Chapter 12 Forces and Motion Section 12.2 Newton’s First and Second Laws of Motion Section 12.3 Newton’s Third Law of Motion and Momentum.

Chapter 12 Forces and Chapter 12 Forces and MotionMotion

Section 12.2 Newton’s First and Section 12.2 Newton’s First and Second Laws of MotionSecond Laws of Motion

Section 12.3 Newton’s Third Law Section 12.3 Newton’s Third Law of Motion and Momentumof Motion and Momentum


Aristotle, Galileo, and NewtonAristotle, Galileo, and Newton **It took about 2000 years to develop the **It took about 2000 years to develop the

understanding of the relationships understanding of the relationships between force and motion.between force and motion.

AristotleAristotle Incorrectly proposed that force is required Incorrectly proposed that force is required

to keep an object moving at constant to keep an object moving at constant speedspeed

Error held back progress in the study of Error held back progress in the study of motion for almost 2000 yearsmotion for almost 2000 years


GalileoGalileo Studied how gravity produces constant Studied how gravity produces constant

accelerationacceleration Concluded that moving objects not Concluded that moving objects not

subjected to friction or any other force subjected to friction or any other force would continue to move indefinitelywould continue to move indefinitely


NewtonNewton Built on the work of other scientists (like Built on the work of other scientists (like

Galileo)Galileo) Published his results many years later in a Published his results many years later in a

book entitled Principia (first had to define book entitled Principia (first had to define mass and force)mass and force)

Then introduced his laws of motionThen introduced his laws of motion

Isaac Newton (1642 – 1727)

During the years of 1665 and 1666, Newton developed three laws

which describe all sates of motion – rest, constant motion and

accelerated motionNewton’s 1st Law – Inertia

Newton’s 2nd Law – Acceleration cannot occur without force

Newton’s 3rd Law – Action/Reaction

Newton also proved the existence of gravity.

Newton is considered the founder of modern physics!

FACTS:

•Born Jan. 4th, 1642 in Woolsthorpe, Lincolnshire, England.

•1687 – proposed the idea of gravity.

•1705 –f first scientist to be be knighted in England.

•Early 1700’s – invented the form of math that became today’s calculus.

•Died in March 31st, 1727 in London, England.

•Buried in the famous Westminster Abbey


Newton’s First Law of MotionNewton’s First Law of Motion **Newton summarized his study of force **Newton summarized his study of force

and motion in several laws of motion.and motion in several laws of motion. Key Concept: According to Newton’s first Key Concept: According to Newton’s first

law of motion, the state of motion of an law of motion, the state of motion of an object does not change as long as the net object does not change as long as the net force acting on the object is zero.force acting on the object is zero.


Newton’s First Law of MotionNewton’s First Law of Motion Unless an unbalanced force actsUnless an unbalanced force acts, an , an

object at rest remains at rest, and an object at rest remains at rest, and an object in motion remains in motion object in motion remains in motion with the same speed and direction.with the same speed and direction.

Ex. Ball (at rest is kicked; slows down Ex. Ball (at rest is kicked; slows down from friction between the ball and the from friction between the ball and the ground)ground)

First law aka the law of inertiaFirst law aka the law of inertia.. Def.-the tendency of an object to resist Def.-the tendency of an object to resist

a change in its motiona change in its motion


InertiaInertia property of matter that resists property of matter that resists a change in motiona change in motion An object with great mass has high inertiaAn object with great mass has high inertia


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Newton’s Second Law of MotionNewton’s Second Law of Motion **How do unbalanced forces affect the **How do unbalanced forces affect the

motion of an object?motion of an object? An unbalanced force causes an object’s An unbalanced force causes an object’s

velocity to change (the object velocity to change (the object accelerates).accelerates).

The more force used, the more The more force used, the more acceleration there is.acceleration there is.

Newton: the acceleration of an object Newton: the acceleration of an object depends on its massdepends on its mass


Newton’s Second Law of MotionNewton’s Second Law of Motion Mass-a measure of the inertia of an Mass-a measure of the inertia of an

object and depends on the amount of object and depends on the amount of matter the object containsmatter the object contains

Key Concept: According to Newton’s Key Concept: According to Newton’s second law of motion, the acceleration of second law of motion, the acceleration of an object is equal to the net force acting an object is equal to the net force acting on it divided by the object’s mass.on it divided by the object’s mass.

Ie. Doubling an object’s mass will cut its Ie. Doubling an object’s mass will cut its acceleration by half.acceleration by half.

F = ma


F = maF: force (N)m: mass (kg)a: accel (m/s2)

1 N = 1 kg ·m/s2

am

F

a

Fm


The greater the The greater the mass of an object, mass of an object, the greater the the greater the force required to force required to change its change its motion.motion.


The greater the The greater the acceleration of acceleration of an object, the an object, the greater the force greater the force required to required to change its change its motion.motion.


Newton’s Second Law of MotionNewton’s Second Law of Motion The acceleration of an object is in the The acceleration of an object is in the

same direction as the net force.same direction as the net force. Newton’s 2Newton’s 2ndnd law also applies when a net law also applies when a net

force acts in the direction opposite to the force acts in the direction opposite to the object’s motion. object’s motion. (The force causes a (The force causes a deceleration that reduces the speed)deceleration that reduces the speed)

Ex. Seat belt, volleyballEx. Seat belt, volleyball

Effects of a Force on Effects of a Force on AccelerationAcceleration

Figure 13


Figure 13


Figure 13

Section 12.2 Newton’s First and Section 12.2 Newton’s First and Second LawsSecond Laws

Weight and MassWeight and Mass ****Mass and weight are not the same, but are related.Mass and weight are not the same, but are related. Weight-the force of gravity acting on an objectWeight-the force of gravity acting on an object An object’s weight is An object’s weight is the product of the objects mass the product of the objects mass

and acceleration due to gravity acting on it.and acceleration due to gravity acting on it. Weight=Mass x Acceleration due to gravity, or Weight=Mass x Acceleration due to gravity, or

W=mg; g=9.8 m/sW=mg; g=9.8 m/s22

(F)orce or (W)eight is expressed in (F)orce or (W)eight is expressed in NewtonsNewtons; ; Acceleration due to grativty (a or g) is expressed in Acceleration due to grativty (a or g) is expressed in m/sm/s22

Mass is expressed in Mass is expressed in kilogramskilograms..


Weight and MassWeight and Mass Mass and weight are proportional; Mass and weight are proportional;

doubling mass, doubles the object’s doubling mass, doubles the object’s weightweight

Key Concept: Mass is a measure of the Key Concept: Mass is a measure of the inertia of an object; weight is a measure of inertia of an object; weight is a measure of the force of gravity acting on an object.the force of gravity acting on an object.


F = 1000 kg x 0.05 m/s/sF =

F = 2000 kg x 0.05 m/s/sF =

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CalculationsCalculations What force would be required to accelerate What force would be required to accelerate

a 40 kg mass by 4 m/sa 40 kg mass by 4 m/s22??

GIVEN:

F = ?

m = 40 kg

a = 4 m/s2

WORK:

F = ma

F = (40 kg)(4 m/s2)

F = 160 N

m

F

a

CalculationsCalculations A 4.0 kg shotput is thrown with 30 N of A 4.0 kg shotput is thrown with 30 N of

force. What is its acceleration?force. What is its acceleration?

GIVEN:

m = 4.0 kg

F = 30 N

a = ?

WORK:

a = F ÷ m

a = (30 N) ÷ (4.0 kg)

a = 7.5 m/s2

m

F

a

CalculationsCalculations Mrs. J. weighs 557 N. What is her mass? Mrs. J. weighs 557 N. What is her mass?

GIVEN:

F(W) = 557 N

m = ?

a(g) = 9.8 m/s2

WORK:

m = F ÷ a

m = (557 N) ÷ (9.8 m/s2)

m = 56.8 kg

m

F

a

A A 0.2 N0.2 N B B 0.8 N0.8 N CC 1.5 N 1.5 N D D 6.0 N 6.0 N

The frog leaps from its resting position at the lake’s bank onto a lily pad. If the frog has a mass of 0.5 kg and the acceleration of the leap is 3 m/s2, what is the force the frog exerts on the lake’s bank when leaping?

Formula chart says F=ma, m is mass in kg, a is acceleration in m/s2.

So, .5 kg x 3 m/s2= 1.5 N


Newton’s Third LawNewton’s Third Law ****A force can’t exist alone. Forces A force can’t exist alone. Forces

always exist in pairs.always exist in pairs. Key Concept: According to Newton’s Key Concept: According to Newton’s

third law of motion, whenever one third law of motion, whenever one object exerts a force on a second object exerts a force on a second object, the second object exerts an object, the second object exerts an equal and opposite force on the first equal and opposite force on the first object.object.

The two forces are called: action and The two forces are called: action and reaction.reaction.


Action and Reaction ForcesAction and Reaction Forces Action force-the force exerted by the first Action force-the force exerted by the first

objectobject Reaction force- the force exerted by the Reaction force- the force exerted by the

second objectsecond object Both forces are equal in size and opposite Both forces are equal in size and opposite

in directionin direction Ex. Pushing on a wallEx. Pushing on a wall


Action-Reaction Forces and MotionAction-Reaction Forces and Motion Not all action and reaction forces produce Not all action and reaction forces produce

motion (pushing a wall).motion (pushing a wall). Action-Reaction Forces Do Not CancelAction-Reaction Forces Do Not Cancel

**Net force is not zero with action reaction **Net force is not zero with action reaction forces.forces.

b/c action and reaction forces do not act on b/c action and reaction forces do not act on the same object (swimmer in water)the same object (swimmer in water)

Only when equal and opposite forces act on Only when equal and opposite forces act on the same object do they result in a net the same object do they result in a net force of 0.force of 0.


Problem:Problem:

How can a horse How can a horse pull a cart if the cart pull a cart if the cart is pulling back on is pulling back on the horse with an equal but opposite the horse with an equal but opposite force? force?

NO!!!

Aren’t these “balanced forces” resulting Aren’t these “balanced forces” resulting in no acceleration?in no acceleration?


forces are equal and opposite but act on forces are equal and opposite but act on differentdifferent objects objects

they are not “balanced forces”they are not “balanced forces” the movement of the horse depends on the movement of the horse depends on

the forces acting the forces acting on the horseon the horse

Explanation:Explanation:


Action-Reaction PairsAction-Reaction Pairs

The hammer exerts a The hammer exerts a force on the nail to force on the nail to the right.the right.

The nail exerts an The nail exerts an equal but opposite equal but opposite force on the hammer force on the hammer to the left.to the left.


Action-Reaction PairsAction-Reaction Pairs

The rocket exerts a The rocket exerts a downward force on the downward force on the exhaust gases.exhaust gases.

The gases exert an The gases exert an equal but opposite equal but opposite upward force on the upward force on the rocket.rocket.FG

FR


Action-Reaction PairsAction-Reaction Pairs Both objects accelerate.Both objects accelerate. The amount of acceleration depends The amount of acceleration depends

on the mass of the object.on the mass of the object.

a Fm

Small mass Small mass more more accelerationacceleration

Large mass Large mass less less accelerationacceleration

F = ma

m

F

a



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MomentumMomentum Def.-the product of an objects mass and its Def.-the product of an objects mass and its

velocityvelocity ****An object with more momentum is hard to An object with more momentum is hard to

stop.stop. Key Concept: An object has a large Key Concept: An object has a large

momentum if the product of its mass and momentum if the product of its mass and velocity is large.velocity is large.

Momentum for any object at rest is 0.Momentum for any object at rest is 0. Momentum= Mass x Velocity (kg * m/s)Momentum= Mass x Velocity (kg * m/s)


MomentumMomentum quantity of motionquantity of motion

p = mvp: momentum (kg ·m/s)m: mass (kg)v: velocity (m/s)m

p

v


Conservation of MomentumConservation of Momentum What happens when objects collide?What happens when objects collide? Under certain conditions, collisions obey Under certain conditions, collisions obey

the law of conservation of momentum.the law of conservation of momentum. ***Conservation of momentum means that ***Conservation of momentum means that

momentum does not increase or decrease.momentum does not increase or decrease. If a system is closed the momentum is If a system is closed the momentum is

conservedconserved Closed system-other objects and forces Closed system-other objects and forces

cannot enter or leave a systemcannot enter or leave a system


Conservation of MomentumConservation of Momentum Objects within the system can exert forces Objects within the system can exert forces

on one another.on one another. Law of conservation of momentum-law Law of conservation of momentum-law

stating that the total momentum of a stating that the total momentum of a system does not change if no net force system does not change if no net force acts on the systemacts on the system

Key Concept: In a closed system, the loss Key Concept: In a closed system, the loss of momentum of one object equals the of momentum of one object equals the gain in momentum of another object—gain in momentum of another object—momentum is conserved.momentum is conserved.

Section 12.3 Newton’s Third Law of Section 12.3 Newton’s Third Law of Motion and MomentumMotion and Momentum

Law of Conservation of MomentumLaw of Conservation of Momentum The total momentum in a group of objects The total momentum in a group of objects

doesn’t change unless outside forces act on doesn’t change unless outside forces act on the objects.the objects.

pbefore = pafter

MomentumMomentum Find the momentum of a bumper car if it Find the momentum of a bumper car if it

has a total mass of 280 kg and a velocity of has a total mass of 280 kg and a velocity of 3.2 m/s. 3.2 m/s.

GIVEN:

p = ?

m = 280 kg

v = 3.2 m/s

WORK:

p = mv

p = (280 kg)(3.2 m/s)

p = 896 kg·m/s

m

p

v

MomentumMomentum The momentum of a second bumper car is The momentum of a second bumper car is

675 kg·m/s. What is its velocity if its total 675 kg·m/s. What is its velocity if its total mass is 300 kg? mass is 300 kg?

GIVEN:

p = 675 kg·m/s

m = 300 kg

v = ?

WORK:

v = p ÷ m

v = (675 kg·m/s)÷(300 kg)

v = 2.25 m/s

m

p

v

Newton’s LawsNewton’s Laws

11stst Law Law:: (inertia: objects tend to do what they are (inertia: objects tend to do what they are doing)doing) cannon ball will rest until a force is put on itcannon ball will rest until a force is put on it ball will roll straight until ramp puts a force on itball will roll straight until ramp puts a force on it

22ndnd Law Law:: (f = m x a) (f = m x a) greater force put on ball accelerates it moregreater force put on ball accelerates it more greater mass of ball but greater force on watergreater mass of ball but greater force on water

33rdrd Law Law:: (every action has an equal but opposite (every action has an equal but opposite reaction)reaction) ball moves right, cannon recoils leftball moves right, cannon recoils left ball move down, water splashes upball move down, water splashes up

Chapter 12 Forces and Motion Section 12.2 Newton’s First and Second Laws of Motion Section 12.3 Newton’s Third Law of Motion and Momentum.

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