Friction
Mohammad Rashik Zaman
Lecturer in Physics
Basic Science Division
World University of Bangladesh
Objectives
How friction affects movement of a body
Static friction and dynamic friction
Friction
o Friction is the resistance to motion of one object moving relative to another. It is not a fundamental force, like
gravity or electromagnetism. Instead, scientists believe it is the result of the electromagnetic attraction between
charged particles in two touching surfaces.
o Cause of Friction
 Friction is a force that resists the relative motion between two objects or materials.
The causes of the resistive force of friction are:
1. Molecular Adhesion
2. Surface Roughness
3. Plowing effect
Cause of Friction
1. Adhesion is a molecular force that arises when two materials are brought close to each other.
2. Surface roughness is a factor in friction when the materials are rough enough to cause serious abrasion.
3. When one or more of the materials are relatively soft, much of the resistance to movement is caused by deformations
or a plowing effect.
Types of friction
o We will classify the friction as mentioned below.
External friction
o External friction
External friction is basically defined as the friction due to the interaction between surfaces of two solid bodies in
contact.
External friction will be further classified in two types of friction as mentioned below.
1. Static friction
2. Dynamic friction
Static friction
o Static friction
Static friction will come in to picture when surfaces of two solid bodies in contact are at rest but there is tendency for a
relative motion.
o What are the laws/formula of static friction?
Fs =μs*N [where μs is coefficient of static friction and N is the normal force]
As per the above formula, static friction is directly proportional to the normal force.
Greater the weight of the object greater is the static friction.
o What is maximum static friction?
Maximum static friction is also called as limiting friction. As explained above, it is the
maximum value that static friction can attain. It is the value of friction when the object
is just about to move.
Dynamic friction
o Dynamic friction
Kinetic friction, also known as dynamic friction or sliding friction, occurs when two objects are moving relative to
each other and rub together (like a sled on the ground)
Dynamic friction will come in to picture when surfaces of two solid bodies in contact are in relative motion.
o What are the laws/formula of kinetic friction?
Force of kinetic friction = (coefficient of kinetic friction)(normal force)
Fk= μ k N
Where, Fk = force of kinetic friction, μk = coefficient of kinetic friction
o What is maximum kinetic friction?
The maximum force of friction which allows one body to slide upon another is called as limiting friction.
Graphically-Static and Kinetic Friction
o The frictional force between two stationary surfaces is called the static friction, fs.
o The frictional force between two sliding surfaces is called the kinetic friction, fk.
Mathematical problem
Q1. A large block of ice is being pulled across a frozen lake. The block of ice has a mass of 300 kg. The coefficient of
friction between two ice surfaces is small: μk = 0.05. What is the force of friction that is acting on the block of ice?
Solution:
On a flat surface, the normal force on an object is given by N = mg.
With this, we can find the force of friction as follows:
Ff =μN
Ff =μmg
Substituting the values in the above equation we get,
Ff =0.05 × 300 kg × 9.8 m/s2 = 157 kg-m/s2 or 157 N.
The force of friction acting in the opposite direction as the block of ice is pulled across the lake is 157 N
Mathematical problem
Q2. A man has to push his boat on the shore across the mud to get to the water. The coefficient of friction between the
boat and the mud is given by μ = 0.400. If the boat has a mass of 40 kg, calculate the magnitude of the force of friction
acting on the boat.
Solution:
On a flat surface, the normal force on an object is N = mg.
Using this, we can calculate the force of friction as follows:
F = μN,
F = μ mg Substituting the values in the equation,
we get
F = (0.400)(40.0 kg)(9.80 m/s2)
F = 156.8 N
The frictional force acting on the boat is 156.8 N
Friction in incline Plane
When a object is on an incline plane, the calculation of the frictional force will be different. Suppose, an
object creates a angle 𝜽, Then the frictional force will be
𝒇 = 𝝁𝑵𝒄𝒐𝒔𝜽
For Static
𝒇𝒔 = 𝝁𝒔 𝑵𝒄𝒐𝒔𝜽
For Kinetic
𝒇𝒌 = 𝝁𝒌 𝑵𝒄𝒐𝒔𝜽
Process how the angle derived
Mathematical Problem
A 1.5 kg block is on a 15° frictionless incline plane
What is the normal force?
Mathematical Problem
A 1.5 kg block is on a 15° frictionless incline plane
What is the force downhill?
Mathematical Problem
A 1.5 kg block is on a 15° frictionless incline plane
What is the acceleration?
Rest!!
Conclusions
hank you