Table of Contents
Introduction to Friction –
It has been proved, that the surfaces of the bodies are not perfectly smooth. If you view the perfectly smooth surface under a microscope, you will find roughness and irregularities, which may not be detected by an ordinary touch or by naked eyes.
For example – If one substance is placed over another substance of the same or different material and if you try to slide across each other then there will be an opposing force, which will act in the opposite direction of the movement of the substance, which is called the force of friction or simply friction. So, in simple words, we can say that the relative motion between two parts produced friction, or friction is a force produced between two surfaces when they try to move against each other.
Advantages Of Friction –
- All types of motions are possible only due to friction. If friction is not present, we can’t be able to walk properly. That’s why people fall on smooth and wet surfaces because it has low friction.
- We use brakes to stop the vehicles and this is possible only due to friction.
- Asteroids are burnt in the atmosphere before reaching Earth due to friction.
- Transmission of powers by belts, gears, etc. is also due to friction.
Disadvantages Of Friction –
- Friction produces unnecessary heat.
- The main reason for the wastage of energy is friction.
- Friction slows down the motion of moving objects.
- Wear and Tear caused by Friction.
Types Of Friction –
Friction is generally two types:
⦁ Static friction
⦁ Dynamic friction
Static Friction –
It is the friction experienced by the bodies when they are at rest.
Dynamic Friction or Kinetic Friction –
It is the friction experienced by the bodies when they are in motion. Dynamic friction is also called kinetic friction and they are less than static friction.
Dynamic friction is further divided into three types –
⦁ Sliding friction – It is the friction, experienced by a body when it slides over another.
⦁ Rolling friction – It is the friction, experienced between the surfaces which have balls or rollers and rolls over another surface.
⦁ Pivot friction – It is the friction, experienced by a body, due to the motion of rotation.
Friction Between Lubricated and Unlubricated Surfaces.
When the lubricant is applied between two surfaces in contact, then the friction decreases. Which helps in reducing the heat and wear of the surface.
The friction experienced between two dry and unlubricated surfaces in contact is known as dry or solid friction. It is due to the high surface roughness.
Limiting Friction –
The maximum value of force that is needed when a body just begins to slide or move over the surface of the other body is known as limiting force of friction or simply limiting friction.
For example – Suppose a brick is placed over the floor and someone is applying F1 force, but the brick is not moving which means the F1 force is balanced by the force of friction.
In this case F1= F
Brick remains at rest.
Where – Applied force = F1 and Friction Force = F
Now the person increases the applied force from F1 to F2 and the bricks start moving in the direction of the applied force which means the friction force has been overcome by the applied force. So, the applied force which overcomes the friction force is known as the limiting friction.
In this case F2 > F
Brick moves in direction of applied force.
Where – Applied force = F2 and Friction Force = F
It may be noted that when the applied force is less than the limiting friction, the body remains at rest, and the friction into play is called static friction which may have any value between zero and limiting friction.
Laws Of Friction –
Laws of Static Friction –
Following are the laws of static friction:
⦁ The force of friction always acts in a direction, opposite to that in which the body tends to move.
⦁ The magnitude of the force of friction is exactly equal to the force, which tends the body to move.
⦁ The magnitude of the limiting friction (F) bears a constant ratio to the normal reaction (R) between the two surfaces.
Mathematically it can be written as – F/R = Constant
⦁ The force of friction is independent of the area of contact, between the two surfaces.
⦁ The force of friction depends upon the roughness of the surfaces.
Laws of Kinetic or Dynamic Friction –
Following are the laws of kinetic or dynamic friction:
⦁ The force of friction always acts in a direction, opposite to that in which the body is moving.
⦁ The magnitude of the kinetic friction bears a constant ratio to the normal reaction between the two surfaces. But this ratio is slightly less than that in the case of limiting friction.
⦁ For moderate speeds, the force of friction remains constant. But it decreases slightly with the increase in speed.
Laws of Solid Friction –
Following are the laws of solid friction:
⦁ The force of friction is directly proportional to the normal load between the surfaces.
⦁ The force of friction is independent of the area of the contact surface for a given normal load.
⦁ The force of friction depends upon the material of which the contact surfaces are made.
⦁ The force of friction is independent of the velocity of sliding of one body relative to the other body.
Laws of Fluid Friction –
Following are the laws of fluid friction:
⦁ The force of friction is almost independent of the load.
⦁ The force of friction reduces with the increase of the temperature of the lubricant.
⦁ The force of friction is independent of the substances of the bearing surfaces.
⦁ The force of friction is different for different lubricants.
Coefficient of Friction –
It is defined as the ratio of the limiting friction (F) to the normal reaction (R) between the two bodies. It is generally denoted by μ.
Mathematically, the coefficient of friction can be written as – μ = F/N