7 Laws of Motion

7/31/2019 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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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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Motion


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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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Motion

a = Fnet

/ m

Fnet = m * a


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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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When an ice-

skater pushesagainst therailing, therailing pushesback and thisforce causes herto move away.
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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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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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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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Consider the propulsion of a fishthrough the water.

Consider the motion of a car on the wayto school.


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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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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.

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7 Laws of Motion