forces chapter 11 table of contents section 1 laws of motion section 2 gravity section 3 newton ’...

ForcesChapter 11

Table of ContentsTable of Contents

Section 1 Section 1 Laws of MotionLaws of Motion

Section 2 Section 2 GravityGravity

Section 3 Section 3 Newton’s Third LawNewton’s Third Law

Section 1 Laws of Motion

ObjectivesObjectives

IdentifyIdentify the law that says that objects the law that says that objects change their motion only when a net force change their motion only when a net force is applied.is applied.

RelateRelate the first law of motion to important the first law of motion to important applications, such as seat belt safety applications, such as seat belt safety issues.issues.

CalculateCalculate force, mass, and acceleration force, mass, and acceleration by using Newton’s second law.by using Newton’s second law.

Chapter 11

BellringerBellringerIn some cases, an applied force is balanced by In some cases, an applied force is balanced by an opposite force, and there is no change in an opposite force, and there is no change in motion. In other cases, an applied force is not motion. In other cases, an applied force is not balanced by an opposite force, and the result is balanced by an opposite force, and the result is acceleration in the direction of the applied force. acceleration in the direction of the applied force. Look at the following illustrations, and identify Look at the following illustrations, and identify the forces and motion in the forces and motion in each one.each one.(Illustrations are shown on the next slide.)(Illustrations are shown on the next slide.)

Section 1 Laws of MotionChapter 11

BellringerBellringerSection 1 Laws of MotionChapter 11

1. In one drawing, no motion is likely to occur. 1. In one drawing, no motion is likely to occur. Which drawing is it?Which drawing is it?2. In which diagram are the forces clearly balanced? 2. In which diagram are the forces clearly balanced? How does this relate to your answer to item 1? How does this relate to your answer to item 1? If more force is exerted by the person, does the If more force is exerted by the person, does the opposite force increase to match the new force, stay opposite force increase to match the new force, stay the same or decrease?the same or decrease?

Newton’s First LawNewton’s First Law Newton’s first law of motionNewton’s first law of motion states states

thatthat an object at rest remains at rest and an object at rest remains at rest and an object in motion stays in motion an object in motion stays in motion unless it experiences an unbalanced unless it experiences an unbalanced force.force.

Objects tend to Objects tend to maintain their state of maintain their state of motion.motion.

Skateboard demoSkateboard demo


InertiaInertia is the tendency of an object to is the tendency of an object to resist being moved or, if the object is resist being moved or, if the object is moving, to resist a change in speed or moving, to resist a change in speed or direction until an unbalanced force acts direction until an unbalanced force acts on the object.on the object.

Basically inertia is the name given to Basically inertia is the name given to describe Newton’s 1describe Newton’s 1stst law. law.

Newton’s First LawNewton’s First Law Inertia Inertia is related to an object’s mass.is related to an object’s mass. Mass is a measure of inertia.Mass is a measure of inertia. The bigger the mass the bigger the The bigger the mass the bigger the

objects inertia.objects inertia.


Seat belts and car seats provide Seat belts and car seats provide protection.protection.• Because of inertia, you slide toward the side Because of inertia, you slide toward the side

of a car when the driver makes a sharp turn.of a car when the driver makes a sharp turn.• When the car you are riding in comes to a When the car you are riding in comes to a

stop, your seat belt and the friction between stop, your seat belt and the friction between you and the seat stop your forward motion.you and the seat stop your forward motion.

Video – Newtons 1Video – Newtons 1stst Law 6min Law 6min

Newton’s Second LawNewton’s Second Law Newton’s second law of motionNewton’s second law of motion states states

that the unbalanced force acting on an that the unbalanced force acting on an object equals the object’s mass times its object equals the object’s mass times its acceleration.acceleration.

Force equals mass times acceleration.Force equals mass times acceleration.Force Force == mass mass acceleration acceleration

F F == ma ma Force is measured in newtons (N).Force is measured in newtons (N).

1 N = 1 kg 1 N = 1 kg 1 m/s 1 m/s22

Video – Newtons 2nd Law 1minVideo – Newtons 2nd Law 1min


Newton’s Second LawNewton’s Second Law

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

1 N = 1 kg ·m/s2

am

F

a

Fm

Newton’s Second LawNewton’s Second Law


Math SkillsMath SkillsNewton’s Second Law Newton’s Second Law Zookeepers lift a stretcher Zookeepers lift a stretcher

that holds a sedated lion. The total mass of the that holds a sedated lion. The total mass of the lion and stretcher is 175 kg, and the lion’s lion and stretcher is 175 kg, and the lion’s upward acceleration is 0.657 m/supward acceleration is 0.657 m/s22. What is the . What is the unbalanced force necessary to produce this unbalanced force necessary to produce this acceleration of the lion and the stretcher?acceleration of the lion and the stretcher?

1. List the given and unknown values.1. List the given and unknown values.Given: Given: mass, mmass, m = 175 kg = 175 kg

acceleration, aacceleration, a = 0.657 m/s = 0.657 m/s22

Unknown: Unknown: force, F = ? force, F = ? NN


Math SkillsMath Skills2. Write the equation for Newton’s 2. Write the equation for Newton’s

second law.second law.force = mass force = mass accelerationaccelerationF = maF = ma

3. Insert the known values into the 3. Insert the known values into the equation, and solve.equation, and solve.FF = 175 kg = 175 kg 0.657 m/s 0.657 m/s22

FF = 115 kg = 115 kg m/s m/s22 = 115 N = 115 N


Newton’s Second LawNewton’s Second Law Newton’s second law can also be Newton’s second law can also be

stated as follows:stated as follows:The acceleration of an object is The acceleration of an object is proportional to the net force on the proportional to the net force on the object and inversely proportional to the object and inversely proportional to the object’s mass.object’s mass.

acceleration force

mass

a F

m


Mike's car, which weighs 1,000 kg, is out of gas. Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s/s. Using and he makes the car go 0.05 m/s/s. Using Newton's Second Law, you can compute how Newton's Second Law, you can compute how much force Mike is applying to the car. much force Mike is applying to the car.

Answer = _____________Answer = _____________

50 newtons 50 newtons

How much force would the lady have How much force would the lady have to apply to the van?to apply to the van?

Video- Newtons 2Video- Newtons 2ndnd law 2min law 2min

Consider the motion of a Hot Wheels car Consider the motion of a Hot Wheels car down an incline down an incline

This animation depicts some additional This animation depicts some additional information about the car's motion. The information about the car's motion. The velocity and acceleration of the car are velocity and acceleration of the car are depicted by vector arrows. The direction of depicted by vector arrows. The direction of these arrows are representative of the these arrows are representative of the direction of the velocity and acceleration direction of the velocity and acceleration vectors. Note that the velocity vector is vectors. Note that the velocity vector is always directed in the same direction always directed in the same direction which the car is moving which the car is moving

Section 2 Gravity

ObjectivesObjectives ExplainExplain that gravitational force becomes stronger that gravitational force becomes stronger

as the masses increase and rapidly becomes weaker as the masses increase and rapidly becomes weaker as the distance between the masses increases.as the distance between the masses increases.

EvaluateEvaluate the concept that free-fall acceleration the concept that free-fall acceleration near Earth’s surface is independent of the mass of near Earth’s surface is independent of the mass of the falling object.the falling object.

DemonstrateDemonstrate mathematically how free-fall mathematically how free-fall acceleration relates to weight.acceleration relates to weight.

DescribeDescribe orbital motion as a combination of two orbital motion as a combination of two motions.motions.

Chapter 11

Law of Universal GravitationLaw of Universal Gravitation Sir Isaac Newton (1642–1727) generalized Sir Isaac Newton (1642–1727) generalized

hishis observations on gravity observations on gravity in a law now in a law now known as theknown as the law of universal gravitation.law of universal gravitation.

Universal Gravitation EquationUniversal Gravitation Equation F G

m1m

2

d 2

• mm11 and and mm22 are the masses of the two objects are the masses of the two objects• dd is the distance between the two objects is the distance between the two objects• GG is a constant for gravity (6.67 X 10 is a constant for gravity (6.67 X 10-11-11 N m N m22/kg/kg2)2)

Video- universal gravitation Bill Nye- 2minVideo- universal gravitation Bill Nye- 2minVideo – universal gravitation- 2minVideo – universal gravitation- 2min

Section 2 GravityChapter 11

Got Gravity???Got Gravity???

Law of Universal Law of Universal GravitationGravitation All matter is affected by gravity.All matter is affected by gravity.

• Two objects, whether large or small, always Two objects, whether large or small, always have a gravitational force between them.have a gravitational force between them.

• When something is very large, like Earth, When something is very large, like Earth, the force is large.the force is large.

Gravitational force Gravitational force increases as mass increases as mass increases. More Mass= More Gravityincreases. More Mass= More Gravity

Video – gravitation 2minVideo – gravitation 2min Gravitational force Gravitational force decreases as decreases as

distance increases. More distance = distance increases. More distance = less gravityless gravity


Kite flying link- 3.5 minKite flying link- 3.5 min

Law of Universal GravitationLaw of Universal Gravitation


Greater distance from Earth= Greater distance from Earth= Less GravityLess Gravity

Free Fall and WeightFree Fall and Weight Free fallFree fall is the motion of a body when is the motion of a body when

only the force of gravity is acting on the only the force of gravity is acting on the body.body.

Free-fall acceleration near Earth’s Free-fall acceleration near Earth’s surface is constant.surface is constant.• If we disregard air resistance, all objects If we disregard air resistance, all objects

near Earth accelerate atnear Earth accelerate at 9.8 m/s 9.8 m/s22..• Freefall acceleration Freefall acceleration is often abbreviated as is often abbreviated as

the letter the letter g, g, so so gg = 9.8 m/s = 9.8 m/s22..


Elephant and feather fallingElephant and feather fallingWith air resistance and withoutWith air resistance and without

Free Fall and WeightFree Fall and Weight

WeightWeight is equal to mass times free-fall is equal to mass times free-fall acceleration.acceleration.

weightweight = = mamass x ss x gravitygravity

w = mgw = mg


Weight is different from mass.Weight is different from mass.• Mass Mass is a measure of the amount of is a measure of the amount of

matter in an object. Mass of an object is matter in an object. Mass of an object is always the same anywhere in the always the same anywhere in the universe.universe.

• Weight Weight is the pull of gravity on an is the pull of gravity on an objects mass.objects mass.

• Gravity can change so weight can Gravity can change so weight can change.change.

Weight of 150 lb person on Weight of 150 lb person on EarthEarth

(At sea level)(At sea level)

Mercury- 55 lbsMercury- 55 lbs Venus- 130 lbsVenus- 130 lbs Jupiter- 380 lbsJupiter- 380 lbs

Top of Mt. EverestTop of Mt. Everest = 148 pounds= 148 pounds

BellringerBellringerRecall that weight is defined as a measure of the gravitational Recall that weight is defined as a measure of the gravitational force exerted on an object. Use knowledge you have about force exerted on an object. Use knowledge you have about gravity to answer the questions in the following situations:gravity to answer the questions in the following situations:1.1. Elvis is a student whose mass is 70 kg. On Earth’s Elvis is a student whose mass is 70 kg. On Earth’s surface, Elvis weighs about 690 N. Suppose Elvis surface, Elvis weighs about 690 N. Suppose Elvis could stand on the surface of the following bodies in could stand on the surface of the following bodies in the solar system. In the blanks provided, match Elvis’ the solar system. In the blanks provided, match Elvis’ weight with the letter of the appropriate body. (Note weight with the letter of the appropriate body. (Note that Earth has a mass of 6.0 x 10that Earth has a mass of 6.0 x 102424 kg.) kg.)PlanetPlanet Elvis’ weightElvis’ weighta.a. Jupiter ( Jupiter (mm = 1.9 x 10 = 1.9 x 102727 kg) kg) 780 N _______780 N _______b.b. Venus ( Venus (mm = 4.9 x 10 = 4.9 x 102424 kg) kg) 113 N _______113 N _______c.c. Neptune ( Neptune (mm = 1.0 x 10 = 1.0 x 102626 kg) kg) 260 N _______260 N _______d.d. Mercury ( Mercury (mm = 3.3 x 10 = 3.3 x 102323 kg) kg) 1800 N _______1800 N _______e.e. Earth’s moon ( Earth’s moon (mm = 7.4 x 10 = 7.4 x 102222 kg) kg) 620 N _______620 N _______


Bellringer, Bellringer, continuedcontinued

2.2. Suppose Elvis is in orbit around Venus Suppose Elvis is in orbit around Venus at a distance twice as far from the at a distance twice as far from the planet’s center as the surface of Venus is. planet’s center as the surface of Venus is. Would you expect his weight to be greater Would you expect his weight to be greater than, less than, or equal to his weight on than, less than, or equal to his weight on the surface of the planet?the surface of the planet?


Free Fall and WeightFree Fall and Weight Velocity is constant when air Velocity is constant when air

resistance balances weight.resistance balances weight. The constant velocity of a falling The constant velocity of a falling

object when the force of air object when the force of air resistance is equal in magnitude and resistance is equal in magnitude and opposite in direction to the force of opposite in direction to the force of gravity is called thegravity is called the terminal terminal velocity.velocity.

Video- falling objects 6minVideo- falling objects 6min


Terminal VelocityTerminal Velocity


QuestionQuestion When a skydiver jumps from a

helicopter, his terminal velocity before opening the parachute reaches approximately 320 km/h.

Why does the rate of descent of the skydiver slow when the parachute opens? After all, the skydiver still has the same mass.

AnswerAnswer When the skydiver’s parachute is unopened, his

surface area is much less than when the parachute is open. Therefore, the air resistance is much less and the skydiver will fall at a faster rate. Once the parachute opens, the surface area of the parachute creates far more resistance with the air and the skydiver’s descent is slowed. The force of gravity is exactly the same, and the mass has not changed, but the air resistance is greater.

Free Fall and MotionFree Fall and Motion Orbiting objects are in free fall.Orbiting objects are in free fall. The moon stays in orbit The moon stays in orbit around Earth because around Earth because Earth’s Earth’s

gravitational force gravitational force provides provides a pull on the moon.a pull on the moon.

Two motions combine to cause Two motions combine to cause orbiting. orbiting.

Orbiting =Forward motion + gravity.Orbiting =Forward motion + gravity.


Two Motions Cause Two Motions Cause OrbitingOrbiting


Projectile Motion and Projectile Motion and GravityGravity Projectile motionProjectile motion is the curved path an is the curved path an

object follows when thrown, launched, or object follows when thrown, launched, or otherwise projected near the surface of otherwise projected near the surface of Earth.Earth.


Projectile MotionProjectile Motion


Projectile motion hasProjectile motion has two components— two components—horizontal(Forward) and vertical horizontal(Forward) and vertical motion(gravity/ Free fall). The 2 motions motion(gravity/ Free fall). The 2 motions combine to form the actual curved path of combine to form the actual curved path of the object. the object. The two components are independent of The two components are independent of each other. each other.

Projectile Motion and Projectile Motion and GravityGravity


What is projectile motion?What is projectile motion?

Periodic MotionPeriodic Motion Periodic motion is motion that repeats itself. Periodic motion is motion that repeats itself.

((A projectile motion, since it doesn't repeat, is A projectile motion, since it doesn't repeat, is not periodic).not periodic). Periodic motion is performed, for Periodic motion is performed, for example, by a rocking chair, a bouncing ball, a example, by a rocking chair, a bouncing ball, a vibrating tuning fork, a swing in motion, and vibrating tuning fork, a swing in motion, and the Earth in its orbit around the Sun.the Earth in its orbit around the Sun.

Rotation vs. RevolutionRotation vs. Revolution RotationRotation means an object spinning on an means an object spinning on an

axis. axis. Ex- top spinning or the Earth rotates 1 time Ex- top spinning or the Earth rotates 1 time in 24 hours.in 24 hours.

RevolutionRevolution- is the elliptical path an object - is the elliptical path an object takes. Ex- Earth travels around Sun 1 time takes. Ex- Earth travels around Sun 1 time in 365 days.in 365 days.

http://www.uwsp.edu/geo/faculty/ritter/images/atmosphere/energy/elliptical_orbit.jpg

Section 3 Newton’s Third Law

ObjectivesObjectives ExplainExplain that when one object exerts a that when one object exerts a

force on a second object, the second force on a second object, the second object exerts a force equal in size and object exerts a force equal in size and opposite in direction on the first object.opposite in direction on the first object.

ShowShow that all forces come in pairs that all forces come in pairs commonly called action and reaction commonly called action and reaction pairs.pairs.

RecognizeRecognize that all moving objects have that all moving objects have momentum.momentum.

Chapter 11

BellringerBellringerYou have learned that forces You have learned that forces account for changes in the account for changes in the motion of objects. Using what you motion of objects. Using what you have learned, explained what have learned, explained what happens in the following happens in the following situation:situation:

An ice skater holding a basketball An ice skater holding a basketball is standing on the surface of a is standing on the surface of a frozen pond. The skater throws frozen pond. The skater throws the ball forward. At the same the ball forward. At the same time, the skater slides on the ice time, the skater slides on the ice in the opposite direction.in the opposite direction.

Section 3 Newton’s Third LawChapter 11

Bellringer, Bellringer, continuedcontinued

1.1. Is the force on the ball Is the force on the ball greater than, less than, or greater than, less than, or equal to the opposite force on equal to the opposite force on the skater?the skater?

2.2. Is the acceleration of the Is the acceleration of the ball greater than, less than, or ball greater than, less than, or equal to the acceleration of equal to the acceleration of the skater? (Hint: Remember the skater? (Hint: Remember Newton’s Second Law.)Newton’s Second Law.)

3.3. Explain your answers. Explain your answers.


Action and Reaction ForcesAction and Reaction Forces Newton’s third law of motionNewton’s third law of motion states states

that for every that for every actionaction force, there is an force, there is an equal and opposite equal and opposite reactionreaction force. force.

Forces always occur inForces always occur in action-reaction action-reaction pairs.pairs.

Action-reaction force pairs Action-reaction force pairs are equal in are equal in size and opposite in direction.size and opposite in direction.

Video- Newtons 3Video- Newtons 3rdrd Law 1min Law 1min


http://images.google.com/imgres?imgurl=http://www.sciencetoymaker.org/balloon/images/newton.gif&imgrefurl=http://www.sciencetoymaker.org/balloon/links.html&h=443&w=394&sz=9&hl=en&start=38&sig2=SfsSu8su_uidvOU1GBvkLw&um=1&tbnid=MpfGc8ZVPKMPEM:&tbnh=127&tbnw=113&ei=DoCeR7r0AqOUgQLttfHYBw&prev=/images%3Fq%3Dnewtons%2Bsecond%2B%2Blaw%26start%3D21%26ndsp%3D21%26svnum%3D10%26um%3D1%26hl%3Den%26safe%3Dactive%26rls%3DDGUS,DGUS:2006-23,DGUS:en%26sa%3DN

Action and Reaction Action and Reaction ForcesForces

Force pairs Force pairs do not act on the same do not act on the same object.object.

When one object exerts anWhen one object exerts an action force action force on a second object, the second object on a second object, the second object exerts aexerts a reaction force reaction force on the first on the first object.object.

Video- Newtons 3Video- Newtons 3rdrd law 3min law 3min


Equal forces don’t always have equal Equal forces don’t always have equal effects.effects.• For example, the For example, the actionaction force of Earth force of Earth

pulling on an object and causing it to fall is pulling on an object and causing it to fall is much more obvious than the equal and much more obvious than the equal and opposite opposite reactionreaction force of the falling object force of the falling object pulling on Earth.pulling on Earth.

• Earth has a much bigger mass and therefore Earth has a much bigger mass and therefore is effected very little by the smaller objects is effected very little by the smaller objects gravity pulling on the Earth.gravity pulling on the Earth.

• Video- Newtons 3 laws of motion 5minVideo- Newtons 3 laws of motion 5min

MomentumMomentum MomentumMomentum is a quantity defined as the is a quantity defined as the

product of the mass and velocity of an product of the mass and velocity of an object.object. Momentum(p) = mass Momentum(p) = mass velocity velocity

p = mvp = mv

All moving objects have momentum.All moving objects have momentum.• For a given velocity, the more mass an For a given velocity, the more mass an

object has, the greater its momentum is.object has, the greater its momentum is.• Likewise, the faster an object Likewise, the faster an object

is moving, the greater its is moving, the greater its

momentum is.momentum is.


MomentumMomentum MomentumMomentum

quantity of motionquantity of motion

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

p

v

Question??Question?? Which is more important for you to Which is more important for you to

wear your seatbelt, on a bus or in a wear your seatbelt, on a bus or in a car?car?

Why??Why?? Bus has more mass and therefore Bus has more mass and therefore

cannot be moved as easily in an cannot be moved as easily in an accident. accident.

Car has less mass and therefore is Car has less mass and therefore is easier to move in a collision.easier to move in a collision.

Math SkillsMath SkillsMomentumMomentum Calculate the momentum of a Calculate the momentum of a

6.00 kg bowling ball moving at 10.0 m/s 6.00 kg bowling ball moving at 10.0 m/s down the alley toward the pins.down the alley toward the pins.

1. List the given and unknown values.1. List the given and unknown values.Given: Given: mass, mmass, m = 6.00 kg = 6.00 kg

velocity, v = velocity, v = 10.0 m/s down the 10.0 m/s down the alleyalley

Unknown: Unknown: momentum, p momentum, p = ? kg • m/s (and = ? kg • m/s (and direction)direction)


Math Skills, Math Skills, continuedcontinued

2. Write the equation for momentum.2. Write the equation for momentum.momentum = mass x velocitymomentum = mass x velocityp = mvp = mv

3. Insert the known values into the 3. Insert the known values into the equation, and solve.equation, and solve.pp = = mvmv = 6.00 kg = 6.00 kg 10.0 m/s 10.0 m/s

pp = 60.0 kg • m/s down the alley = 60.0 kg • m/s down the alley


MomentumMomentum Force is related to change in Force is related to change in

momentum.momentum.• When you force an object to change its When you force an object to change its

motion, you force it to change its motion, you force it to change its momentum.momentum.

Momentum is conserved in collisions. Momentum is conserved in collisions. • The law of conservationThe law of conservation of momentum of momentum

states that the total amount of momentum states that the total amount of momentum in an isolated system is conservedin an isolated system is conserved..


Rocket PropulsionRocket Propulsion


Conservation of momentum explains Conservation of momentum explains rocketrocket propulsion.propulsion.

MomentumMomentum Momentum is transferred. Momentum is transferred.

• When a moving object hits a second object, When a moving object hits a second object, some or all of the momentum of the first some or all of the momentum of the first object is transferred to the second object.object is transferred to the second object.

• Momentum can be transferred in collisions, Momentum can be transferred in collisions, but thebut the total momentum total momentum before and after a before and after a collision is the same.collision is the same.

Pool table- Cue ball is Pool table- Cue ball is

moving fast and transfers moving fast and transfers

its momentum to the 2its momentum to the 2ndnd ball. ball.


pbefore = pafter

Copyright © by Holt, Rinehart and Winston. All rights reserved.

ResourcesChapter menu

Conservation of MomentumConservation of Momentum A 5-kg cart traveling at 4.2 m/s strikes a A 5-kg cart traveling at 4.2 m/s strikes a

stationary 2-kg cart and they connect. Find stationary 2-kg cart and they connect. Find their speed after the collision. their speed after the collision.

BEFORECart 1:m = 5 kgv = 4.2 m/s

Cart 2 :m = 2 kgv = 0 m/s

AFTERCart 1 + 2:m = 7 kgv = ?

p = 21 kg·m/s

p = 0

pbefore = 21 kg·m/s pafter = 21 kg·m/s

m

p

vv = p ÷ mv = (21 kg·m/s) ÷ (7 kg)v = 3 m/s

Copyright © by Holt, Rinehart and Winston. All rights reserved.

ResourcesChapter menu

Conservation of MomentumConservation of Momentum So…now we can solve for velocity. So…now we can solve for velocity.

GIVEN:

p = -1000 kg·m/s

m = 250 kg

v = ?

WORK:

v = p ÷ m

v = (-1000 kg·m/s)÷(250 kg)

v = - 4 m/s (4 m/s backwards)

m

p

v

Concept MappingConcept Mapping


Understanding ConceptsUnderstanding Concepts

1. 1. What is the net force on a 2.0 What is the net force on a 2.0 kg weight hanging motionless on a kg weight hanging motionless on a string?string?

A.A. 0.0 N0.0 N

B.B. 2.0 N2.0 N

C.C. 9.8 N9.8 N

D.D. 19.6 N19.6 N

Standardized Test PrepChapter 11

Understanding Understanding Concepts, Concepts, continuedcontinued

1. 1. What is the net force on a 2.0 What is the net force on a 2.0 kg weight hanging motionless on a kg weight hanging motionless on a string?string?

A.A. 0.0 N0.0 N

B.B. 2.0 N2.0 N

C.C. 9.8 N9.8 N

D.D. 19.6 N19.6 N


Understanding Concepts, Understanding Concepts, continuedcontinued

2. 2. What is the source of the force that What is the source of the force that causes a jet airplane to accelerate causes a jet airplane to accelerate forward?forward?

F.F. gravitational pullgravitational pull

G.G. air pressure on the wingsair pressure on the wings

H.H. exhaust gases pushing against the exhaust gases pushing against the engineengine

I.I. exhaust gases pushing against the exhaust gases pushing against the atmosphereatmosphere



3. 3. Why does a skydiver not accelerate Why does a skydiver not accelerate downward after reaching terminal velocity?downward after reaching terminal velocity?

A.A. The force of gravity is inactive on the The force of gravity is inactive on the skydiver at skydiver at terminal velocity.terminal velocity.

B.B. Air resistance exceeds the force of gravity.Air resistance exceeds the force of gravity.

C.C. Air resistance balances the force of gravity.Air resistance balances the force of gravity.

D.D. The force of gravity decreases as the The force of gravity decreases as the skydiver descends.skydiver descends.



3. 3. Why does a skydiver not accelerate Why does a skydiver not accelerate downward after reaching terminal velocity?downward after reaching terminal velocity?

A.A. The force of gravity is inactive on the The force of gravity is inactive on the skydiver at skydiver at terminal velocity.terminal velocity.

B.B. Air resistance exceeds the force of gravity.Air resistance exceeds the force of gravity.

C.C. Air resistance balances the force of gravity.Air resistance balances the force of gravity.

D.D. The force of gravity decreases as the The force of gravity decreases as the skydiver skydiver descends.descends.



4. 4. The ancient Greek scientist, The ancient Greek scientist, Aristotle, claimed that the speed of a Aristotle, claimed that the speed of a falling object depends on its weight. falling object depends on its weight. But you can disprove his hypothesis But you can disprove his hypothesis by dropping a pen and a baseball by dropping a pen and a baseball simultaneously and observing when simultaneously and observing when they hit the floor.they hit the floor.Why do falling objects not act as Why do falling objects not act as Aristotle thought they would?Aristotle thought they would?


Understanding Concepts, Understanding Concepts, 4. 4. The ancient Greek scientist, Aristotle, claimed that The ancient Greek scientist, Aristotle, claimed that

the speed of a falling object depends on its weight. But the speed of a falling object depends on its weight. But you can disprove his hypothesis by dropping a pen and you can disprove his hypothesis by dropping a pen and a baseball simultaneously and observing when they hit a baseball simultaneously and observing when they hit the floor.the floor.

Why do falling objects not act as Aristotle Why do falling objects not act as Aristotle thought they would?thought they would?

Answer:Answer:The acceleration due to gravity The acceleration due to gravity depends on the total masses of the object depends on the total masses of the object and Earth. Because Earth is so much and Earth. Because Earth is so much larger than either of the objects, the larger than either of the objects, the acceleration depends on the mass of acceleration depends on the mass of Earth and is the same for both objects.Earth and is the same for both objects.



5. 5. Analyze why you would weigh Analyze why you would weigh less on the surface of Mars, even less on the surface of Mars, even though your body remains exactly though your body remains exactly the same size and shape.the same size and shape.



5. 5. Analyze why you would weigh Analyze why you would weigh less on the surface of Mars, even less on the surface of Mars, even though your body remains exactly though your body remains exactly the same size and shape.the same size and shape.

Answer: Answer: Weight is a measure of the Weight is a measure of the force of gravity on an object. The force of gravity on an object. The mass of your body stays the same, mass of your body stays the same, but the force on it is less on Mars.but the force on it is less on Mars.


Reading SkillsReading SkillsThe United States Air Force trains astronauts in a large jet The United States Air Force trains astronauts in a large jet airplane, which is known as the “Vomit Comet” because airplane, which is known as the “Vomit Comet” because many people get airsick during its flight. The plane many people get airsick during its flight. The plane accelerates upward and then falls back toward Earth, in the accelerates upward and then falls back toward Earth, in the form of an arc. At the peak of its flight, the passengers seem form of an arc. At the peak of its flight, the passengers seem to float inside the plane, and objects around them appear to to float inside the plane, and objects around them appear to be unaffected by gravity for about 20 seconds during each be unaffected by gravity for about 20 seconds during each arc.arc.

6. Describe the forces that are acting on the 6. Describe the forces that are acting on the passengers as the plane begins its passengers as the plane begins its acceleration upward towards the top of the acceleration upward towards the top of the arc.arc.


Reading Skills, Reading Skills, continuedcontinued[See previous slide for reading passage.][See previous slide for reading passage.]

6. 6. Describe the forces that are acting on Describe the forces that are acting on the passengers as the plane begins its the passengers as the plane begins its acceleration upward towards the top of acceleration upward towards the top of the arc.the arc.

Answer: Answer: The forces acting on the passengers The forces acting on the passengers are gravitational pull of Earth and an are gravitational pull of Earth and an additional downward force against the additional downward force against the airplane, which is moving upward.airplane, which is moving upward.


Interpreting GraphicsInterpreting Graphics

7. 7. When a ball is falling When a ball is falling toward Earth, what is the toward Earth, what is the reaction force to the pull reaction force to the pull of gravity on the ball?of gravity on the ball?F.F. air pressure pushing air pressure pushing up up on the ballon the ballG.G. force of the ground force of the ground

against the ballagainst the ballH.H. upward pull of the upward pull of the ball ball on Earthon EarthI.I. pull of gravity on the pull of gravity on the

ball toward Earthball toward Earth


Interpreting Graphics, Interpreting Graphics, continuedcontinued

7. 7. When a ball is falling toward When a ball is falling toward Earth, what is the reaction force to Earth, what is the reaction force to the pull of gravity on the ball?the pull of gravity on the ball?F.F. air pressure pushing up air pressure pushing up on on the ballthe ballG.G. force of the ground force of the ground against against the ballthe ballH.H. upward pull of the ball upward pull of the ball on on EarthEarthI.I. pull of gravity on the pull of gravity on the ball ball toward Earthtoward Earth


The force of gravity on you from a The force of gravity on you from a pound of pound of leadlead compared with the compared with the force from a pound of force from a pound of feathersfeathers??

a) The force from a) The force from leadlead will be larger will be larger than the force from feathers.than the force from feathers.

b) The force from b) The force from feathersfeathers will be will be larger than the force from lelad.larger than the force from lelad.

c) The forces will be the c) The forces will be the samesame..

forces chapter 11 table of contents section 1 laws of motion section 2 gravity section 3 newton ’...

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