Everyday Forces Everyday Forces Holt PhysicsHolt Physics

Chapter 4 Section 4Chapter 4 Section 4

Pages 141-148Pages 141-148

Everyday Forces Everyday Forces

WeightWeight – the magnitude of the force – the magnitude of the force of gravity acting on an objectof gravity acting on an object

Everyday Forces Everyday Forces

FFgg = mg = mgFFgg = force due to gravity (in Newtons) = force due to gravity (in Newtons)

m = mass of object (in kilograms)m = mass of object (in kilograms)

g = acceleration due to gravity (-9.81 m/sg = acceleration due to gravity (-9.81 m/s22))

* Mass ≠ Weight* Mass ≠ Weight

Everyday Forces Everyday Forces

Normal ForceNormal Force – (F – (Fnn) – a contact force ) – a contact force exerted by one object on another in exerted by one object on another in a direction perpendicular to the a direction perpendicular to the surface of contact. On a horizontal surface of contact. On a horizontal surface, Fsurface, Fnn = mg = mg

Everyday Forces Everyday Forces

Normal force diagrams- Normal force diagrams-

Everyday Forces Everyday Forces

Frictional Forces – Contact forces which Frictional Forces – Contact forces which oppose applied forcesoppose applied forces

These are the surfaces in contact at a These are the surfaces in contact at a microscopic level.microscopic level.

Everyday ForcesEveryday Forces1. Static friction - (Fs) - force exerted1. Static friction - (Fs) - force exerted

on a motionless object by theon a motionless object by the

environment to environment to

resist an resist an

external force external force

attempting to attempting to

cause a change cause a change

in motionin motion

Everyday ForcesEveryday Forces2. Kinetic friction – (Fk) – force2. Kinetic friction – (Fk) – force

exerted on a moving object thatexerted on a moving object that

opposes opposes

motionmotion

Everyday ForcesEveryday Forces * Fk < Fs due to surface adhesion * Fk < Fs due to surface adhesion

between two surfaces in contactbetween two surfaces in contact Coefficient of friction (μ) – a ratio of Coefficient of friction (μ) – a ratio of

frictional force to normal force frictional force to normal force between two objectsbetween two objects

Coefficient of FrictionCoefficient of Friction

The coefficient of friction is a ratio of The coefficient of friction is a ratio of force of friction to the normal force force of friction to the normal force acting between two objects.acting between two objects.

This completely depends on the This completely depends on the objects in contact.objects in contact.

Different experimental values have Different experimental values have been calculated based on the been calculated based on the materials in contact materials in contact (page 144: table 4-2)(page 144: table 4-2)

Coefficient of Friction EquationsCoefficient of Friction Equations

Coefficient of static frictionCoefficient of static friction

μμss = = FFs,maxs,max

FFnn

Coefficient of kinetic frictionCoefficient of kinetic friction

μμkk = = FFkk

FFnn

Everyday ForcesEveryday Forces

3. Air resistance is 3. Air resistance is

a form of friction a form of friction

as velocity as velocity

increases so does increases so does

air resistanceair resistance