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(with illustrations)

written by Stanley Udegbunam || Jan 15th, 2021

What is the direction of Static Friction?

Static Friction acts opposite the direction of the net applied force.

This friction type exists between two surfaces at rest relative to each other.

As a result of this, the direction of static friction cannot be directly visualized like kinetic friction.

Nevertheless, friction force in whatever form, is a resistive force and will always act opposite the direction of motion.

In a hurry?

Here’s a quick infographic summary

Let’s dive deep by analyzing different case scenarios.

CASE 1

Consider a wooden box of weight 200N sitting undisturbed on the floor with a coefficient of static friction, 0.3N.

The maximum value of static friction = Coefficient of Friction × Normal force

Fs = µs × N

Normal force = weight of the object

Maximum value of static friction = 0.3 x 200 = 60N

The maximum value of static friction is also known as limiting friction force.

Let’s draw the free body diagram

Q1. What is the direction of static friction when there’s no applied force?

Since no force is applied, static friction force will not exist or have a zero value, a state called static friction zero.

Static friction is a vector meaning it possesses both magnitude and direction.

It is direction is dependent on the direction of the applied force.

When there’s no applied force, static friction has no direction.

At static friction zero, it has no magnitude and direction.

CASE 2

Let’s consider a block of 500N on a wooden basement with a coefficient of friction of 0.6

Maximum value of static friction = 0.6 x 500 = 60N

A lady tries to move the block over a distance by applying a force of 40N which is less than the maximum value of static friction.

Q2. What will be the direction of static friction when the applied force is less than the maximum value of static friction?

The direction of static friction will be opposite the direction of the applied force.

Since the applied force is less than the maximum value of static friction, the object won’t move.

For motion to occur, the applied force must be greater than the limiting static friction.

FREE BODY DIAGRAM

CASE 3

Using the same scenario above, let’s assume the lady applies a force of 60N which is the same as the maximum static friction value.

Q3. What will be the direction of static friction if the applied force is the same as the maximum value of static friction?

The direction of static friction will still be opposite that of the applied force.

At this point, the force applied is equal to the limiting force which is the transition phase of the stationary position.

This transition phase is known as the threshold of motion.

Any additional force applied regardless of its magnitude will cause the object to move.

FREE BODY DIAGRAM

CASE 4

I guess you know where we are headed by now.

Let’s assume the lady applies a force of 70N which happens to be greater than the maximum value of static friction.

Q4. What will be the direction of static friction if the applied force is greater than the maximum value of static friction?

Since the applied force is greater than the limiting friction, the object will move in the direction of the force and static friction ceases to exist.

Once the threshold of motion is exceeded by the applied force, the opposing force changes from static friction to kinetic friction.

Therefore, the static friction has no direction because the state of the object has been changed.

FREE BODY DIAGRAM

From the 4-case scenario, you can see that we can only talk about the direction of static friction when the object is in a stationary position.

And it actually makes sense because static friction acts between surfaces at rest relative to each other.

Anything beyond that means exceeding the scope of its definition.

Let’s have a quick question summary:

1. What is the direction of static friction when there’s no applied force?

Since no force is applied, static friction force will not exist or have a zero value, a state called static friction zero.

Since the static friction has no value, it, therefore, possesses no direction.

1. What is the direction of static friction when the applied force is less than the maximum value of static friction:

The direction of static friction will be opposite the direction of the applied force.

1. What will be the direction of static friction if the applied force is the same as the maximum value of static friction?

The direction of static friction will still be opposite that of the applied force.

1. What will be the direction of static friction if the applied force is greater than the maximum value of static friction?

The force applied will cause motion, static friction will cease to exist and possess no direction.

Still curious about static friction? see our guides:

1. STATIC FRICTION : The Complete Guide

(with illustrations)

written by Stanley Udegbunam || Jan 4th, 2020

Static friction is the friction force between two surfaces at rest relative to each other.

It is the friction that exists between the contacting surfaces of static objects.

In today’s article, we’ll be looking at the 6 major causes of static friction.

Without further ado, let’s get started.

Static friction is caused by:

1. The interlocking of surface irregularities
2. Material Deformation of static object.
3. Intermolecular bond due to surface adhesion.
4. Stickiness of contact surface.
5. Surface roughness or degree of smoothness.
6. Surface abrasion between contacting pairs.

1. INTERLOCKING OF SURFACE IRREGULARITIES

All objects have tiny rough planes regardless of how smooth they may appear.

It might not be visible to the naked eyes but can be detected using microscopic lenses.

These minute rough planes are called surface irregularities.

The interlocking of these surface irregularities give rise to static friction.

Surface irregularity are also called Asperities; a common term in material science that defines the unevenness of surfaces.

Even if the surface is polished, it still contains tiny bits of unevenness.

Apparently, there is no known material surface that is completely devoid of surface irregularity.

Surface asperity consists of:

1. Surface Hills (mountains)
2. Surface Valleys (grooves)

Due to this microscopic surface unevenness, the actual contact area is slightly less than what is expected when viewed with the naked eyes.

Deductive implication;

2. MATERIAL DEFORMATION OF STATIC OBJECT

Deformation is another cause of static friction.

The deformation can be elastic or plastic. The force of friction will increase if the two surfaces are pressed harder.

This means an increase in weight will lead to a corresponding increase in static friction.

The weight of an object directly affects the material deformation.

The deformation will continue until the contact area is great enough to support the load.

Excess weight acting on an object can lead to a permanent distortion of the object’s shape or orientation.

This non-reversible deformation is called plastic deformation and large deformation can leads to one object sinking into the other.

Generally, soft material deforms under pressure.

Although such deformations are elastic in nature, it still increases the resistance to motion.

Some common examples of elastic deformation due to friction includes:

• Slight inward depression of our feet when standing on a rug.
• Weight dip when we lie on the bed
• Sitting on an office leathered chair
• Cuddling a pillow
• Rubber tire flattens out at the area of contact with the road.

Want more examples? see this article: 50 + Detailed Examples of Static Friction.

The higher the material deformation, the greater the static friction between the contact surfaces.

3. INTERMOLECULAR BOND DUE TO SURFACE ADHESION

Surface Adhesion is the driving force of static friction.

Adhesion is the force of attraction between molecules of different substances.

When two objects or surfaces are brought into contact, the adhesive or electromagnetic forces attracts the molecules of the two objects together.

A common example of surface adhesion is rain droplets on window surfaces just after rainfall or morning dew.

The force of adhesion between the liquid droplets and the glass surface prevents it from sliding down thereby maintaining a stationary position.

The static nature of the object makes it possible for stronger molecular bonding which in turn leads to a rigid conjoined surface asperity.

In surface science, the term adhesion almost always refers to dispersive adhesion.

Dispersive adhesion involves the interaction between the molecules of the liquid and the molecules of the solid.

4. STICKINESS OF CONTACT SURFACES

Sticky surfaces also give rise to static friction.

The composition of some solid materials greatly increases their adhesion and makes them even “sticky” to the touch.

For two objects to stick together their surfaces must come in contact as closely, and at as many points, as possible.

This stickiness greatly increases static friction.

Rubber and adhesive tape are examples of sticky materials that have this type of friction.

A common example of sticky surface is the household mouse glue.

Since sticky surfaces enhance static friction, sticky materials like the sticky pad are used in automobiles to keep objects intact.

Another example of sticky material is the sticky pad used in automobiles to keep objects intact.

Sticky pad is a friction device used to prevent objects from sliding on a surface, by effectively increasing the friction between the object and the surface.

It is placed on car dashboards and helps prevents car accessories from slipping over the surface.

The pad has a large friction coefficient and a multifaced sticky property.

This means it maintains complete stickiness with the dashboard surface and also with the item laid on it.

Sticky pads are efficient automobile resources since the acceleration of vehicles can cause objects on dashboards to slip off.

5. DEGREE OF SURFACE ROUGHNESS OR SMOOTHNESS

In many cases, rough surface is referred to as the primary cause of friction.

But this is not always true since adhesion, deformation and other friction causes listed above still have a role to play.

Nevertheless, the degree of the surface roughness greatly affects friction.

An increase in surface roughness will lead to higher static friction because rough surfaces greatly oppose motion in the macroscopic level and creates more contact points for irregularity interlocking.

This increased con-joined asperities, strengthens the adhesive bond which in turn leads to a higher static friction.

If two surfaces in contact are extremely rough, the surface irregularities will be easily locked together.

This is quite similar to surface irregularity mention above, but a difference exists between surface irregularity and surface roughness.

Surface roughness is the type of surface unevenness that we can see with our naked eyes, while surface irregularities is surface unevenness that’s seen only with the help of a magnifying lens.

Nevertheless, the degree of roughness of the contact surface largely affects the extent of coin-joined asperities.

A material placed on smooth surfaces has a lower static friction than materials placed on rough surfaces.

Smoothening rough surface reduces the interlocking of asperities thus causing a decrease in static friction.

The smoother the surface the lower the friction force and vice versa.

To smoothen a rough surface, you can either use sandpaper or apply a coat of high coverage primer.

SURFACE ABRASION BETWEEN CONTACTING PAIR

Abrasion is the process of scraping off or wearing away material surfaces by means of surface contact.

Surface abrasion hinders surface friction by reducing the material roughness.

When surface abrasion occurs, particles of the materials are dislodged from their surfaces. This reduces the chances needed for asperity interlocking and weakens static friction.

When we attempt to move any object, we have to apply a force to overcome interlocking.

For the object to slide over another surface, the adhesive force between the contact molecules must be broken. This process involves the abrasion of contact points.

This means that the interlocked areas must be broken apart or plastically deformed before the object can move.

Once movement is initiated, some abrasion continues to occur, but at a much-reduced level than the static position.

Still curious about static friction? see our guide: STATIC FRICTION : The Complete Guide

7 major Factors that Increase Static Friction

written by Stanley Udegbunam || Jan 4th, 2020

AFRILCATE

Have you been wondering what causes static friction to increase?

Then you just bumped into the right website. In today’s article, we will be talking about 9 major factors that increase static friction.

Without further ado, let’s get started.

The 7 factors that increase static friction are:

1. High surface irregularity
2. A well deformed material
3. High intermolecular bond
4. Increased surface roughness
5. Increased Object weight
6. High coefficient of static friction
7. High contact surface stickiness

1. High surface Irregularity

Surface irregularities are tiny rough planes that exists on surfaces.

This tiny surface roughness may not visible to the naked eyes but can easily be detected with microscopic lens.

Let’s see what this surface roughness looks like with a microscope:

Surface irregularity is also called asperity.

If a book is placed on the table, both the asperity of the table and that of the table surface will be interlocked together.

That’s why our article on causes of static friction; clearly states that the interlocking of surface irregularities leads to static friction.

microscopic view of contact surface

Surface irregularity/asperity consists of surface mountains and valleys.

The mountains of the asperity are the higher points of the irregularity or elevation, while the valleys are the lower points or depressions.

Now the question is: how do high surface irregularities increase static friction?

Surface irregularity is directly proportional to static friction force because the minute surface roughness allows for the interlocking of hills and valleys of asperities.

The greater the unevenness of contact surfaces, the greater the static friction.

Therefore, high surface irregularity will increase the static friction force.

Let’s take the book from the table above and place it on another table with a higher degree of microscopic surface unevenness.

We will call this new table, table B, and the initial one table A.

Comparing both table surfaces:

Since table B have a higher surface irregularity than table A, you should expect the same book placed on surface B to experience a higher static friction.

It’s worth knowing that all objects have microscopic rough planes regardless of how smooth or polished they may appear.

2. A Well Deformed Material

Deformation greatly increases static friction.

The higher the material deformation, the greater the static friction between the contact surfaces.

Consider these two soccer balls:

PERFECT CONDITON

SLIGHTLY DEFORMED

Which of these balls will have a greater reluctance to move when tossed into motion with the same applied force and on the same surface?

Ball 1 will have a lower reluctance to motion.

The deformed nature of ball 2 allows for higher contact areas between both surfaces and this creates a high reluctance to motion.

Once the object is set in motion, rolling friction takes over.

Just like static friction, the effect of rolling friction is also more evident in deformed objects.

3. High Intermolecular Bond

Surface Adhesion is the force of attraction between molecules of different substances.

When two objects or surfaces are brought into contact, the adhesive or electromagnetic forces attract the molecules of the two objects together.

A common example of surface adhesion is rain droplets on window surfaces just after rainfall or morning dew.

The force of adhesion between the liquid droplets and the glass surface prevents it from sliding down thereby maintaining a stationary position.

These forces of adhesion are weak intermolecular forces popularly known as the van der waal forces.

There are 5 different mechanisms of adhesion, they include:

1. Mechanical Adhesion: here, the materials fill the voids or pores of the surfaces and hold surfaces together by interlocking eg sewing
2. Chemical Adhesion: Two materials in contact may form a compound at the joint.
3. Dispersive Adhesion: the materials are held together by van der waals forces
4. Electrostatic Adhesion: A potential difference is formed at the point of contact.
5. Diffusive Adhesion: involves the merging of material at the joint by diffusion.

In surface science, the term adhesion almost always refers to dispersive adhesion.

Dispersive adhesion involves the interaction between the molecules of the liquid and the molecules of the solid.

The strength of surface adhesion is largely dependent on:

• the contact angle,
• the shape of the liquid droplet and
• the surface area upon which the two-material contact.

A higher intermolecular bond will lead to a higher adhesive force and a corresponding increase in static friction.

4. Increased Surface Roughness

An increase in surface roughness will lead to higher static friction because rough surfaces hinder motion at the macroscopic level and create more contact points for irregularity interlocking.

This increased con-joined asperity strengthens the adhesive bond which in turn leads to higher static friction.

This is quite similar to the surface irregularity mention above, but a difference exists between both factors.

While surface irregularity refers solely to the joining of materials on the microscopic level, surface roughness refers to surface unevenness at the macroscopic level.

Surface roughness is the type of surface unevenness that we can see with our naked eyes, while surface irregularity is surface unevenness that’s seen only with the help of a magnifying lens.

Nevertheless, the degree of roughness of the contact surface largely affects the extent of coin-joined asperities.

A material placed on rough surfaces has higher static friction than materials placed on smooth surfaces.

5. Increased Object Weight

The weight of an object is directly proportional to static friction.

Depending on the nature of the material in contact, the object’s weight can affect the degree or extent of the material deformation.

The normal force which is the resisting force acting on the object also depends on the weight of the object.

An increase in object weight will lead to a corresponding increase in static friction.

6. High Contact Surface Stickiness

Surface stickiness increases static friction.

An example of such material is the sticky pad used in automobiles to keep objects intact.

It is placed on car dashboards and it help prevents car accessories from slipping over the surface.

The pad has a large friction coefficient and a multifaced sticky property.

This means it maintains complete stickiness with the dashboard surface and also with the item laid on it.

Pads with a high level of stickiness tend to grip objects firmly.

Another common household example is mouse glue.

The glue board traps moving objects passing across its surface.

Let’s digress a little bit;

Rats in Africa especially in Ghana and Nigeria are getting smarter by the day.

When caught on mouse glue, they quickly urinate on it and struggle to spread it around the contact points to weaken the effect of the glue until they set loose.

As a result of this, the mouse glue is hardly used twice because it’s messed up and rendered ineffective by the 1st rat encounter.

Seldomly, you might encounter some household rat that’s strong enough to even run with the glue board.

As a result of this, mouse glue companies in Africa are continually increasing the efficiency of the trap by using high-level adhesive components strong enough to even trap humans.

A high level of material stickiness increases static friction because the sticky element present in the material has a high adhesive property.

This adhesive property makes the material to be deformed when pressed alongside creating contact entries at the points of application of the force.

The interlocking of both surfaces rapidly increases, static force is increased and a larger resisting force will be needed to break loose.

7. A high Coefficient of Static Friction

The coefficient of static friction is a numerical value that quantifies the degree of stickiness between a static object and its contact surface.

It is the ratio of the Static friction between two contact surfaces to the normal force acting on the body.

The coefficient of static friction is denoted by µs and the formula is:

 µs =  Fs/N

Where,

• Fis the static frictional force
• N is the normal force, sometimes it’s denoted by η

This means that the static friction force Fs =  N x µs

The means that the static friction is affected by both the normal force and the coefficient of friction.

The coefficient of static friction is increased by either:

1. Increasing the normal force acting on the object or
2. By increasing the coefficient of static friction

Normal force (N) = Mass (m) x Acceleration due to gravity (g)

Therefore increasing the object weight will increase static friction as discussed earlier.

Coefficient of static friction is a material property.

Increasing the coefficient of static friction entails reviewing the nature of the contact materials and also the surface condition of the surface.

For example:

The static friction coefficient of copper on mild steel on a dry surface is 0.53,

but the static friction coefficient of copper on cast iron is 1.05.

If you wish to increase static friction by increasing the static friction coefficient, you should pay proper attention to the materials under consideration.

Our guide on Static friction coefficient will give you more insight into the coefficient of static friction.

It has a comprehensive friction coefficient table where you get to see the static friction coefficient of different material types and on different surface conditions.

See it here: COEFFICIENT OF STATIC FRICTION

We are gradually coming to the end of this article and you must be wondering why it’s necessary to increase static friction even when it opposes motion right?

Well, the opposition to motion rendered by static friction, can be helpful and needed sometimes.

In automobiles, friction is needed to bring cars to a complete stop when the brake is stepped upon.

The automobile brake manufacturers take these factors discussed above when designing the brake especially the material type in order to have a fast response when depressed.

Unlike popular opinion, the presence of static friction can really be a lifesaver in some case scenario.

Still curious about static friction? see our guide: STATIC FRICTION : The Complete Guide