Principles of Dairy Machine Design

Module 1. Statics and dynamics

Lesson 2
FRICTION, LAW OF FRICTION

2.1 Introduction

Friction is a part of our everyday life. Nearly every movement we make involves friction, and we have instinctively learned to take advantage of friction, or the lack of friction, since our childhood. Simple devices that rely on friction are everywhere around us.

2.2 Friction

The opposing force, which acts in the opposite direction of the movement of the body or particle, is called the force of friction or simply friction.

2.2.1 Static friction: It is the friction, experienced by a body, when at rest (Fig 2.1 )

Example

Fig. 2.1 Static friction

2.2.2 Dynamic friction

1. It is the friction, experienced by a body, when in motion (Fig. 2.2).

2. Dynamic friction is also called kinetic friction.

Fig. 2.2 Dynamic friction

2.3 Law of Friction

2.3.1 Law of static friction

The forces of friction always act in a direction, opposite to that in which the body tends to move, if the forces of friction would have been absent.

The magnitude of the force of friction is exactly equal to the force, which tends the body to move.

The force of friction is independent of the area of contact between the two surface. The force of friction depends upon the roughness of the surfaces. The magnitude of the limiting friction bears a constant ratio to the normal reaction between the two surfaces.

Mathematically,

F/R=Constant

2.3.2 Law of dynamic friction

• The force of friction always acts in a direction opposite to that in which the body is moving.
• The magnitude of kinetic limiting friction (F) bears a constant ratio to the normal reaction (R) between the two surfaces, known as coefficient of friction (μ).

2.3.2.1 Angle of Friction: : Consider a component of weight ‘W’ resting on an inclined plane, as shown in Fig. ( 2.3).

Fig. 2.3. Angle of friction

Let the angle of inclination (θ) be gradually increased, till the component just starts sliding down the plane. This angle of inclined plane, at which a body just begins to slide down the plane, is called the angle of friction. The body is in equilibrium under (1) Weight (W) of the body, acting vertically down wards.(2) Frictional force (F) acting up wards along the plane. (3) Normal reaction (R) acting at right angle to the plane.

2.4 Co- efficient of Friction

It is the ratio of the limiting friction (F) to the normal reaction, (R) between the two bodies, and is generally denoted by μ. Such that

μ= tan θ =F/R

F= μR

μ = co-efficient of friction.

θ = angle of friction.

F=frictional force

R=Normal reaction between the two bodies.

P= Force

Fig. 2.4 Coefficient of friction

2.4.1 Limiting force of friction

Let a component rests on a horizontal plane be acted upon by a gradually increasing force P parallel to the plane and passing through the centre of gravity of the body.

Initially, when P is small, the body does not move note it is on the point of motion. The body is said to be in non-limiting equilibrium. The magnitude of the force of friction is equal to P.

As P increases, a state is reached when the component is on the point of motion. The component is said to be in limiting equilibrium and force of friction developed is called the limiting force of friction.