Definition of Friction

According to newton’s first law of motion, a body moving uniformly along a straight line would continue to do so unless an external force applied on it.
But in real life examples like a rolling ball on a floor stops after some time.
Similarly, when we stop paddling our bicycle, it comes to rest after travelling a certain distance.
All these examples shows that there is some (invisible) force that opposes the motion of one body over the other. This opposing force is known as friction.
Hence,friction can be defined as an opposing force that comes in to play when one body actually moves or even tries to move over the surface of another body.

Type of Friction

Friction may be classified in to two types.
  1. Internal Friction

  2. Which arises one account of relative motion between every two layers of liquid. Internal friction is also called referred to as viscosity of the liquid.
  3. External Friction

  4. Which arise when two bodies in contact with each other try to move or there is an actual relative motion between the two. The external friction is also called contact friction.
    Further, external friction is of three types.
  1. Static Friction

  2. The opposing force that comes into play when one body tends to move over the surface of another, but the actual motion has yet not started is called static friction.
  3. Limiting Friction

  4. It is the maximum opposing force that comes into play, when one body is just at the verge of moving over the surface of the other body.
  5. Kinetic Friction or Dynamic Friction

  6. It is the opposing force that comes into play when one body is actually moving over the surface of another body.
    Kinetic friction is always slightly less than the limiting friction. This is because, once the motion starts actually, inertia of rest has been overcome.
Further, kinetic or dynamic friction may be of two types
  1. Sliding Friction

  2. The opposing force that comes into play when one body actually sliding over the surface of the other body is called sliding friction.
    For example, when a flat book is moved over the flat surface of a table, the opposing force is sliding friction.
  3. Rolling Friction

  4. The opposing force that comes into play when one body is actually rolling over the surface of the other body is called rolling friction.
    For example, when a wheel, a circular disc or a ring or a sphere (example ball) or a cylinder rolls over a surface, the force that opposes it, then that force is known as the rolling friction.

Coefficient of Friction

According to first law of limiting friction,
F ∝ R or F = µR
Here R is normal reaction
When a body presses against a surface, the surface deforms even if it appears to be rigid. The deformed surface pushes on the body with a normal force R that is perpendicular to the surface. This is called normal reaction.
Also, µ is a constant of proportionality and is called coefficient of limiting friction between the two surfaces in contact.
µ = F / R
µ has no units.
Coefficient of limiting friction between any two surfaces in contact is defined as the ratio of the force of limiting friction and normal reaction between them.

The value of µ depends on

  • Nature of the surfaces in contact. For example, dry or wet, rough or smooth, polished or non – polished.
  • Material of the surface in contact. For example, when two polished metal surfaces are in contact, µ ⁓ 0.2. When these surfaces are lubricated, µ ⁓ 0.1.

  • Between two smooth wooden surfaces, µ varies between 0.2 and 0.5.
    When a body is actually moving over the surface of another body, we replace F by Fk, the kinetic friction and µ by µk.
    µk = Fk / R
    µk is called the coefficient of kinetic or dynamic friction.
    As Fk < F, therefore µk is always less than µ, that is coefficient of kinetic energy or dynamic friction is always less than the coefficient of limiting friction.

    Why friction is necessary evil?

    Friction is called a necessary evil. It is important because we cannot do anything without it. But at the same time, it is an evil because it involves unnecessary waste of energy.

    Advantages of Friction

  • Walking will not possible without friction. Our foot pressing the ground for walking will slip, when there is no friction.
  • No two bodies will stick to each other without it.
  • Brakes of the vehicles will not work without friction.
  • Nuts and bolts for holding the part of machinery together will not work.
  • Writing on black board or on paper will also not be possible without friction.
  • Cleaning with sand paper will not be possible without friction.

  • And so on.

    Disadvantage of friction

  • Friction always opposes the relative motion between any two bodies in contact. Therefore, extra energy has to be spent in overcoming friction. Thus friction involves unnecessary expense of energy. That is why output is always less than the input.
  • Friction causes wear and tear of the parts of machinery in contact. Thus their life time reduces.
  • Friction forces result in the production of heat, which causes damage to the machinery. Hence it is concluded that friction is necessary evil.

  • Methods of changing friction

    Some ways to reduce friction are :

  • By Polishing

  • Polishing makes the surface smoother. Therefore, friction reduces.

  • By Lubrication

  • Lubricants like oil, grease etc. Fill up the irregularities of the surfaces, making them smoother. Hence friction decreases.

  • By proper selection of materials

  • Friction depends on nature of material of the surface in contact. For example, tyres are made of rubber. This is because friction between rubber and concrete is much less than friction between iron and concrete.

  • By Streamlining

  • Friction due to air is considerably reduced by streamlining the shape of the body moving through air. For example, jets, airplanes, fast moving cars etc.

  • By using ball bearings

  • Using ball bearings, sliding friction is converted into rolling friction which is much less. Ball bearings commonly used in bicycles, fans, fidget spinner etc.

    Some ways to increase friction :

  • On a rainy day, we throw some sand on the slippery ground. This increases friction between our feet and the ground. The chance of slipping reduce.
  • Similarly, sand is spread on tracks covered with snow. Force of friction between the wheels and the track increase and driving becomes safer.
  • In the manufacture of tyres, synthetic rubber is preferred because its coefficient of friction with the road is large. Proper threading of the tyres also increases the force of friction between the tyres and the road.
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