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
Equilibrium and Linear Motion
“When you’re over the hill, that’s when you pick
up speed.”
—Quincy Jones
Net Force
• Net force is
the combination of all forces that change an object’s
state of motion.
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
CHECK YOUR NEIGHBOR
A cart is pushed to the right with a force of 15 N while being
pulled to the left with a force of 20 N. The net force on the cart is
A.
B.
C.
D.
5 N to the left.
5 N to the right.
25 N to the left.
25 N to the right.
Net Force
CHECK YOUR ANSWER
A cart is pushed to the right with a force of 15 N while being
pulled to the left with a force of 20 N. The net force on the cart is
A.
B.
C.
D.
5 N to the left.
5 N to the right.
25 N to the left.
25 N to the right.
Net Force
Vector quantity
• a quantity whose description requires both
magnitude (how much) and direction (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 zero
• 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 equal in
magnitude and opposite in direction
when added, cancel to zero
bag of flour remains at rest
The Equilibrium Rule
CHECK YOUR NEIGHBOR
The equilibrium rule, F = 0, applies to
A.
B.
C.
D.
vector quantities.
scalar quantities.
both of the above.
neither of the above.
The Equilibrium Rule
CHECK YOUR ANSWER
The equilibrium rule, F = 0, applies to
A.
B.
C.
D.
vector quantities.
scalar quantities.
both of the above.
neither of the above.
Explanation:
Vector addition takes into account + and - quantities that can cancel to
zero. Two forces (vectors) can add to zero, but there is no way that two
masses (scalars) can add to zero.
Support Force
Support force (normal 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
force
support
The Support Force
CHECK YOUR NEIGHBOR
When you stand on two bathroom scales with one foot on each
scale and with your weight evenly distributed, each scale will
read
A.
B.
C.
D.
your weight.
half your weight.
zero.
more than your weight.
The Support Force
CHECK YOUR ANSWER
When you stand on two bathroom scales, with one foot on each
scale and with your weight evenly distributed, each scale will
read
A.
B.
C.
D.
your weight.
half your weight.
zero.
more than your weight.
Explanation:
You are at rest on the scales, so F = 0. The sum of the two upward
support forces is equal to your weight.
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 steady speed across a floor, it is
in dynamic equilibrium.
Equilibrium of Moving Things
CHECK YOUR NEIGHBOR
A bowling ball is in equilibrium when it
A.
B.
C.
D.
is at rest.
moves steadily in a straight-line path.
both of the above
none of the above
Equilibrium of Moving Things
CHECK YOUR ANSWER
A bowling ball is in equilibrium when it
A.
B.
C.
D.
is at rest.
moves steadily in a straight-line path.
both of the above
none of the above
The Force of Friction
Friction
• occurs when objects rub against one another
• applies to solids, liquids, and gases
• acts in a direction to oppose 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
CHECK YOUR NEIGHBOR
The force of friction can occur
A.
B.
C.
D.
with sliding objects.
in water.
in air.
all of the above
The Force of Friction
CHECK YOUR ANSWER
The force of friction can occur
A.
B.
C.
D.
with sliding objects.
in water.
in air.
all of the above
Comment:
Friction can also occur for objects at rest. If you push horizontally on
your book and it doesn’t move, then friction between the book and the
table is equal and opposite to your push.
The Force of Friction
• Friction is a Passive Force
– It exists as a response to an Applied Force
– 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