# Sliding Friction

by Ron Kurtus

The most common type of friction encountered is sliding friction. This is the resistance to motion when you try to move or slide a solid object along the surface of another solid object.

When the external force pushing an object is not great enough to cause motion, the resistance is called static sliding friction. Once the objects are in motion with respect to each other, the resistance is called kinetic or dynamic sliding friction.

Between static sliding friction and dynamic sliding friction is the transitional phase of sliding friction.

Interestingly enough, the static coefficient of sliding friction is greater than the kinetic coefficient, and thus more force is required to start to move an object than is needed to keep it moving.

Questions you may have include:

• What is the sliding friction equation?
• What is static sliding friction?
• What happens in the transitional phase of sliding friction
• What kinetic sliding friction?

This lesson will answer those questions. Useful tool: Units Conversion

## Sliding friction equation

The equation for sliding friction (as opposed to rolling friction or fluid friction) is:

Ff = μN

where:

• Ff is the resistive force of friction
• μ is the coefficient of friction for the two surfaces (Greek letter "mu")
• N is the normal or perpendicular force pushing the two objects together
• μN is μ times N

Ff and N are measured in units of force, which are pounds or newtons.

An external force greater than the friction force is required to slide an object along the surface.

## Static sliding friction

When the external force pushing an object is not great enough to cause it to slide, the resistance is called static sliding friction. In other words, the object will not move when:

Fe < Fss

where

• Fe is the external force parallel to the sliding surface
• < means "is less than"
• Fss is the resistive force of static sliding friction

### Static sliding friction equation

The equation for static sliding friction is:

Fss = μssN

where:

• Fss is the resistive force of sliding friction
• μss is the coefficient of sliding friction for the two surfaces (Greek letter "mu")
• N is the normal or perpendicular force pushing the two objects together

## Transitional phase

When the external force equals the static sliding friction resistance (Fe = Fss), the object can break loose and start moving. The static friction becomes kinetic or dynamic sliding friction, which—surprisingly—has a lower coefficient of friction.

This change from static to kinetic occurs rapidly but is not instantaneous.

Since the kinetic sliding friction is less than the static friction, the external force is greater than the kinetic friction.

## Kinetic sliding friction

Once an object is sliding along a surface, the resistance is called kinetic or dynamic sliding friction.

### Kinetic sliding friction equation

The kinetic sliding friction equation is:

Fks = μksN

where:

• Fks is the kinetic sliding resistive force of friction
• μks is the kinetic coefficient of sliding friction for the two surfaces (Greek letter "mu")

### Coefficients of friction different

The kinetic coefficient of sliding friction is less than the friction when the object is stationary or static.

μks < μss

This means that it is easier to slide a moving object than it is to get it to start moving.

### Relationship to external force on sliding object

If an external force is acting on the object, it can accelerate, remain at a constant velocity, or slow down, according to the strength of the external force.

#### Acceleration

When the external force is greater than the kinetic sliding friction, the object will accelerate. This is the case when a stationary object breaks from the static to kinetic sliding mode.

#### Constant velocity

If the external force equals the kinetic sliding friction, the object will continue sliding at a constant velocity.

#### Slowing down and stopping

If the external force is less than the kinetic sliding friction, or has been reduced to zero, the object will slow down and ultimately stop moving.

## Summary

Sliding an object along the surface of another object results in sliding friction. When the resistance to sliding is greater than the force pushing the object, it is called static friction. When the external force equals the static sliding friction, the object starts moving and transitions from the static to kinetic mode. Once the object is sliding, the resistance is call kinetic friction.

The external force acting on the object can cause it to accelerate, remain at a constant velocity, or slow down, according to the strength of the external force.

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## Resources and references

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Friction Resources - Extensive list

Friction Concepts - HyperPhysics

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