newton's Second Law
Where is the external force.
- Acceleration occurs when the external force is greater than the force of kinetic friction
- Slowing Down (or Stopping) occurs when the force of kinetic friction is greater than that of the external force
- This is also follows Newton's first law of motion as there exists a net force on the object
- Constant Velocity occurs when there is no net force on the object, that is the external force is equal to force of kinetic friction.
Motion on an Incline Plane
One of the most common physics problems in introductory physics classes is a block subject to friction as it slides up or down an incline plane.
In this case the force of gravity is accounted for and is given by:
The force of friction opposes the motion of the block and the normal force (perpendicular to the surface is given by:
To find the coefficient of kinetic friction on an incline plane, one must find the moment where the force parallel to the plane is equal to the force perpendicular; this occurs when the block is moving at a constant velocity at some angle
Here it is found that:
where is the angle at which the block begins moving at a constant velocity
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