# What is Frictional Force?

We often rub our hands together in the cold weather for warmth. This heat, which is released by rubbing our hands is due to the phenomenon of Friction. Friction, coupled with concepts of Force, forms an important topic of Physics, which is not only taught in class 8 Science but questions based on this concept are also framed in various entrance exams. With applications rooted in our daily lives, it is essential to understand what is Frictional Force. Read through the blog to gain a conceptual understanding of the concept.

## What is Frictional Force?

The force which comes into play whenever two objects come in contact and slide over each other is known as the frictional force. In simpler words, the force which obstructs motion when coming in contact with another object is known as Friction or Frictional Force. This force greatly varies as per the texture of the surface which is in contact.

Also Read: Basic Physics Formulas & Notes for Competitive Exams

## Factors Affecting Frictional Force

Here are some of the major factors affecting frictional force.

• The surface texture and quantity of force impelling them together have the greatest influence on the frictional force.
• The quantity of frictional force is affected by the angle and position of the object.
• When an object is put flat against another object, the frictional force is equal to the object’s weight.
• When an object is pressed against the surface, the frictional force increases and exceeds the weight of the object.

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## Examples of Frictional Force

• You must have observed that a rolling ball stops itself. Have you ever wondered why? The frictional force between the ball and the ground increases with time and there comes a moment when friction restricts the motion of the ball.
• The force of attraction or adhesion between the two objects acts as a major cause of existing friction between the points of contact. Every object, no matter how smooth it appears to the naked eye, has an irregular or rough surface with minute extensions. Friction arises when these irregularities come in contact. This also explains why you end up slipping in old and worn-out footwear. In comparison to new footwear, which has a rough sole, soles in old slippers smoothen out due to more usage.
• The reason why you are able to drive a car smoothly is because of the frictional force acting between the tire surface and the road.
• When the tip of the pen makes contact with the surface of the paper, a frictional force is created. While writing with a ballpoint pen, rolling friction is present, whereas when writing with a pencil, sliding friction is present.
• Heat is formed when the matchstick’s head is pushed against a rough surface, and this heat transforms red phosphorous to white phosphorous. White phosphorous is extremely flammable, and the matchstick catches fire. Matchsticks can sometimes fail to ignite due to the presence of water as water reduces friction.
• Friction is in charge of nailing nails into a wall. As the nail is driven into the wall, the material close to the nail compresses. This applies pressure on the nail. The friction that changes the normal force exerted by the compressed layers of the wall into the resistant shear force is referred to as this force. As a result of the friction, nails and screws are held to the walls.

### Other Examples Include:

• Pulling a luggage trolley
• Pushing a heavy carton on the floor
• Rock climbing
• Playing a game of bowling
• Combing hair
• Walking
• Flying an aeroplane
• Sliding on a garden slide

Now that you are familiar with what is the frictional force, let us dive deeper into the concept and check out some more amazing facts about it.

## Frictional Force Formula

Understanding the formula of frictional force is important for solving a variety of problems based on this concept. Mentioned below is the basic formula of friction:

F = Frictional Force
μ= Coefficient of Friction
N = Force

If two objects are placed against each other, the frictional force between the two objects will be equal to the weight of the object. If we move one object against the surface of the other, then F between the two increases and becomes more than the weight of the object. Using the aforementioned formula, we can calculate the total F that an object exerts on the other surface/object.

### Calculating the Force of Friction

• Normal Force – The force that is exerted when an object is resting on a surface is the “normal force”. According to Newton’s Law of motion, the force of the motionless object in the flat surface must resist the force due to gravity, or else the object will move. In most cases, this normal force can be formulated as:

N = mg

where m is the mass of the object and g is the acceleration due to gravity.

• Right Coefficient – The object and the particular situation defines a coefficent. When the object is not moving across the surface, you formulate this situation with static friction (μstatic). In case, the object is moving across the surface then the coefficient of sliding friction (μslide) is used.

## Different Types of Frictional Force

Now that you are clear with what is the frictional force, let us understand its types. There are two types of frictional forces namely, Dry Friction and Fluid Friction. As the name suggests, Dry Friction occurs on solid structures whereas Fluid Friction is observed in liquid surfaces or the bodies which move through the liquid. Mentioned below is a brief summary of the same.

### Dry Friction

The interaction between two solid objects which are in contact and in motion (Kinetic Friction) or when they are stationary (Static Friction) can be determined by the Dry Friction. Kinetic and Static frictions are directly proportional to the normal force between two bodies. Apart from Static and Kinetic, other dry friction forces are Sliding and Rolling friction.

### Fluid Friction

The force which resists the flow of fluid can be regarded as the fluid friction. In this case, both the fluids act as resistance in between the two surfaces. If the fluid substance in contact has less friction, it’s a greasy material and if they have higher resistance, they are called viscous fluids.

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## Application of Friction

Friction is essential in many everyday processes. When two items rub together, for example, some of the energy of motion is transformed into heat. This is why rubbing two sticks together will eventually result in the formation of a fire. Friction is also to blame for the wear and tear on bicycle gears and other mechanical components. As a result, lubricants, or liquids, are frequently used to reduce friction — and thereby wear and tear — between moving parts.

## Problems on Frictional Force

Problem 1 – A 50 N of force is applied to the 6 kg of box. If the coefficient of friction is 0, 3, calculate acceleration of the box?

Solution 1– Fnormal = 60 N – 40 N = 20 N

Friction force is –  Ffriction = µ.Fnormal = 0, 3.20N = 6N

Net force in –Y to Y = zero,

But, in –X + X direction net force is not zero

Fnet = m.a

Fnet = m.a

Fx – Ffriction = m.a

Fx – Ffriction = m.a

30 N – 6 N = 6 a

a = 4m/s²

Problem 2 – A block of mass M = 10 kg is placed on a surface which is inclined at angle θ = 45°.

Mentioning that μs = 0.5 is the coefficient of static friction is between block and the surface.

• What will be the minimum force F required to prevent slipping?
• What will be the maximum force F that can be applied without causing the block to slip?

Solution 2 – The minimum force essential to stop slipping is the minimum force that will inhibit the block from sliding down the incline.

Fmin = 10 g sin(45°) – 10 g cos(45°) x 0.5.

The maximum force that can be applied without causing the block to slip is the maximum force that can be applied without causing the block to slide up the incline.

Fmax = 10 g sin(45°) + 10 g cos(45°) x 0.5.

Fmin = 34.65 N, Fmax = 103.94 N