conceptual physics fundamentals chapter 3: equilibrium and friction edited for twhs physics
TRANSCRIPT
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