7 Laws of Motion
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Transcript of 7 Laws of Motion
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NEWTONS LAWS OF
MOTION
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Introduction In previous discussions, the variety of
ways by which motion can be
described (words, graphs, diagrams,numbers, etc.) was discussed.
There are also ways in which motioncan be explained.
Explanation why objects move as theydo
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Introduction Isaac Newton (a 17th century scientist)
put forth a variety of laws which explain
why objects move (or don't move) asthey do.
These three laws have become known
as Newton's three laws of motion.
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Introduction It was known for sometime that if an
object was moving against nature, then
a force of some kind was responsible.
It is only possible because of an outsideforce.
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What is a Force? Force can be defined as a push or a
pull.
(Technically, force is something thatcan accelerate objects.)
For example, when you throw a
baseball, you apply a force to the ball.
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What is a Force? Force is measured by N (Newton).
A force that causes an object with amass of 1 kg to accelerate at 1 m/s isequivalent to 1 Newton.
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Net Force Net force is the sum of all forces acting
on an object.
For example, in a tag of war, when oneteam is pulling the tag with a force of100 N and the other with 80 N, the net
force would be 20 N at the direction ofthe first team (100 N - 80 N = 20 N).
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Early Findings When you slide your book on floor it
will stop soon.
When you slide it on icy surface, it willtravel further and then stop.
Galileo believed that when you slide a
perfectly smooth object on a frictionlessfloor the object would travel forever.
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Early Findings (The New Idea) Isaac Newton developed the idea of
Galileo further.
He concluded that an object willremain at rest or move withconstant velocity when there is no
net force acting on it. This is called Newton's First Law of
Motion, or Law of Inertia.
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Inertia The tendency of a body to maintain its
state of rest or of uniform motion in a
straight line is called inertia.
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Newtons First Law of Motion An object at rest tends to stay at rest and an
object in motion tends to stay in motion with
the same speed and in the same directionunless acted upon by an unbalanced force.
There are two parts to this statement - onewhich predicts the behavior of stationary
objects and the other which predicts thebehavior of moving objects.
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Newtons First Law of Motion
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Newtons First Law of Motion It is the natural tendency of objects to
keep on doing what they're doing.
All objects resist changes in their stateof motion.
In the absence of an unbalanced force,
an object in motion will maintain itsstate of motion.
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Applications the head of a
hammer can be
tightened onto thewooden handle bybanging the bottom
of the handleagainst a hardsurface.
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Applications to dislodge ketchup from the bottom of a
ketchup bottle, the bottle is often turned
upside down, thrust downward at a highspeed and then abruptly halted.
while riding a skateboard (or wagon orbicycle), you fly forward off the board when
hitting a curb, a rock or another object whichabruptly halts the motion of the skateboard.
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Applications
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Applications
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Applications
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Situational Analysis Consider some of your experiences in
an automobile.
Have you ever observed the behavior ofcoffee in a coffee cup filled to the rimwhile starting a car from rest or while
bringing a car to rest from a state ofmotion?
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Situational Analysis Coffee tends to "keep on doing what it is
doing."
When you accelerate a car from rest, theroad provides an unbalanced force on thespinning wheels to push the car forward; yetthe coffee (which is at rest) wants to stay atrest.
While the car accelerates forward, the coffeeremains in the same position; subsequently,the car accelerates out from under the coffee
and the coffee spills in your lap.
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Situational Analysis On the other hand, when braking from
a state of motion the coffee continues
to move forward with the same speedand in the same direction, ultimatelyhitting the windshield or the dashboard.
Coffee in motion tends to stay inmotion.
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SEAT BELTSA person in motion tends to stay in
motion with the same speed and in the
same direction ... unless acted upon bythe unbalanced force of a seat belt.
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SEAT BELTS If the car were to abruptly stop and the
seat belts were not being worn, then
the passengers in motion wouldcontinue in motion.
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SEAT BELTSAssuming a negligible amount of friction
between the passengers and the seats,
the passengers would likely bepropelled from the car and be hurledinto the air. Once they leave the car,
the passengers becomes projectiles andcontinue in projectile-like motion.
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SEAT BELTS Seat belts are used to provide safety for
passengers whose motion is governed by
Newton's laws. The seat belt provides the unbalanced force
which brings you from a state of motion to astate of rest.
Perhaps you could speculate what wouldoccur when no seat belt is used.
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MOTORCYCLES Why are motorcycles not equipped with
safety harnesses?
Is this a gross oversight made bymotorcycle manufacturers?
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MOTORCYCLESAbsolutely not!
While no transportation accident is safe, it is
the goal of the manufacturers of all roadwayvehicles to produce a vehicle whichmaximizes the safety of its riders.
In the case of a motorcycle, it is believed thatthe rider's safety is maximized by notstrapping the rider to the motorcycle
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CRITICAL THINKING In which mode of land transportation
more safe to ride, car or motorcycle?
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CRITICAL THINKING In a car accident, the safest place to be
is in the car; yet the worst place to be
is on the motorcycle. The reason?
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CRITICAL THINKING Cars are four-wheeled vehicles which
have a stable platform capable of
resisting sideways motion and resistingtipping over.
As such, being strapped to the a car in
an accident is an advantageous strategyfor maximizing passenger safety.
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CRITICAL THINKING On the other hand, a motorcycle is a
single-track vehicle (two wheels) which
are prone to tipping over and slidinginto and underneath the obstacleswhich they hit.
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Mass as a Measure of InertiaAll objects resist changes in their state
of motion.
All objects have this tendency - theyhave inertia.
But do some objects have more of a
tendency to resist changes than others?
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Mass as a Measure of Inertia The tendency of an object to resist changes
in its state of motion varies with mass.
Mass is that quantity which is solelydependent upon the inertia of an object.
The more inertia which an object has, themore mass it has.
A more massive object has a greatertendency to resist changes in its state ofmotion.
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Situational Analysis Suppose that there are two seemingly
identical bricks at rest on the table
one brick consists of mortar and
the other brick consists of Styrofoam.
Without lifting the bricks, how could
you tell which brick was the Styrofoambrick?
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Situational AnalysisYou could give the bricks an identical
push in an effort to change their state
of motion. The brick which offers the least
resistance is the brick with the least
inertia - and therefore the brick with theleast mass.
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Situational Analysis
CAUTION: do not trythese demonstrations athome.
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Mass is NOT Volume Mass and volume are entirely different.
Volume is the measure of the space an
object occupy.
An object may have a huge mass butmay or may not have a huge volume.
Example: pillow
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Mass is NOT Weight Mass is more fundamental than weight.
Mass is a measure of the amount of
material in an object. It depends on the quantity and quality
of atom the object is composed of.
Weight depends on the objects location. Mass and weight are proportional to
each other but they are not equal.
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SUMMARY Newton's first law of motion predicts the
behavior of objects for which all existing
forces are balanced. The first law - sometimes referred to as the
law of inertia - states that if the forces actingupon an object are balanced, then the
acceleration of that object will be 0 m/s2. Objects at equilibrium (the condition in
which all forces balance) will not accelerate.
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SUMMARYAccording to Newton, an object will only
accelerate if there is a net or
unbalanced force acting upon it. The presence of an unbalanced force
will accelerate an object - changing
either its speed, its direction, or both itsspeed and direction.
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Newtons Second Law of
Motion Newton's second law of motion pertains
to the behavior of objects for which all
existing forces are not balanced. The second law states that the
acceleration of an object is dependent
upon two variables - the net forceacting upon the object and the mass ofthe object.
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Newtons Second Law of
Motion The acceleration of an object depends
directly upon the net force acting upon the
object, and inversely upon the mass of theobject.
As the force acting upon an object isincreased, the acceleration of the object is
increased. As the mass of an object is increased, the
acceleration of the object is decreased.
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Newtons Second Law of
Motion
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Newtons Second Law of
Motion Newton's second law of motion can be
formally stated as follows:
The acceleration of an object as producedby a net force is directly proportional to themagnitude of the net force, in the samedirection as the net force, and inverselyproportional to the mass of the object.
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Newtons Second Law of
Motion
a = Fnet
/ m
Fnet = m * a
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Newtons Second Law of
Motion Newtons second law relates the description
of motion to the cause of motion, force.
It is one of the most fundamentalrelationships in physics. From Newtons
second law we can make a more precisedefinition of force as an action capable ofaccelerating an object.
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Situational AnalysisThe bobsled accelerates because the
team exerts a force.
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Situational Analysis Consider the force required to push a cart
whose friction is minimal.
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Newtons Third Law of Motion A force is a push or a pull upon an object
which results from its interaction with anotherobject.
Forces result from interactions!
Some forces result from contact interactions(normal, frictional, tensional, and applied
forces are examples of contact forces) andother forces are the result of action-at-a-distance interactions (gravitational, electrical,and magnetic forces).
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Newtons Third Law of MotionAccording to Newton, whenever objectsA and B interact with each other, they
exert forces upon each other. When you sit in your chair, your body
exerts a downward force on the chair
and the chair exerts an upward force onyour body.
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Newtons Third Law of Motion There are two forces resulting from this
interaction - a force on the chair and a
force on your body. These two forces are called actionand
reactionforces and are the subject of
Newton's third law of motion.
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Newtons Third Law of Motion Formally stated, Newton's third law is:
For every action, there is an equal and
opposite reaction.
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Newtons Third Law of Motion The statement means that in every
interaction, there is a pair of forces
acting on the two interacting objects. The size of the forces on the first object
equals the size of the force on the
second object.
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Newtons Third Law of Motion The direction of the force on the
first object is opposite to the
direction of the force on thesecond object.
Forces always come in pairs -
equal and opposite action-reaction force pairs.
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Situational AnalysisExample:
If your hand pushes against the edge
of a desk, the desk pushes back againstyou hand.
If a hammer strikes a nail, thehammer exerts a force on the nail, thenail exerts a force back on the hammer.
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Situational Analysis
When an ice-
skater pushesagainst therailing, therailing pushesback and thisforce causes herto move away.
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Situational Analysis
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Situational Analysis
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Situational Analysis
Consider the flying motion of birds.
A bird flies by use of its wings.
The wings of a bird push airdownwards.
Since forces result from mutual
interactions, the air must also bepushing the bird upwards.
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Situational Analysis
The size of the force on the air equals thesize of the force on the bird; the direction of
the force on the air (downwards) is oppositethe direction of the force on the bird(upwards).
For every action, there is an equal (in size)
and opposite (in direction) reaction. Action-reaction force pairs make it possible
for birds to fly.
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Situational Analysis
A common misconception is that rocketsaccelerate because the gases rushing out the
back of the engine push against the groundor the atmosphere.
What happen, instead is that a rocket exerts
a strong force on the gases, expelling them;and the gases exert an equal and oppositeforce on the rocket.
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Situational Analysis
Consider the propulsion of a fishthrough the water.
Consider the motion of a car on the wayto school.
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Situational Analysis
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Situational Analysis
While driving down the road, a firefly strikesthe windshield of a bus and makes a quite
obvious mess in front of the face of thedriver.
This is a clear case of Newton's third law ofmotion. The firefly hit the bus and the bus
hits the firefly. Which of the two forces is greater: the force
on the firefly or the force on the bus?
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Situational Analysis
Trick Question! Each force is the same size.For every action, there is an equal reaction.
The fact that the firefly splatters only meansthat with its smaller mass, it is less able towithstand the larger acceleration resultingfrom the interaction.
Besides, fireflies have guts and bug guts havea tendency to be splatterable. Windshieldsdon't have guts.