Conceptual Physics
Fundamentals
Chapter 3:
EQUILIBRIUM AND FRICTION
Edited for TWHS Physics
This lecture will help you understand:
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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 flour
two 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 zero
bag 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 equilibrium
example: 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