Friction and its Causes- Physics Guide for Class 8

Information about Friction and its Causes


Friction and its Causes


Class 8


Class 8 Physics

Topics Covered

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

Idea of Friction

  • It is a matter of common experience that when we roll a ball along the ground, the ball does not continue to 'keep on moving' for long. It slows down and finally comes to rest. 
  • Now consider this situation, we are riding a bicycle, and after attaining a good speed, we stop pedalling. The bicycle would be seen to gradually slow down and would stop after covering a certain distance. To make it move with a constant speed, even along a straight level road, we have to 'keep on' pedalling it.
  • We also know that we need an external force to change the speed, or direction of motion, of an object that may be initially at rest or in motion.
  • In terms of our ideas about the effects of force, we can now realise that there must be a force between the rolling ball (or the moving bicycle) and the ground. We call this force as the force of friction.
  • This force opposes any relative motion between two objects that are in contact with each other. It is due to the friction, between the ground and the surface of ball/wheel of bicycle, that the rolling ball/moving bicycle stops (after moving some distance) when the externally applied force has been removed. 
  • It is clear from above that the force of friction is a contact force.


  • We can now say that whenever an object moves, or tends to move over the surface of another object, there is a force acting between the two surfaces in contact. We call this force as the force of friction, or simply 'friction'. 
  • We also understand that this force is a contact force and always opposes, or tends to oppose, any relative motion between the two surfaces in contact.
  • The force of friction is always directed along the surfaces in contact, i.e. it acts along the 'tangential direction.

Activity 1 
To explore the relation between the force of friction and nature of the surfaces in contact. 
    • Take four matchboxes (or toy cars), sand paper, a plastic sheet, an aluminium foil, handmade paper, a wooden tray, a plastic tray, a metal tray and a sheet of waxed paper. 
    • Cover the first matchbox with sand paper, the second one with the plastic sheet, the third one with aluminium foil and the fourth one with handmade paper.
    • Put all the covered matchboxes along a line at one end of a wooden tray.
    • Gradually lift upwards and tilt that end of the wooden tray towards which the matchboxes have been kept.
    • Observe the order in which the matchboxes start moving. Also, observe the order in which they slide down to reach the other end of the wooden tray.
    • Next cover the wooden tray with a sheet of waxed paper and repeat the above steps. Do you observe any change now? Replace the wooden tray, first with a plastic tray and then with a metallic tray and again repeat the above steps. What do you observe?

    Cause of Friction

    The Activity l (and similar other observations) show that the force of friction depends on the nature of the two surfaces in contact.
    • The more is the roughness of the two surfaces, that are in contact, the more is the force of friction between them. We can, therefore, associate friction with the roughness of the surfaces in contact.
    • All surfaces have some roughness on them. Even the surfaces, which appear to be very smooth to the unaided eye, are seen to have a large number of minute irregularities (bumps or depressions), when seen under a powerful microscope.
    • The view of an apparently smooth looking surface, through a powerful microscope, invariably shows it to be uneven (rough) having ups (mountains) and downs (valleys) in it.
    We can now have a simple understanding/explanation of the cause of friction.
    • When two surfaces are put in contact, the irregularities (ups/downs) of one surface get somewhat interlocked with the irregularities of the other surface. This may be regarded somewhat similar to the interlocking of the teeth of two saws.
    • We have to apply a force to unlock this interlocking of the two surfaces (in contact) and, thereby, to enable them to move with respect to each other.
    • It is this interlocking of irregularities that may be viewed as the basic cause of a built-in opposition to any relative motion between the two surfaces in contact.
    • It is this opposition that we observe as the force of friction (or just friction), between them. 

    Factors affecting Friction

    The force of friction, between two surfaces in contact, depends on the extent of their roughness or smoothness. The force of friction is greater where rougher surfaces are involved. The smoother the surfaces, the smaller is the force of friction between them. 
    We can, therefore, say: The force of friction between two surfaces, depends on the nature of the surfaces in contact. 
    Activity 2 
    • Tie a string around a wooden block/board. Pull the block by a spring balance as shown in the figure. 
    • Note down the reading on the spring balance, when the block just begins to move. It gives a measure of the force of friction between the surface of the block and the floor. 
    • Now, keep a book on the block. Again pull the block by the spring balance. Note down the reading. Do we observe any difference in the reading of the spring balance in the above two cases? 
    • When a body moves over a horizontal surface, it presses down against the surface by a force equal to its 'weight'.
    The force of friction increases with increase in the weight of the body. Hence, in the second case, reading on the spring balance would be more. The force of friction is thus, seen to depend on the magnitude of the force (weight) pressing the two surfaces together. We can now say that the force of friction depends on:
    1. nature of the two surfaces in contact. 
    2. force pressing the two surfaces together. 

    Important Points

    • It is interesting to note that the force of friction between two surfaces does not vary with:
      (i) their apparent area of contact
      (ii) their speed relative to each other after the start of motion. 
    • When one surface is placed over another surface, humps of their molecules press against each other and get interlocked. The pressure values, at the points of contact are, therefore, high; this results in small 'joints' being formed there due to the strong (adhesive) intermolecular forces between the molecules of the surfaces in contact. These 'joints' have to be broken apart before one surface can slide over the other surface. We usually observe, and talk of, this effect in terms of the more convenient concept of the 'force of friction'.
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