Friction - Definition, Types of Friction, Applications, FAQs

Friction, about friction. The theory of friction is easy to understand. The concept of friction is very important in our daily life, friction makes our life easy as well as tough. If we push a glass on the table with a certain initial velocity, it eventually comes to rest. What is the reason why glass stops after a certain velocity? Aristotle said every moving object should come to rest and Galileo said but what is the reason that a moving object comes to rest there is something that stops the object. A force that stops the moving object after a certain distance. This is friction, what is friction in science? Friction is a force. Meaning of the friction is the resistance represented by a single force..

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What is the definition of friction?

So, the Definition of friction is the opposing force that comes into play tangentially between two surfaces so as to destroy the relative motion acting between two surfaces is called friction. Whenever the surfaces of the body slide over the surface of another body each body exerts the frictional force on the other which is parallel to the surface in contact. It is always opposed to relative motion. Friction produces heat or sound or both. Here we discuss, what are the factors affecting friction or the factors on which friction depends on, angle of repose, angle of friction, the application of friction.

In fig 1, a block is placed on a horizontal surface if we apply a force F on the block in the forward direction then the friction opposes the motion and is applied in the backward direction. It is represented by f.

This is the introduction of friction.

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Types of friction:

Static friction: When we apply a small horizontal force F, the block does not move. The friction force that comes into play when a body is just about to start ( but the body is not moving ) over the surface of another body is called static frictional force.

If F is applied force on the body and the body remains at rest then static friction f_s=F.

• If there are no force acts on a body and the body is at rest then f_s=0
• It is a self-adjusting force. ( means it changes itself by the applied force ).

Limiting friction: The maximum value of the static frictional force which comes into play when a body just begins to slide over the surface of another body is called limiting frictional force.

• The force of friction is tangential or parallel to the surface in contact and its direction is opposite to the direction in which the body would start moving.
• For the given surfaces, the limiting frictional force f is

f_s∝ R

f_s = μ_sR

where μs is called the coefficient of static or limiting friction.

The coefficient of static friction is defined as the ratio of limiting friction to the normal reaction.

μ_s= f_s/R

The static friction is greater than zero and smaller than limiting friction. 0<f_s>flim.

kinetic friction: When the motion starts the frictional force acting between the surfaces decreases so that the smallest force f is equal to maintain uniform motion. The force acting between the two surfaces in relative motion is called the Kinetic frictional force fk. It is less than the limiting friction( f_lim >fk ) because once the motion start block can easily move and there is less friction force. It is proportional to the normal reaction R

f_k∝ R

f_k = μ_kR

The graph between frictional force and applied force:

A graph between the F and f is shown in fig., part AB of the graph represents the static friction fs . Its value increases linearly with the applied force F.

Point B represents the limiting friction film

Beyond B, the force of friction is seen to decreases slightly. The CD represents the kinetic friction fk. CD represents that kinetic friction does not change with applied force, it remains constants.

The angle of friction: Angle of friction is defined as the angle which the resultant P makes with the normal reaction R in the state of limiting friction is called the angle of friction. It is represented by θs.

tanθ_s= f_s/R

but μ_s = f_s/R so, tanθ_s = μ_s

Thus, the coefficient of static friction is equal to the tangent of the angle of friction.

The angle of repose: In fig, a block is placed on an inclined plane surface making an angle θ with the horizontal. The gravity force mg on the block is directed vertically downward and the normal reaction R is directed upward normal to the inclined plane. The gravity force mg can be resolved into two components, mg sinθ parallel to the inclined plane and mgcosθ perpendicular to it. Due to the parallel components mgsinθ, the block has a tendency to slide down the plane but the static frictional force fs exerted on the block by the plane is stopping it from sliding. Since the block is in equilibrium, the net force on the block parallel to the plane and also perpendicular to the plane is zero. i.e.,

f_s = mgsinθ ⇒ f_s - mgsinθ = 0 ……(i)

R = mgcosθ ⇒ R = mgcosθ = 0. …….(ii)

On increasing the angle of inclination θ, the component mgsinθ increases, but simultaneously fs increases to become equal to mgsinθ. When fs increases to its limiting value, the block is just to slide. Let the angle of inclination of the plane is θ_s. Then,

f_s =μ_sR …..(iii)

Now, using eqn (iii) in eqn (i) and (ii)

μ_sR= mgsinθ_s …..(iv)

R = mgcosθ_s ……(v)

Dividing eqn (iv) by eqn (v), we get

μ_s = tanθ_s

The coefficient of friction between ‘a’ and ‘b’ is μ. The minimum force F with which A will be pushed such that B will not slip down.

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Soln: We know f =μR …..(i)

The force acting on the block A is F

F = ma

F = (M+m)a …..(ii)

The normal force acting in the direction of force

R = ma ……(iii)

It was given in the question that A will be pushed with min. Force such that B will be

Not slip down. The static friction acts between A and B . In Fig 2 friction force is equal to mg

f = mg …..(iv)

From eqn (i) and (iv), we get

f= μR = mg

Put the value of R from eqn (iii) in the above eqn

f =μma = mg

a = g/μ

Putting this value in eqn (ii)

F = (M+m)g/μ

The angle of repose is defined as the angle of the inclined plane (θs) with a horizontal such body placed on it that has just begun to slide down is called the angle of repose.

Once the block starts sliding, it continues to slide at a constant speed at the slightly smaller angle of inclination θk (<θs). In this position

μ_k = tanθ_k

Types of kinetic friction

There are two types of kinetic friction

• Sliding friction: When two objects are sliding over one another, the friction between them is called sliding friction. For example, a block is sliding over a horizontal surface, or figure skates.
• Rolling friction: The opposing force that comes into play when a body, such as a wheel, discs, or cylinder rolls over a surface. It is a common experience that much less effort is needed to roll a drum of coal tar than to drag it along over a surface. It means that rolling friction is much less than sliding friction because a very small portion of the rolling object comes in contact with the ground at any instant and this portion is momentarily at rest. Hence no work is done against friction. This is the reason why vehicles run on wheels.
  

Rolling friction is directly proportional to the normal reaction R and the load and inversely proportional to the radius of curvature. Consider the formula:

f_rolling = μ_rR/r

where μr is called the coefficient of rolling friction.

• Work done by the frictional force is always negative by its definition.
• Internal friction: Internal friction is an opposing (internal) force that comes into play when there is the movement of particles in the material.
• Define cone of friction: If the resultant reaction rotates about the normal reaction force, it will form a cone, called a cone of friction.
• Surface friction: Surface friction is the opposition to the air flow that has motion along the earth’s surface.

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Factors affecting the friction:

• Friction depends on the surfaces in contact, it depends on the nature of the surface on their roughness (or smoothness).
  
• The frictional force depends upon the irregularities interlocking between the surfaces in contact.
• In fig, there are two blocks of the same material and mass 2kg and 5kg placed on the same surface. Now we applied the same force on both blocks. The lighter block goes the farthest distance, it covers more distance than the heavier one. It tells us friction depends upon the force with which the two surfaces are pressed together. If two surfaces are pressed harder the friction will be more. Because of the block of heavier weight, it presses on the table with greater force. Since the surface of the block and the surface of the table are pressed together harder and the irregularities between the surfaces are more that causes more friction. So force with which two surfaces are pressed together.
  

Application of friction:

In our daily life, there are many applications of friction. Friction makes our life easy and also makes our life difficult.

• Walking is possible due to friction.
• Two bodies stick together due to friction.
• Brakes work on the basis of friction.
• Writing is not possible without friction.
• The transfer of motion from one part of the machine to another part through the belts is possible by friction.
  
  

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