Conceptual Physics Fundamentals

Chapter 3: EQUILIBRIUM AND FRICTION

Edited for TWHS Physics

This lecture will help you understand:

• Net Force• The Equilibrium Rule• Equilibrium of Moving Things• The Force of Friction

Net Force• Net force is– the combination of all forces that change an object’s state

of motion.

– it is a ________________ of the forces.example: If you pull on a box with 10 N and a friend pulls oppositely with 5 N,

the net force is 5 N in the direction you are pulling.

Net Force

Vector quantity• a quantity whose description requires both

__________ (how much) and _______ (which way)• can be represented by arrows drawn to scale, called

vectors– length of arrow represents magnitude and arrowhead

shows direction

examples: force, velocity, acceleration

The Equilibrium Rule

The equilibrium rule• the vector sum of forces acting on a non-accelerating

object equals ________• in equation form: F = 0

The Equilibrium Rule

example: a string holding up a bag of flourtwo forces act on the bag of flour:

–tension force acts upward–weight acts downward

In Equilibrium, the Forces are ________ in magnitude and ____________ in direction

when added, the sum is zerobag of flour remains at rest

Support Force

Support force (______________) is an upward force on an object that is opposite to the force of gravity.example: a book on table a compresses atoms in the table, and the compressed atoms produce the support force

Equilibrium of Moving Things

Equilibrium• a state of no change with no net force acting– static equilibrium

example: hockey puck at rest on slippery ice

– dynamic equilibriumexample: hockey puck sliding at constant speed on

slippery ice

Equilibrium of Moving Things

Equilibrium test• whether something undergoes changes in motion

example: A refrigerator at rest is in static equilibrium. If it is moved at a _____________ across a floor, it is in dynamic equilibrium.

The Force of Friction

Friction• occurs when objects rub against one another• applies to solids, liquids, and gases• acts in a direction to __________ motion

example: When an object falls down through air, the force of friction (air resistance) acts upward.

The Force of Friction• depends on the kinds of material and how much they are

pressed together• is due to tiny surface bumps and to “stickiness” of the atoms

on a material’s surface

example: friction between a crate on a smooth wooden floor is less than that on a rough floor

The Force of Friction

• Friction is a Passive Force– It exists as a response to an _____________– As the Applied Force increases, the Friction Force

also increases, until it reaches its maximum– When the Applied Force is greater than the

maximum Friction Force, the Fnet X will be unbalanced, and the object will slide.

– The equation for maximum Force of Friction is μ●FN = coefficient of Friction x Normal Force