  # Static Friction vs. Kinetic Friction: What's the Difference? Edited by Huma Saeed || By Sumera Saeed || Updated on October 5, 2023
Static friction resists the initiation of motion between two surfaces, while kinetic friction opposes the ongoing motion between moving surfaces. ## Key Differences

Static friction emerges when two surfaces are not in motion relative to each other. This type of friction must be overcome to initiate movement, essentially acting as the force that holds objects stationary. On the contrary, kinetic friction, sometimes referred to as dynamic friction, manifests when two surfaces are sliding past each other, providing a resistive force against the motion already in progress.
In practical applications, static friction is what keeps a parked car from sliding down a hill – it works against the gravitational pull. Once the car begins to move, kinetic friction takes over, opposing the sliding between the tires and the road. Thus, static friction prevents the initial slide, and kinetic friction opposes the motion once it has begun.
Interestingly, static friction generally tends to be greater than kinetic friction for a given set of materials. This means that it often takes more force to initiate motion (overcoming static friction) than to keep it in motion (overcoming kinetic friction). As a practical observation, once an object is set in motion, it typically requires less effort to keep it moving, showcasing the role of kinetic friction as a slightly lesser opposing force.
In physics, static and kinetic friction are described by the equations: F_s ≤ μ_sN and F_k = μ_kN respectively, where F_s is the maximum static frictional force, F_k is the kinetic frictional force, μ_s and μ_k are the coefficients of static and kinetic friction, and N is the normal force. These equations underline the core difference, where static friction provides a variable resistance up to a maximum threshold, and kinetic friction presents a consistent force once motion has commenced.
From an experimental perspective, static friction can be observed in various day-to-day activities, such as pushing a heavy object from rest. It resists the initiation of movement until a sufficient force is applied. Kinetic friction, meanwhile, is witnessed once the object is in motion, providing a constant resistance that must be counteracted to maintain a steady movement, thereby illustrating practical encounters with these frictional forms in our daily lives.

## Comparison Chart

### Definition

Resists the initiation of motion
Opposes ongoing motion

### Occurrence

When objects are stationary
When objects are sliding past each other

### Magnitude

Generally greater than kinetic friction
Generally less than static friction

F_s ≤ μ_sN
F_k = μ_kN

### Practical Example

Preventing a stationary object from moving
Slowing down a sliding object

## Static Friction and Kinetic Friction Definitions

#### Static Friction

Inhibits the initiation of sliding between surfaces.
Static friction keeps the vase secure on the shelf.

#### Kinetic Friction

Resistance against the continuous sliding of objects.
Kinetic friction between the skis and snow affects the skier’s speed.

#### Static Friction

A friction form resisting the start of relative motion.
Static friction stops the boulder from rolling down the hill.

#### Kinetic Friction

Acting on objects already in relative motion.
Kinetic friction works against the moving skateboard.

#### Static Friction

A friction type that acts on stationary objects.
Static friction holds the car in place on the sloped road.

#### Kinetic Friction

Impedes the persistence of sliding among surfaces.
The kinetic friction between the sliding box and the ground will eventually stop it.

#### Static Friction

Resisting force against starting movement.
The static friction between the box and the floor must be overcome to start pushing it.

#### Kinetic Friction

A friction type hindering ongoing relative movement.
Kinetic friction resists the rolling motion of the tire across the pavement.

#### Static Friction

Force that opposes initial motion.
Static friction prevents the book from sliding off the tilted table.

#### Kinetic Friction

Force that opposes ongoing motion.
Kinetic friction slows down the sled as it slides down the hill.

## FAQs

#### Can kinetic friction act on an object at rest?

No, kinetic friction only acts on objects already in motion.

#### How are coefficients of static and kinetic friction determined?

They are determined experimentally, often using a tilted plane and observing at what angle an object starts to slide (static) or continues sliding (kinetic).

#### Why does static friction not act on moving objects?

Static friction acts to prevent the initiation of motion, so it doesn’t act on objects already moving.

#### Do materials with high static friction also have high kinetic friction?

Not necessarily, the relationship between static and kinetic friction coefficients can vary between materials.

#### Can static friction occur without a normal force?

No, static friction depends on the normal force acting perpendicular to the surfaces in contact.

#### Can static and kinetic friction be equal for certain materials?

Yes, in some scenarios or materials, the coefficients of static and kinetic friction may be very close or equal.

#### How is rolling friction different from kinetic friction?

Rolling friction involves objects rolling over each other, while kinetic friction involves sliding, even though both involve motion.

#### How does temperature affect static and kinetic friction?

Temperature can alter material properties, potentially changing the coefficients of static and kinetic friction.

#### Why do we often shimmy or shake objects to overcome static friction?

Shaking can provide additional force or reduce the contact area, making it easier to overcome static friction.

#### How do static and kinetic friction play roles in automotive braking?

Static friction helps keep a stationary vehicle in place, while kinetic friction acts to slow and eventually stop a moving vehicle when braking.

#### Is static friction always higher than kinetic friction?

Typically, yes. Static friction usually requires more force to overcome compared to kinetic friction.

#### How do lubricants affect kinetic friction?

Lubricants usually decrease kinetic friction by reducing the interaction between sliding surfaces.

#### Does air resistance impact kinetic friction?

No, kinetic friction pertains to surfaces in contact, while air resistance is a separate force.

#### Can an object be subjected to both static and kinetic friction simultaneously?

No, an object experiences static friction when at rest and kinetic friction once in motion, not simultaneously.

#### Is it possible to have frictionless surfaces, thus eliminating static and kinetic friction?

While theoretically conceivable, practically achieving completely frictionless surfaces is nearly impossible.

#### What is the basic difference between static and kinetic friction?

Static friction resists the start of motion, while kinetic friction opposes motion that’s already happening.

#### In what units is friction measured?

Friction is a force, so it is measured in Newtons (N) in the International System of Units (SI).

#### How does surface area impact static and kinetic friction?

Surprisingly, friction is independent of surface area as it's defined per unit area.

#### Can zero-gravity environments influence static and kinetic friction?

While gravity affects normal force, which in turn affects friction, static and kinetic friction still occur in zero-gravity environments.

#### Why do we feel heat when we rub our hands together, in terms of kinetic friction?

The kinetic friction between our hands converts mechanical energy into thermal energy, creating heat.  