Friction

• Friction results from relative motion between objects.

• Frictional forces are forces that resist or oppose motion.

Types of Friction

• Static friction

• Sliding friction

• Rolling friction

Types of Friction

• Air friction

• Viscous friction

CAN I THINK ….?

Friction

Ff = μ. RN

Normal force (N)

Coefficient of friction

Friction force (N)

Calculate force of friction

• A 10 N force pushes down on a box that weighs 100 N. • As the box is pushed horizontally, the coefficient of sliding friction is

0.25.• Determine the force of friction resisting the motion. Also find F at

the point of sliding ?

Sliding Friction

Fs = sFn

Normal force (N)

Coefficient of sliding friction

Friction force (N)

COMPARISON

STATIC

• Ff OPPOSITE TO F

• Ff = F

• Ff < Fl

• Ff / Rn = CONSTANT

= μ = tanφ

Ff ≠ f ( AREA)

SLIDING

• Fs OPPOSITE TO MOTION

• F > Fl

• Fs < Fl

• Fs / Rn = CONSTANT

= μ = tanφ

• Ff ≠ f ( VELOCITY)

Table of friction coefficients

I THINK…. I CAN THINK….

Calculate using friction

• A steel pot with a weight of 50 N sits on a steel countertop.

• How much force does it take to start the pot sliding?

Calculate using friction

• The engine applies a forward force of 1,000 Newton to a 500-kg car.

• Find the acceleration of the car if the coefficient of rolling friction is 0.07.

LAWS OF FRICTION• Basic properties of friction have been described as laws:• Amontons' 1st Law: The force of friction is directly

proportional to the applied load.• Amontons' 2nd Law: The force of friction is independent

of the apparent area of contact.• Coulomb's Law of Friction: Kinetic friction is independent

of the sliding velocity.• Amontons' 2nd Law is an idealization assuming perfectly

rigid and inelastic materials. For example, wider tires on cars provide more traction than narrow tires for a given vehicle mass because of surface deformation of the tyre.

ENERGY IN FRICTION

• According to the law of conservation of energy, no energy is destroyed due to friction, though it may be lost to the system of concern. Energy is transformed from other forms into heat

• When an object is pushed along a surface, the energy converted to heat is given by:

• E = μ ∫ RN . x dx

WORK IN FRICTION• In the reference frame of the interface between two

surfaces, static friction does no work, because there is never displacement between the surfaces. In the same reference frame, kinetic friction is always in the direction opposite the motion, and does negative work.

• The work done by friction can translate into deformation, wear, and heat that can affect the contact surface properties (even the coefficient of friction between the surfaces). This can be beneficial as in polishing. The work of friction is used to mix and join materials such as in the process of friction welding.

FRICTION AN ENGINEERING PROBLEM

• Bearing seizure or failure may result from excessive wear due to work of friction. As surfaces are worn by work due to friction, and surface finish of an object may degrade until it no longer functions properly.

APPLICATION

• SCREW JACK / LIFTING MACHINE• INCLINED PLANE • WEDGE

FRICTION ANGLE• The angle of repose or, more precisely,

the critical angle of repose is the steepest angle of descent or dip of the slope relative to the horizontal plane when material on the slope face is on the verge of sliding. This angle is given by the number (0°-90°) If the coefficient of static friction is known of a material, then a good approximation of the angle of repose can be made with the following function.

• μ = tan φ , Where:μ is the coefficient of static frictionφ is the angle of repose

TYPICAL PROBLEMS…. WITH INCLINED PLANE

W

F

W COS θ

W SIN θ θ

RN

CAN I INTROSPECT…….?

TYPICAL SITUATIONS

• NUMERICALS……..

• On to the white board………