Chapter 5 Friction Friction A force developed between

Chapter 5 Friction

Friction • A force developed between two surfaces as they move relative to each other

Types of Friction • Two types – Kinetic Friction – Static Friction • Kinetic friction – Friction force that acts on an object while it is in motion

Static Friction • Acts on an object before it is in motion • Greater than kinetic frictional force • It takes more force to get an object moving than it does to keep it moving

Static Friction (Cont’d) • As a force is applied to an object, static frictional force starts to increase • At some point, the amount of force applied to an object exceeds the amount of static frictional force

Static Friction (Cont’d) • There are two factors that determine the maximum static force applied by an object: – Normal Force – Coefficient of Friction • Normal Force – The reaction which occurs perpendicular to the moving object and the surface

Static Friction (Cont’d) • Coefficient of Friction – Describes the ratio of the force of friction between two bodies and the force pressing them together (Normal Force)

Frictional Force Maximum Frictional Force: Ff = µ * N Ff = Maximum Frictional Force µ = Coefficient of Friction N = Normal Force

Force Needed to Overcome Friction How much force is needed to set the object in Motion? Ff = µ * N Force = 0. 74 * 500# Force = 370# Force needed to overcome static friction = 371#

Frictional Force and Inclines • When an object is placed on a surface that slopes, the amount of force needed to overcome static friction decreases

Frictional Force and Inclines (Cont’d) Example: For an object weighing 500# starting at an angle of 5 degrees having a coefficient of friction of 0. 74 Frictional force = Normal force * Coefficient of Frictional force = 368. 59 To push the object up the incline the force needed = 412. 16 To push the object down the incline the force needed = 325. 014

Frictional Force and Inclines (Cont’d) If the angle is increased by one degree Frictional force = Normal force * Coefficient of Frictional force = 367. 97 To push the object up the incline the force needed = 420. 23 To push the object down the incline the force needed = 315. 70

Wedges • An object used to develop normal force that can be used to move an object

Belt Friction • Belts are used to transmit power from one rotating shaft to another • The frictional forces between the belt and it’s mating surface help keep the belt from slipping

Belt Friction (Cont’d) Tmax = ToeµB Tmax = the maximum tension needed to get the drum rotating To = the tension on the other side E = base of the natural log (2. 718) µ = coefficient of static friction B = angle of contact between belt and drum in radians