Mechanical Equilibrium
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Transcript of Mechanical Equilibrium
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MechanicalEquilibriumChapter 2
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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?
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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
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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 is needed to change an object’s state of motion
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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?
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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.
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Quantities Vector Quantity
A quantity that needs both magnitude and direction for a complete description
Scalar Quantity A quantity that can be described by magnitude only
and has no direction.
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On your board (OYB): How can you change an object’s state of motion?
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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
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Mechanical Equilibrium Remember that vector quantities take into account
direction So upward forces must be positive and downward
counted as negative
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OYB:How can you express the
equilibrium rule mathematically?
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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
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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
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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?
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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?
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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
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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?
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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…
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OYBHow are static and dynamic
equilibrium different?
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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?
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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
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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
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OYBConsider 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?
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Practice Pages 25-26 Questions 21-31, 37-38, 40-41, 42-48