Hey we know that there are so many dissipative forces are present around us which opposes our efforts e.g. imagine that you want to slide a heavy table on irregular surfaces, in this case you will try it very hard as the irregularities present on the surface opposes your action. This force present here is nothing but frictional force. We are going to discuss the reasons of friction in this article.
Let’s learn the frictional forces in detail…..!
Definition of friction
The property which opposes the relative motion between two surfaces in contact is called friction.
According to physics if we observe any surface under microscope we will find that the surface has irregularities termed as hills and dales. When two surfaces comes in contact these hills and dales get interlocked within each other and the motion is gets opposed see the figure below.
According to modern theory, cause of friction is the force of attraction between molecules of two surfaces in actual contact in addition to the force due to the interlocking between the two surfaces. The force of attraction can be divided as
Cohesive force: Force of attraction between the molecules of identical or same substance is known as cohesive force
Adhesive force: The force of attraction between molecules of different substance is known as adhesive force.
When the surfaces in contact become more and more smooth, the actual area of contact goes on increasing. Due to this, the force of attraction between the molecules increases and hence the friction also increases.
Types of friction
1.) Static friction: The force of friction between two objects such that one object is just moved or tends to move over another surface is known as static friction.
E.g. Fruit seller is trying to pull his lorry over a road. In this case he applied more force in initial instant as the opposition on his lorry is high due to its weight. The minimum force that is applied on the object due to which the lorry just started moving or tends to move is known as limiting force of static friction.
Laws of static friction
1.) The limiting force of static friction is directly proportional to the normal reaction (N) between the two surfaces in contact.
∴ fs α N
∴ fs = μs N …………… … (1)
Where, μs = coefficient of static friction
For every substance the weight of substance is nothing but the normal reaction by surface on the substance.
∴ fs = μs mg …………… (2)
∴ μs = fs/N…………………..(3)
Hence, the coefficient of static friction is defined as the ratio of limiting force of friction to the normal reaction.
2.) The limiting force of friction is independent of the apparent area between the surfaces in contact, so long as the normal reaction remains the same.
3.) The limiting force of friction depends upon materials in contact and the nature of their surfaces.
2.) Kinetic/Sliding friction: The force of friction between two objects such that one object is sliding over another surface is known as kinetic friction.
E.g. In ice skating, long wooden pads are sliding continuously on ice. In this case the friction is considered as kinetic friction.
Laws of kinetic friction
1.) The limiting force of kinetic friction is directly proportional to the normal reaction (N) between the two surfaces in contact.
∴ fk α N
∴ fk = μk N …………… … (1)
Where, μk = coefficient of kinetic friction
For every substance the weight of substance is nothing but the normal reaction by surface on the substance.
∴ fk = μk mg …………… (2)
∴ μk = fk/N…………………..(3)
Hence, the coefficient of kinetic friction is defined as the ratio of limiting force of friction to the normal reaction.
2.) Force of kinetic friction is independent of shape and apparent area of the surfaces in contact.
3.) The Force of kinetic friction depends upon the nature and material of the surfaces in contact.
4.) The magnitude of the force of kinetic friction is independent of the relative velocity between the object and the surface provided that the relative velocity is neither too large nor too small.
3.) Rolling motion: Friction between two bodies in contact when one body is rolling over the other is called rolling friction.
Motion of a body over a surface is said to be rolling motion if the point of contact of the body with the surface keeps changing continuously
Remember that,
Static friction > Kinetic friction > Rolling friction
Advantages of friction
- We can walk due to friction between ground and feet.
- We can hold object in hand due to static friction.
- Brakes of vehicles work due to friction; hence we can reduce speed or stop vehicles.
- Climbing on a tree is possible due to friction.
Disadvantages of friction
- Friction opposes motion.
- Friction produces heat in different parts of machines. It also produces noise.
- Automobile engines consume more fuel due to friction.
Lets solve the following example to understand the friction…!
Ex.1) Heavy stone of mass 50 kg is to be moved on irregular surface of coefficient of friction 0.33. Find the limiting force of static friction
Solution:
Here, m = 50 kg, μs =0.33 , g=10 m/s2
We have,
fs = μs mg
∴ fs = 0.33 × 50 × 10
∴fs = 165 N