You will recall that energy is the ability to do work. And that we can we  have different forms of energy in which mechanical energy is one. Mechanical energy is subdivided into two, viz: Kinetic energy and potential energy.

Under this  scheme, we shall be dwelling on “Friction”.

The generation of motion or speed(power) in one part of a machine can be transmitted to another part of the machine with the aid of drive mechanism. There are various types of drive mechanisms. The one to select depends solely on the nature of the design of the machine and the distance at which the power is to be transmitted . The drive mechanisms include the use of belt, chains and gears.

Practical Objectives

At the end of this scheme, you should be able to:

1. define friction

2.state three causes of friction.

3. state at least three types of friction.

4. Effects of friction

5. Application of friction

6. state four laws of friction.

Learning Materials

Cardboard and plywood to make gears, gear box system. bicycle and clock driving systems.


Friction is bound to take place when there is contact between two surfaces that are in motion or one in motion relative to the other. The friction may be more severe if the surfaces are  rough. As a result of that, heat develops  leafing to wear and tear of the surfaces. Friction is not limited to machines but also take  place between the soles of our shoes and ground. Only that of the machines can be reduced by lubrication.

Defnition of Friction

Friction can be defined as the force that resists the motion between two surfaces in contact or when one moves relative to the other.

Friction is the force resisting the relative motion of solid surfaces, fluid layers and material elements sliding against each other.

Causes Of Friction

1. Surface Roughness

Friction occurs because the surfaces of materials being rubbed together are not completely smooth.

Grooves or treads on one or both sliding surfaces can increase the friction, especially if the treads  have sharp edges and are not parallel with the line of motion. Critical examples of these are found in automobile and bicycle tyres.

2. Molecular Adhesion

Another cause of Friction is the molecular adhesion or attraction. This can be found in the the following:

(a) Non  Levelled-Smooth  Surfaces: if the two surfaces care levelled- smooth and flat, the friction from surface roughness is quite very less. Once the friction from molecular attraction  occurs, it can be greater than the effect of friction that occurs if the surfaces were relatively rough.

(b) Sticky Materials: Rubber is an example of a material that can have friction caused by molecular attraction.

(c) Fluids: commonly undergo molecular adhesion, thereby increasing the friction. This adhesion force is often seen in the capillary effect. This is where water will be pulled up a glass tube by the forces of molecular adhesion. That same force can slow down fluid motion.

3. Deformations

Materials that are soft will deform when under pressure. This also increased the resistance to motion. For instance, when one stands on a rug, he sinks in slightly, which causes resistance to dragging of feet along the rug’s surface.

Different Types of Friction

1. Dry Friction: opposes relative lateral motion of two solid surfaces in contact. Dry friction us subdivided into:

(a) Static friction, which is the type between non- moving surfaces, and

(b) Kinetic friction, which is the type between moving surfaces.

2. Fluid friction: describes the friction between layers within a viscous fluid that are moving relative to each other.

3. Lubricated Friction: refers to fluid friction where a fluid separates two solid surfaces.

4. Skin friction: the force opposing the motion of a solid body through a fluid . This is similar to drag.

5. Internal friction: is the force opposing motion between the components making up a solid material while it undergoes deformation.

Effects of Friction

1. It produces heat, that helps in heating parts of any object or to warm ourselves.

2. It also causes loss in power.

3. It produces noise during any kind of operation.

4. It’s because of friction that we’re able to walk, run, play, etc.

Application of Friction

Friction is an important factor in many engineering disciplines.

1. Transportation

Automobile  brakes  inherently rely on friction, slowing a vehicle by converting its kinetic energy into heat. Incidentally, dispersing this large amount of heat safely is one technical challenge in designing brake systems. Disk brakes rely on friction between a disc and brake pads that are squeezed transversely against the rotating disc. In drum brakes, brake shoes or pads are pressed outwards against a rotating cylinder (brake drum) to create friction. Since braking discs can be more efficiently cooled than drums, disc brakes have better stopping performance.

Rail  adhesion  refers to the grip wheels of a train have on the rails.

Road  slipperiness  is an important design and safety factor for automobiles

Split friction is a particularly dangerous condition arising due to varying friction on either side of a car.

Road texture affects the interaction of tires and the driving surface.

2. Measurement

• A tribometer  is an instrument that measures friction on a surface.

•A profilograph  is a device used to measure pavement surface roughness.

3. Household Usage

Friction is used to heat and ignite match sticks  (friction between the head of a matchstick and the rubbing surface of the match box).

• Sticky pads are used to prevent object from slipping off smooth surfaces by effectively increasing the friction coefficient between the surface and the object.

Laws of Friction

There are four laws of friction:

1. Friction depends on the quantity( magnitude) of the pressure(force) between the rubbing surfaces in contact

2. Friction depends on the nature ( types) of the surfaces.

3. Friction does not depend on the speed of rubbing.

4. Friction does not depend on the area( part) of the surfaces in contact �7H�K]�