Types of Friction- Physics Guide for Class 8

Information about Types of Friction


Types of Friction


Class 8


Class 8 Physics

Topics Covered

  • Force of Friction
  • Cause of Friction
  • Factors affecting Friction

Static and Sliding Friction

We now understand that force of friction is the force exerted by a surface when an object moves across it, or makes an effort to move across it.
Hence, we can say that there are two types of friction
  1. static friction
  2. sliding friction.
Let us perform some activities that can help us to understand the difference between the two.
Activity 1 
Try to push a large box (to make it move across the floor), by applying a small force say 'F' units.
What do we find?
The box remains at rest. Now increase the applied force to '2F' units.
What do we find now?
  • The box still remains at rest. When an external force acts on the box, the force of friction, known as static friction, comes into play and opposes the motion of the box.
  • This static friction balances the force which we exert on the box and the box remains at rest. When applied force is increased to, say '2F', static friction also increases to '2F' and again opposes the motion of the box.
  • Now, increase the applied force gradually till the box just begins to slide over the horizontal surface (floor).
This activity shows that force of static friction increases with an increase of the applied force. In other words, static friction is a self-adjusting force. However, it can increase only up to a certain limit. The maximum value of the force of static friction comes into play when the body is just sliding over the horizontal surface; it is called the limiting force of friction.

We, thus, conclude that when a force is applied on a body at rest, a force of friction, called static friction, comes into play.
This static friction opposes the applied force. On increasing the applied force, static friction also increases.
However, it can increase only up to a certain maximum value. This maximum value of the force of static friction is called limiting friction.
When the applied force is increased beyond the limiting friction, the body begins to slide over the surface on which it was resting. After this, it is the force of sliding (kinetic) friction that acts between the two surfaces. This force of friction is a little less than the (limiting) force of static friction.
Our day to day experience tells us that it is easier to keep sliding an object (once it has been put in motion) than to make it slide from rest. Let us do an activity to verify this fact. 
Activity 4 
  • Take a wooden tray/block. Place it near the one edge of a table top.
  • Keep a cylindrical pencil/rod on the opposite edge of the table top (using drawing pins).
  • Make sure the pencil/rod is free to rotate about its axis (between the drawing pins) as shown in the figure. (You can also use a pulley for this purpose).
  • Next take the plastic lid of a jar. Make three symmetric holes in it. Put three pieces of string through the holes and tie them together.
  • Next tie them to a longer string from which we can suspend the plastic lid freely. Tie the other end of the longer string to the wooden tray/block such that it passes over the pencil.
  • Take some marbles (marble chips/very small marbles). Add these marbles in the plastic lid one by one until the wooden tray/block starts just sliding.
  • Note down the number of marbles required. The number of marbles, put in the plastic lid, is an indicator of the magnitude of the limiting force of (static) friction.
  • Once the wooden tray/block begins to slide, (gently) take out a small marble from the plastic lid.
What do we observe? Pick up another such small marble. Does the tray/block stop sliding over the surface of the table top? What happens when we remove a sufficient number of marbles from the plastic lid?
  • In the first case, when the tray/block just begins to slide, the force of friction is the limiting (or maximum) value of the force of static friction. Once the tray/block begins to slide, the friction, that exists, is sliding friction.
  • When two/three (small) marbles are (gently) removed from the plastic lid, the tray/ block still keeps on sliding and moves to the other end of the table top (with almost the same speed). It shows that sliding friction is (slightly) less than static friction. 

Static friction

We call the force of friction as 'static friction' when it exists between two surfaces (in contact) between which there is no relative motion.
In other words, the force of friction, which balances the applied force during the stationary state of a body, is static friction.

Sliding friction (or Kinetic friction)

We call the force of friction, between two objects, when one of them is sliding over the surface of the other, as the (force of) sliding friction between them. Sliding friction is (a little) smaller than the static friction between the same two surfaces. 

A Simple Explanation

  • We now observe that, for a given pair of surfaces, static friction is (a little) more than sliding friction. Once motion gets started, the friction becomes slightly less than the maximum, or limiting, value of the force of static friction.
  • This can be understood as follows:
  • We can say that relative motion between two surfaces starts only when the interlockings between their irregularities (ups/downs) have been unlocked.
  • Once motion starts the irregularities act just as an obstruction against their relative motion; they are not interlocked now.
  • In other words, once the motion starts, the 'contact points' on one surface, do not get enough time to lock into the 'contact points' of the other surface.
  • Thus, it is easier to keep on moving an object (when it is already in motion) than to start it from rest.

Rolling Friction

We often see (on bus stands, railway platforms and airports) that even small children are able to carry along their suitcases easily if they are fitted with wheels. Why is it so? Let us again try the previous activity to understand the reason. 

Activity 5
Take a few pencils which are cylindrical in shape. Place them parallel to each other on the table top. Now, place the wooden tray/block over them as shown in the figure. Repeat the steps of the previous activity.
What do we observe? Do we find it easier to move the tray/block in this way than to slide it? Note down the number of marbles required in this case? Observe the difference between the two cases. Do you think that opposition to the motion of the wooden tray/block has been reduced? 
We observe that the pencils start rolling as the wooden tray/block moves. Moving the tray/block in this way is much easier than sliding it. Thus, rolling is seen to reduce the friction.

  • We observe that it is much easier 'to roll', than 'to slide', a body over a given surface. On a railway platform, or bus stop, we often notice that small children can easily pull a suitcase fitted with wheels.
  • It is because of the reduced friction, (associated with rolling), that makes it easy for the child to pull along the suitcase. It Is easy now to appreciate why labourers prefer placing logs under heavy machines/objects while moving them from one place to another place.
  • Rolling friction is, thus, the force of friction that comes into play when one body rolls over the surface of another body. It is (much) less than the (force of) sliding friction. 
We can now conclude that it is more convenient 'to roll' than 'to slide' an object over a given surface. Hence, 
Static friction > Sliding friction > Rolling friction

Wheel: A Revolutionary Invention

  • The realisation, that rolling friction is much less than sliding friction, led man to invent the wheels. The wheel has been considered one of the greatest inventions in the history of mankind.
  • It is much easier to cart a heavy load, put on a trolley with wheels, than to push it. Wheels are used extensively in daily lives for transportation.
  • They save labour and energy to a great extent. This is because of the (very much) reduced friction associated with rolling. 

Important Points

  • Proper inflated tyres roll without sliding since rolling friction is less than the sliding friction. Therefore, there is less dissipation of energy against friction. Hence, proper inflated tyres save fuel.
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