Friction

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Welcome to class! 

In today’s class, we will be talking about friction. Enjoy the class!

Friction

Introduction to friction

When we throw a ball on the floor it starts moving with some velocity. But once it is rolled ideally no force is acting in the direction of motion and according to Newton’s first law the ball should keep on rolling but this does not happen. The ball stops after moving a certain distance so a force must be acting on it. That force is known as friction.

What is friction?

When a solid object moves or attempts to move over the surface of another solid object its motion is always opposed by a retarding force. This force is called friction. Thus, friction is the force-directed opposite to the direction of motion or attempted motion. The frictional force is always parallel to the surface in contact.

Types of friction

• Static friction:

The friction force that comes into play between the two surfaces when one body tends to move on the surface of the other body is called static friction.

The maximum value of static friction is called limiting friction. Example: sliding of a book or a box over the ground.

Static/Stationary Friction

µ_s =   =  ———————————— (1)

N:B

(i) R = w= mg

(ii)F_r = F_m

F_r = µ_s m_g —————————————– (2)

M_g sin

F_r

wcos = m_g cos

Static/Stationary Friction

F_m = m_g sin        ——————————————- (3)

R = w = mg cos  ————————————————- (4)

µ_s =   =  = tan ———————————– (5)

 

F_m = moving force

F_r = limiting frictional force

F_n = force applied

R = Normal reaction between surfaces

W = Weight,

µ_s = Coefficient of Static/Stationary friction.

NB:

(i) Fm = minimum parallel force to the plane required to just slide the object

(II)  = angle of friction

• Kinetic friction:

The frictional force that comes into play between the two surfaces when one body moves on the surface of the other body is called kinetic friction.

Where F¹ is the dynamic friction,

R is the normal reaction between surfaces and µ_k is the coefficient of kinetic friction.

NB: F₁ = F_m

Where F_m x s = ½ mv²

There are two types of kinetic friction. They are:

(a) Sliding friction

The friction that exists between two surfaces when one body is sliding on the other is called sliding friction. For example friction between the woodblock and the road when the woodblock slides on the road.

(b) Rolling friction

The friction that exists between two surfaces when one surface is rolling over the other is called rolling friction. For example friction between the tyre of moving vehicle and the road.

Factors affecting friction

1. Nature of surface: If there is more roughness, more is the friction. if the surface of an object is smooth, less friction is produced.
2. Weight (normal force): If the weight of an object is more, then friction is more and if the weight is less, friction is also less.
3. The surface area in contact: The larger the surface area of contact the more the friction. This means that the more the interlocking the more the friction.

Advantages (applications) of friction

1. Friction between pen and paper enables us to write on the paper.
2. It enables us to hold a pen.
3. Friction between our feet and the ground allows our movements like standing, walking and running.
4. It holds the screw in wood.
5. It allows dishes to stand on the table & many more.
6. Friction between the surface of the road and tyres of our vehicles allow the vehicles to move without slipping 

Disadvantages of friction

1. Friction produces heat, but some of the energy is wasted in overcoming friction.
2. It causes wear and tear of the sole of shoes. For example, the sole of our shoes wears out after prolonged use because of friction with the rough road surface.
3. The heat produced by friction damage rubber parts of the machine.
4. Friction causes moving objects to stop or slow down.

Reduction of friction

Methods to reduce friction are as follows-  

1. In some situations, friction is undesirable. We need to reduce it.
2. In the engines of vehicles, use of lubricants increases the efficiency as parts are not rubbed directly each other because of a thin layer of lubricant.
3.  In many machines, graphite powder, oil, grease are used as a lubricant for reducing friction.
4. Few drops of oil are poured on the hinges of a door for reducing friction, the door moves smoothly.
5. While playing carom, sprinkle the fine powder over it to reduce friction.
6. With the help of lubricant, we can reduce friction. Lubricants make a thin layer over the surface and facilitate smooth movement.

Laws of friction

1. The frictional force opposes the relative motion of two surfaces.
2. The frictional force is parallel to the surfaces in contact.
3. The frictional force is directly proportional to the normal reaction.
4. The frictional force is independent of the area of contact.
5. The frictional force depends upon the nature of the two surfaces in contact and their state of roughness.
6. The kinetic friction is independent of the relative velocities of the surfaces.

 

In our next class, we will be talking about Position, Distance and Displacement.  We hope you enjoyed the class.

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