Friction free study note

1. What is Friction?

In everyday life, friction plays a vital role in almost every activity we perform. Walking, writing, driving vehicles, holding objects, and even the functioning of machines depends on friction. In engineering mechanics, friction is a fundamental concept that helps engineers analyze the behavior of bodies in contact.

Friction is defined as the resisting force that develops between two surfaces in contact when there is a tendency of relative motion between them or when such motion actually occurs.

Importance of Friction in our life:

• Vehicles could not move or stop safely.
• Machines would slip and fail to transmit power
• Structures would become unstable.

Causes of Friction: Although surfaces appear smooth to the naked eye, they are irregular at the microscopic level. These irregularities cause friction when surfaces are in contact. Main causes of the friction is as follows. The combined effect of these factors results in frictional resistance.

• Surface Roughness – Interlocking of microscopic asperities
• Molecular Adhesion – Attraction between molecules of contacting surfaces
• Deformation – Soft surfaces deform under load, increasing resistance.

2. Types of Friction

Friction can be broadly classified into the following three types:

3. Coefficient of Friction

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

Formula

where:

• (μ) = Coefficient of friction
• (F) = Limiting friction
• (N) = Normal reaction

Typical Values

Surface Combination	Coefficient of Friction (Approx.)
Steel on Steel (Dry)	0.50–0.80
Steel on Steel (Lubricated)	0.05–0.15
Rubber on Concrete	0.70–1.00
Wood on Wood	0.25–0.50
Ice on Ice	0.02–0.05

4. Angle of Friction

The angle of friction (ϕ) is the angle between the resultant reaction and the normal reaction when friction reaches its limiting value.

Formula

where:

• (ϕ) = Angle of friction
• (μ) = Coefficient of friction

5. Angle of Repose

The angle of repose is the maximum angle of an inclined plane at which a body just begins to slide under its own weight.

Formula

Therefore,

Angle of repose is equal to the angle of friction.

6. Cone of Friction

The cone of friction is an imaginary cone generated by the resultant reaction at limiting equilibrium.

Conditions

• Resultant inside the cone → No motion
• Resultant on the cone → Impending motion
• Resultant outside the cone → Motion occurs

The concept is widely used in robotics, machine design, gripping mechanisms, and contact analysis.

7. Advantages of Friction

• Enables walking and running.
• Makes vehicle braking possible.
• Provides grip between tyres and roads.
• Allows belt drives and clutches to transmit power.
• Helps screws, nuts, and bolts remain tightened.
• Enables writing with a pen or pencil.

8. Disadvantages of Friction

• Causes wear and tear of machine parts.
• Generates unwanted heat.
• Reduces mechanical efficiency.
• Increases power consumption.
• Produces noise and vibration.
• Shortens component life.

9. Methods to Reduce Friction

• Lubrication (oil or grease)
• Ball and roller bearings
• Surface polishing
• Low-friction materials (e.g., PTFE)
• Streamlined shapes for fluid flow
• Proper alignment of moving parts

10. Methods to Increase Friction

• Surface roughening
• Grooved tyre treads
• Brake linings
• Sand on railway tracks
• High-friction clutch and brake materials

11. Engineering Applications of Friction

• Braking systems
• Clutches
• Belt and rope drives
• Screw jacks
• Wedges
• Bearings
• Conveyor belts
• Tyre-road interaction
• Robotic grippers
• Machine tools

Comparison Between Static and Kinetic Friction

Feature	Static Friction	Kinetic Friction
Acts When	Before motion starts	During motion
Magnitude	Variable up to limiting value	Nearly constant
Formula	(F \le \mu_sN)	(F = \mu_kN)
Relative Size	Higher	Lower

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