Friction: This is the stopping force which opposes the relative movement or the tendency of relative motion of two solid surfaces, fluid layers, and material elements when they slide each other, known as friction. It acts as tangentially along with the contact. Friction always exists in pairs.

On microscopic observation, we find friction occurs due to irregularity in surfaces.

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Types of Friction: Friction may be classified into two main types as follows:

Internal Friction: It is the force which restricts motion between the elements constituting a solid material at the time of deformation.

External Friction: It happens when two bodies which are in contact with each other have the tendency to move, or they are in actual relative motion.

There are many types of friction like Dry friction, Fluid friction, Lubricated friction, Skin friction etc. 

Fluid friction If the friction happens, in between layers of any viscous fluids that are moving with each other, the phenomenon is called fluid friction.

Lubricated friction occurs when any lubricant fluid present in between two solid objects.

Skin friction occurs when a force restricts the movement of fluid across any body’s surface.

Dry Friction: If a force restricts the relative lateral movement of two solid surfaces which are in contact with each other, the phenomenon is called dry friction or external friction, it may be further classified into static friction, limiting friction and kinetic friction. 

Static Friction: It happens in between nonmoving solid surfaces when the tendency of relative motion occurs in the bodies which are in contact and the body is at rest. It is represented by ‘fs’. Static friction never exists on its own. It opposes the impending motion that means it always takes place under the applied force. If there is no applied force, there is no static- friction. Its value is variable and self-adjusting. It has no formula as its value is variable. The maximum value of static friction is known as Limiting friction, represented as ‘flimiting’. The value of static force may be described as below:

0 ≤  fs  ≤ flimiting

Flimiting = μsN (μs is the coefficient of static friction or limiting friction and N is the normal reaction exerted by the surface)

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Laws of Static Friction:

1. The magnitude of the max. Force of the static friction is independent of the area of contact between two surfaces.

2. The magnitude of maximum force is always comparative to the normal force; therefore if the normal force increases, the maximum external force that the object can endure without moving, also increases.

Limiting Friction: It is the maximum force of static friction, i.e. achieved when a body just starts moving over another body’s surface when in contact with each other. The following equation determines its value:

Flimiting = μsN (μs is the coefficient of static friction or limiting friction and N is the normal reaction exerted by the surface)

 Laws of Limiting friction:

There are following four laws for limiting friction:

1. The force of Limiting friction between two objects is directly proportional to the normal reaction:

F ∝ N

Therefore if a body is heavy, the normal reaction, i.e. mg increases, so force increases and more force is needed to move the heavy body in comparison to a lighter body.

2. The direction of limiting friction is always opposite to the direction in which one object is just about to move over another object.

3. Limiting-friction’s force is independent of the area in between two bodies in contact.

4. Limiting friction’s force between any two objects in contact depends on the nature of their material or the roughness and smoothness they possess.


Kinetic Friction:  It happens in between moving surfaces or when motion is there. When we increase the applied force slightly more than limiting friction, the actual motion starts. The force of friction at this point is known as ‘Kinetic friction’. Its value is constant and does not depend on the relative motion and area of contact. Fk denotes it. Kinetic friction fk is directly proportional to N (normal reaction).

Fk ∝ N

Fk = μkN (μk = constant and known as the coefficient of kinetic friction, depends on the surfaces which are in contact). 

If we plot a graph in between the friction and applied force, we find that at the stage of rest, the static function is equal to the applied force as static friction is self-adjustable. Therefore the graph will be a straight line and linear. Just after the stage of limiting friction when the body is just about to start, the value of friction always decreases and then becomes constant, during the motion as shown in the following figure:

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Laws of Kinetic Friction:

There are following four laws of kinetic friction:

1. The kinetic friction always opposes the relative motion and has a constant value. Its value depends on the nature of two surfaces of the objects in contact.

2. The value of kinetic friction fk does not depend on the area of contact; therefore, the normal reaction is the same throughout.

3. The kinetic friction is not entirely dependent on velocity.

4. The value of kinetic friction is directly proportional to the normal reaction between both the surfaces in contact.



(i) The direction of kinetic friction is always opposite from relative motion.

(ii) The value of limiting friction is always more than kinetic friction, i.e. Flimiting.

(iii) The value of the static fraction is more, and it is maximum at limiting fraction, after that the value is constant during the motion. Therefore, the value of the kinetic fraction is always constant.

(iv) As the static friction is variable, there is no formula for the determination of static friction.

(v) Frictional force always opposes the relative motion, not the motion.

FAQs (Frequently Asked Questions)

1. A Boy Pushes a Cardboard of Mass 75.0 kg Box on the Flat Surface of a Floor. The Kinetic Friction’s Coefficient μk is 0.520. If the Boy Exerts 400.0 N Force Forward, then Calculate. The Force of Friction’ Magnitude?

The normal force on any object may be found out by the following  equation

N = mg

By substituting the value of η, i.e. 75.0 x 9.8 in the equation Fk = μk N, we will have (taking g as 9.8 m/s2)

Fk = (0.520)(75.0)(9.80) = 382.20 N

And to calculate the net force that moves the box we have to use the following formula:

Fnet = Fworker – Fk as the net force acting on a body is always the sum of all the forces acting on any object. Here the forces acting on the given body are the force by the man plus the kinetic friction in the opposite direction. As the forward motion is considered positive, then the net-force will be calculated as below:

Fnet = Fworker – Fk

On substitution of the values in the above equation, we get

Fnet = 400 N – 382.2 N = 17.8 N

2. A Force of 980.0 N is Required to Slide a Sledge Weighing 1200.0 kgf Over a Flat- Surface, Calculate the Coefficient of Friction?

As given fs = 980 N,

Normal-force = mg = 1200 kgf = 1200 x 9.8 N

We know that coefficient of static friction

µs = fs / N

    = 980.0 / 1200.0 x 9.8

   = 0.83

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