Limiting Friction and Normal Reaction: Understand Physics In-depth

Princeton,NJ/ 360prwire/ November 22/

By definition, Friction is the resistance present between two bodies when they move against one another. Every physical material in the world causes friction. This is true for all states of matter- solid, liquid and gas.

Before we understand the Limiting Friction definition, let us understand friction as a concept in itself.

What is Friction?

Friction is caused due to different reasons between different objects in motion. The following are the three main causes of friction:

  • Molecular adhesion: A force caused at the molecular level of two objects when they come in contact.
  • Surface roughness: The rougher a surface, the more friction it causes because the rough particles contact more frequently.
  • Plowing effect: When two soft surfaces are brought in contact, they sink into each other, causing resistance and thus causing friction.

Types of friction

There are several types of friction that can happen between two bodies. Here is a list of the kinds of friction:

  • Dry friction: Friction caused between two solid surfaces. It can be further divided into kinetic friction, where the two objects are in motion or static friction, where the two objects are not in motion. 
  • Liquid friction: Friction caused between two liquids that have a viscosity to their physical attributes against one another.
  • Lubricated friction: Friction can be reduced by the use of lubricants but never erased. Thus lubricated friction is the friction caused between the surfaces of two lubricated solids.
  • Internal friction: Friction caused between the elements of any solid object.

Application of friction

The force of friction can be used for various applications. The following are some of the most common applications of the force of friction:

  • Friction is commonly applied in various aspects of engineering, especially in transportation.
  • The tyres of many automobiles are made with rougher surfaces that have deep crevices to ensure their grip on the road increases. This is possible due to friction, and the more friction, the easier it is to apply brakes at even a high-speed level.
  • The railway tracks are made using frictional contact mechanics to keep the train on tracks, even at extremely high speeds.
  • Not just the tyres, but the pavement the tyres run on is also made smoother or rougher in tune with the type of tyres running on them. Smoother pavements are designed to reduce friction and improve the speed of the vehicles, whereas more uneven pavement will slow the vehicles due to increased force of friction.
  • Moreover, to light a matchstick, you have to use friction. When a matchstick head strikes against the side of the box, the friction causes the matchstick to light up.
  • Using sticky pads for putting objects on smoother surfaces increases the force of friction between them which helps to keep the objects stationary.

Understanding the force of limiting friction

Limiting friction is the maximum friction caused between two stationary objects when they contact one another. Therefore to move the objects, the force applied must be more than the force of the limiting friction. This can be calculated by multiplying the normal reaction force and the coefficient of limiting friction.

The normal reaction force is the force an object applies when it is in contact with a flat surface. It can be calculated by using the formula,

          FN = m x g;

Where FN is the normal force, m is the mass of the object, and g is the gravitational force.

The coefficient of limiting friction is the maximum friction ratio of two objects in contact with one another at rest. 

Hence, we can conclude that limiting friction is the maximum friction between two stationary objects, just before they are set in motion by a higher external force. Below is a graph that can help understand the levels of friction between two objects, first at rest and when in motion:

Thus, it is easy to understand that limiting friction is the product of the normal reaction and the coefficient of limiting friction. Limiting friction is the maximum force of friction that can be applied to overcome the normal reaction force of a stationary object to put it in motion.

This coefficient can be calculated for any surface because each surface and angle can impact the force of limiting friction. Therefore, to calculate the coefficient, you can conduct a simple experiment.

Experiment to calculate the coefficient of limiting friction

Objective: To define the relationship between the normal reaction and limiting friction and calculate the coefficient of limiting friction of a wooden block on a perpendicular surface at rest.

Materials required: A book, a scale, and thread.

Theory: In this experiment, we are using gravity to calculate the coefficient of limiting friction. Thus, the equation for an object sliding across a perpendicular surface can be written as:

         FN = Fr x W;

Where Fr is the coefficient of limiting friction,  FN is the force of friction, and W is the object’s weight. 

Therefore, the coefficient of limiting friction is the ratio of the force of friction and the object’s weight. Thus, 

       Fr = FN/W


  • Step 1: Measure the weight of the book on the scales.
  • Step 2: Note the weight in Newtons.
  • Step 3: Tie the thread to the book.
  • Step 4: Slide the book across the table and measure the force applied to move the book by the scales.
  • Step 5: Note the force applied in Newtons.
  • Step 6: Calculate the coefficient of limiting force by dividing the force applied by the object’s weight.

Observations: During the procedure, you can calculate the difference between the force the object applies at rest and the force of the object just as it is in motion. The maximum force you can apply just before the object is set in motion.


Now that the concept of friction and the scientific reason is clear, it is easier to understand the relationship between limiting friction and normal reaction. As explained above, there are many types of friction you can study in physics that affect day-to-day life. Understanding these basic principles can be very helpful in understanding the way certain things work around you.