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Mechanical
Equilibrium
Chapter 2
Equilibrium
 What


is equilibrium?
Balanced budget
Glass thermometer reaching same temp as surroundings
 Equilibrium
is achieved when an object is stable, without
changes in motion.

Jump rope example
 Did
the rope remain straight?
 Is there any way to make the rope stay straight when someone is
pushing on the center?
 What do you think are the conditions necessary for equilibrium?
Net Force
 There
is usually more than one force acting on an
object
 Combination of all forces acting on an object is the
net force—this is what changes its motion
Force
 A force


is a push or a pull
Always required to change the state of motion of an
object
A hockey puck at rest will stay at rest and a puck in
motion will stay in motion
 A force
motion
is needed to change an object’s state of
Tension and Weight
 Spring


scales
Stretching force is called tension
In science, we measure force in terms of Newtons (N)
2


Forces acting on the fish
Equal and opposite
What is the net force?
Vectors
 Forces
are represented by arrows
 http://www.youtube.com/watch?v=A05n32Bl0aY
 When the length of an arrow is scaled to represent
the amount (magnitude) the arrow is called a
vector

Vector is an arrow that represents the magnitude and
direction of a quantity.
Quantities
 Vector

A quantity that needs both magnitude and direction
for a complete description
 Scalar

Quantity
Quantity
A quantity that can be described by magnitude only
and has no direction.
On your board (OYB):
How can you change an
object’s state of motion?
Mechanical Equilibrium
 Mechanical
equilibrium is a state wherein no
physical changes occur; it is a state of steadiness.
 The equilibrium rule states that whenever the net
force on an object is zero, the object is said to be in
mechanical equilibrium.
Expressed mathematically:
∑ F=0
Mechanical Equilibrium
 Remember
that vector quantities take into account
direction

So upward forces must be positive and downward
counted as negative
OYB:
How can you express the
equilibrium rule
mathematically?
Support Force
 What
about my book on the podium?
 Where does the upward force come from?
 Support Force is the upward force that balances
the weight of an object on a surface. (Also
referred to as normal force)
 Book and spring

Spring would push up with as much force as you
push down—book compresses atoms which act like
microscopic springs
Support Force
 Step
on a scale: what two forces act on the scale?
 Forces compress a mechanism, like a spring, that
is calibrated to show the support force, your
weight
OYB:
While standing on the scale what
can be said about the magnitude
of the support force and your
weight?
What is the net force on a
bathroom scale when a 110
pound person stands on it?
OYB:
Suppose you stand on two
bathroom scales with your weight
evenly distributed between the
two scales. That is the reading
on each of the scales? What
happens when you stand with
more of your weight on one foot
than the other?
Equilibrium for Moving Objects
 Equilibrium
is a state of no change, but that
includes more than just not moving.
 Would an object under the influence of only one
force be in equilibrium?
 A desk being pushed along the floor, if it stays at a
constant speed
OYB
 An
airplane flies horizontally at constant speed in
a straight-line direction. Its state of motion is
unchanging. In other words, it is in equilibrium.
Two horizontal forces act on the plane. One is the
thrust of the propeller that pulls it forward. The
other is the force of air resistance (air friction) that
acts in the opposite direction. Which force is
greater?
Equilibrium
 Objects
at rest are said to be in static equilibrium.
 Objects at constant speed in a straight line path are
said to be in dynamic equilibrium.
 In the car of a train…
OYB
How are static and dynamic
equilibrium different?
Vectors
 If
a gymnast is suspended by a single vertical
strand of rope, tension would equal the gymnast's
weight, lets say 300N
 If she hangs by two vertical strands, what could we
expect the tension on the ropes to become?
 What two forces are acting on the gymnast?
Vectors
 Combining
vectors when they are parallel is
simple, if in the same direction, they add
 The sum of two or more vectors is called their
resultant
 To find the resultant of two non-parallel vectors,
construct a parallelogram wherein two vectors are
adjacent sides. The diagonal of the parallelogram
shows the resultant.
 The diagonal is the resultant R
Parallelogram Rule
 http://www.youtube.com/watch?v=zwyUjFzLsuQ
 So
what is the answer to his question?
 Can you break a guitar string by pulling on its
ends? How can you break one?
 Vertical vs. horizontal clothesline
OYB
Consider what would happen if
you suspended a 10-N object
midway along a very tight,
horizontally stretched guitar
string. Is it possible for the string
to remain horizontal without a
slight sag at the point of
suspension?
Practice
 Pages
25-26
 Questions 21-31, 37-38, 40-41, 42-48