Static friction: Definition|Examples|FAQs

Static friction definition

Static friction is a resistive force that keeps an object at rest. In other words, it is the frictional force between a stationary object and the sliding surface. If the object is moving then we call the friction kinetic friction. We encounter static friction quite often in our life, let’s find out with some real-life examples.

You might have tried pushing some heavy objects. Initially, for a few seconds, they would not move, the friction involved here is static friction. But after some time you get enough force to overcome the static friction and push the object forward. When the object moves, kinetic friction comes into action.

In the example, you might have understood that static friction is always greater than kinetic friction. Static friction is generally denoted by f_s and kinetic friction as f_k.

The value of static friction varies between zero to the maximum force required to move the object(limiting friction).

What are the laws/formula of static friction?

F_{s =}μ_s*N [where μ_s is coefficient of static friction and N is the normal force]

As per the above formula, static friction is directly proportional to the normal force. The greater the weight of the object, the greater is static friction.

Static friction is independent of the surface area of contact.It acts along the sliding plane and opposite to the direction of motion.

Static friction is dependent on the irregularities on both surfaces. Static friction arises due to the interlocking of these irregularities.

What is limiting friction?

It is the force that is required to overcome static friction. Any force above the limiting friction sets the object in motion. In other words, it is the maximum value of static friction. It is the point when the object is just about to slide. Kinetic friction starts acting after this point. You can understand the concept by looking at a friction plot.

What are some examples of static friction?

1. Cars parked on a hill.
2. A book is placed on a table. The book will not slide automatically. A force has to be applied to over-static friction.
3. The paperweight on paper keeps the paper from flying. [Remember static friction is directly proportional to the normal force. Here the normal force is the weight of the object]
4. Stationary Football on a field.
5. Lizard standing vertically on a wall surface.

Static friction is a self-adjusting force explain?

Static friction is a self-adjusting force. The static frictional force adjusts itself in order to stop the motion of an object. Till the limiting force is reached, it will try to match the applied force.

But, beyond a limit (limiting friction) it gets converted into kinetic friction. You can understand this concept using a friction plot.

What is the concept of static friction in an inclined plane?

When an object is at rest, there will be many forces acting on it. One such force is the gravitational force. If the object is on an inclined surface then the component of this gravitational force will act on it. The object will tend to slide down the slope. But static friction will try to stop this motion. The limiting friction depends on the incline angle. Hence, the incline angle can also be considered as a factor affecting friction.

What is the difference between static and kinetic friction?

1. Static friction applies to an object before it moves and kinetic friction applies to the object while it is in motion. Static friction is always greater than kinetic friction.
2. The coefficient of static friction is denoted as μ_s and the coefficient of kinetic friction is μ_k. 
3. The maximum static friction (limiting friction) value is the maximum force required to start the motion of an object, and the kinetic friction force value is the maximum force required to keep the object in motion.
4. A parked car is an example of static friction, a moving car is an example of kinetic friction.

Is static friction constant?

Static friction is proportional to the force applied till the value of limiting friction is reached. In other words, static friction increases till you reach a limit and is not constant. After reaching the limiting friction value, it remains fairly constant, known as kinetic friction. This can be understood by the friction plot.

What is maximum static friction?

Maximum static friction is also called limiting friction. As explained above, it is the maximum value that static friction can attain. It is the value of friction when the object is just about to move.

Is rolling friction higher than static friction?

Based on the type of contact, friction can be divided into 2 categories: static and kinetic friction. And based on the state of motion friction can be categorized as static and kinetic friction. So, they cannot be compared. But, since rolling friction comes into action while an object is in motion it is generally less as compared to maximum static friction.

What is the greatest type of friction?

Static friction is the greatest type of friction followed by sliding, rolling, fluid friction, and air resistance.

What is an easy definition of static friction?

The resistive force of an object when at rest.

Why does static friction occur?

Static friction occurs when an object is at rest. At rest, many forces will be acting on the body in a balanced manner. The irregularities in the surface, chemical bonds, gravitational force, etc will contribute towards this.

How does static friction apply to real life? What is the application of static friction?

• Paperweight
• Brakes on a vehicle
• Belts on our waist
• Hats on our head
• Ring on our finger

What is the principle of static friction? What is the mechanism of static friction?

For example: When a book is lying on a table, there will be weak chemical bonds, the interlocking of irregularities, etc. These factors contribute to static friction.

What are the factors affecting static friction?

The static friction force depends on the following factors:

• Irregularities in surfaces
• Any chemical bonds like weak Van der Waal
• Gravitational force
• Electromagnetic forces
• The angle of incline.
• Force of wind

Why is static friction greater than kinetic friction?

To move an object at rest you will have to break all the strong attractive chemical forces and the interlocking between the surfaces. Once the threshold is reached this strong bond is broken and while in motion weak temporary bonds are formed. Hence static friction is always greater than kinetic friction.

How does static friction increase?

Friction is caused due to interlocking and chemical bonding. When you push an object, there is sliding of the 2 surfaces. While sliding the interlocking increases and the bonds are not broken immediately. But after you cross a certain threshold force then the bonds and the interlocking are broken. This is why the coefficient of static friction is always higher than the coefficient of kinetic friction. The limiting friction force depends on the coefficient of static friction and the normal force applied.

What is a Friction plot?

The friction plot shows the value of friction with time. It shows how much force is required to move a body at rest. We can also calculate kinetic friction, how much force is required to keep a moving body in motion.

See Also:

• Examples of static friction
• What are the applications of friction in our daily lives?
• Factors affecting friction
• How can we increase friction? What are the ways of doing it?
• How can we decrease friction? What are the ways of doing it?
• What is a frictional force?
• What is rolling friction?
• What is static friction?
• 50 Examples of friction in our daily lives
• Friction is a necessary evil

Research on static friction:

The authors have studied a theoretical model to define static friction. Authors have used radial basis function neural network (BRFNN) with a beetle antennae search (BAS) algorithm to develop the model for static friction in a robotic joint. [Link: https://www.hindawi.com/journals/mpe/2020/5839195/]

This paper discusses the static friction between steel and different types of polymers call polyamide. You can read this paper to understand how authors have measured friction. [Link: https://doi.org/10.1051/matecconf/201818402004]