Name: _______________________________ Class: ____Name: _______________________________ Class: ____

12.112.1 ForcesForces

Chapter 12Chapter 12

Forces and MotionForces and Motion

What are we going to learn?What are we going to learn?

A soccer ball sits A soccer ball sits at restat rest. You come along and . You come along and kick itkick it, sending it , sending it flyingflying across the field. It across the field. It eventually eventually slowsslows to a to a stopstop. You . You appliedapplied a a force force to start it to start it movingmoving, and then it , and then it stoppedstopped all on its all on its own, right?own, right?

No!No!

ForcesForces cause all cause all changeschanges in in motionmotion. Just as . Just as you you appliedapplied a a forceforce to the ball to to the ball to speedspeed it up it up from from restrest, the , the groundground applied a applied a forceforce to to slowslow it it down to a down to a stopstop. If the ground . If the ground didn’tdidn’t apply a apply a force to the ball, it would force to the ball, it would keep rollingkeep rolling forever forever withoutwithout slowing down or slowing down or stoppingstopping..

Forced to ChangeForced to Change

What’s the BIG Idea?What’s the BIG Idea?

She jumps with enough force to stay in the air long enough to turn before she lands

We will find out the answer as we go along...

What is a Force?What is a Force?

How are Forces Described?How are Forces Described?Force – a Force – a push or pullpush or pull

When one When one objectobject pushes or pushes or pullspulls another another objectobject, the first object , the first object exertsexerts a a forceforce on the on the secondsecond object object

Ex: You exert a Ex: You exert a forceforce on a computer on a computer keykey when when you you push itpush it

Ex: You exert a Ex: You exert a forceforce on a on a chairchair when you pull when you pull it it away from a tableaway from a table

Force is Force is describeddescribed by its by its strengthstrength and by the and by the directiondirection in which it acts in which it acts

PushingPushing to the to the leftleft is a is a different forcedifferent force from from pushingpushing to the to the rightright

The The directiondirection and and strengthstrength can be represented can be represented by an by an arrowarrow

The arrow The arrow pointspoints in the in the directiondirection of the of the forceforce

The arrow The arrow lengthlength tells you the tells you the strengthstrength (ex: the (ex: the longer the arrowlonger the arrow, the , the greater the forcegreater the force))

The The strengthstrength of the force is of the force is measuredmeasured in the SI in the SI unit called a unit called a newton (N)newton (N), after the scientist Sir , after the scientist Sir Isaac NewtonIsaac Newton

1 newton1 newton is the force that is the force that causescauses a a 1-kilogram1-kilogram mass to mass to accelerateaccelerate at a rate of 1 meter per at a rate of 1 meter per second each second second each second (1 m/s(1 m/s22))

A A forceforce can cause a can cause a restingresting object to object to movemove, , or it can or it can accelerateaccelerate a a moving objectmoving object by by changingchanging the object’s the object’s speed or directionspeed or direction

We will find out the answer as we go along...

How do Forces Affect Motion?How do Forces Affect Motion?Often Often more than onemore than one force acts on an force acts on an object object at the at the same timesame time

The The combinationcombination of all the of all the forces forces on an object on an object is called the is called the net forcenet force

The net force The net force determinesdetermines if and if and howhow an an object object will will accelerateaccelerate

When the When the forcesforces on an object are on an object are balancedbalanced, , the net force is the net force is zerozero and there is and there is no changeno change in in the object’s the object’s motionmotion

When an When an unbalanced forceunbalanced force acts on an object, acts on an object, the the object acceleratesobject accelerates

You can You can findfind the the net forcenet force on an object by on an object by addingadding together the together the strengthsstrengths of of all all the the individual individual forcesforces acting on the object acting on the object

Look at the figure belowLook at the figure below

The The bigbig dog dog pushespushes on the box with a on the box with a forceforce of of 16 N16 N to the to the rightright

The The smallsmall dog dog pushespushes on the box with a on the box with a forceforce of of 10 N10 N to the to the rightright

The The net forcenet force on the box is the on the box is the sum sum of these of these forcesforces

In the situation, there was a In the situation, there was a nonzero net forcenonzero net force

A nonzero net force A nonzero net force causescauses a a changechange in the in the object’s motionobject’s motion

What if the Forces on an Object are What if the Forces on an Object are not acting in the same Direction? not acting in the same Direction?

TheThe big big dog pushes with a dog pushes with a force of 20 Nforce of 20 N

The The smallsmall dog pushes with a dog pushes with a force of 10 Nforce of 10 N

The dogs are The dogs are pushing against each otherpushing against each other

When the When the forcesforces on an object act in on an object act in opposite opposite directionsdirections, the strength of the , the strength of the net forcenet force is found is found by by subtractingsubtracting the strength of the the strength of the smallersmaller force force (think negative) (think negative) fromfrom the strength of the the strength of the larger larger force (think positive)force (think positive)

When When forcesforces act in act in opposite directionsopposite directions, the , the net net force is in the force is in the same directionsame direction as the as the larger forcelarger force

In the example, the box will In the example, the box will accelerateaccelerate to the to the rightright

Let’s try a few examples...Let’s try a few examples...

Use what you know about net force to describe Use what you know about net force to describe the motion of the box in the figure. Assume the the motion of the box in the figure. Assume the box starts at rest.box starts at rest.

1. You pull on your dog’s leash to the right with 1. You pull on your dog’s leash to the right with a 12 N force. Your dog pulls to the left with a 6 a 12 N force. Your dog pulls to the left with a 6 N force. Sketch this situation, including labeled N force. Sketch this situation, including labeled force arrows.force arrows.

1.1.

2. What is the net force on the leash? Calculate 2. What is the net force on the leash? Calculate it. Draw and label it.it. Draw and label it.

2. 6 N to the right2. 6 N to the right

Tug of War ClipTug of War Clip

http://techtv.mit.edu/videos/850-mit-http://techtv.mit.edu/videos/850-mit-physics-demo----no-win-tug-of-warphysics-demo----no-win-tug-of-war

What's the definition of balanced forces? What's the definition of balanced forces?

What's the definition of balanced forces? What's the definition of balanced forces?

Equal but opposite forces in strength and Equal but opposite forces in strength and direction resulting in a zero net forcedirection resulting in a zero net force

What's the definition of unbalanced forces? What's the definition of unbalanced forces?

What's the definition of unbalanced forces?What's the definition of unbalanced forces?

Direction and strength of forces acting on an Direction and strength of forces acting on an object result in a non-zero net force which object result in a non-zero net force which causes the object to acceleratecauses the object to accelerate

When forces are unbalanced and in opposite When forces are unbalanced and in opposite directions, what is the combined force equal directions, what is the combined force equal to? What direction is it? to? What direction is it?

When forces are unbalanced and in opposite When forces are unbalanced and in opposite directions, what is the combined force equal directions, what is the combined force equal to? What direction is it?to? What direction is it?

The combined force is equal to the difference The combined force is equal to the difference between the 2 forces between the 2 forces

It will be in the direction of the larger forceIt will be in the direction of the larger force

Use your answers above to explain what Use your answers above to explain what happens in a tug-of-war stand-off.happens in a tug-of-war stand-off.

Use your answers above to explain what Use your answers above to explain what happens in a tug-of-war stand-off.happens in a tug-of-war stand-off.

The 2 sides each exert an opposite force. The 2 sides each exert an opposite force. The side that exerts the most force will cause The side that exerts the most force will cause a movement of the other side toward them. a movement of the other side toward them.

Use your answers above to explain what Use your answers above to explain what happens in a arm-wrestling stand-off. happens in a arm-wrestling stand-off.

Use your answers above to explain what Use your answers above to explain what happens in a arm-wrestling stand-off. happens in a arm-wrestling stand-off.

Each arm wrestler is exerting a force Each arm wrestler is exerting a force opposite the other wrestler. The person with opposite the other wrestler. The person with the most force will cause the other’s arm to the most force will cause the other’s arm to move in the direction of the greater force.move in the direction of the greater force.

How is a tug-of-war stand-off similar to an How is a tug-of-war stand-off similar to an arm-wrestling stand-off? arm-wrestling stand-off?

How is a tug-of-war stand-off similar to an How is a tug-of-war stand-off similar to an arm-wrestling stand-off? arm-wrestling stand-off?

Same: both involve opposite forces, if there is Same: both involve opposite forces, if there is a net zero force there is no winner, if there is a net zero force there is no winner, if there is a net non-zero force the side/person with the a net non-zero force the side/person with the greater force will cause the other to move in greater force will cause the other to move in the direction of the greater forcethe direction of the greater force

How are the two stand-offs different? How are the two stand-offs different?

How are the two stand-offs different? How are the two stand-offs different?

Different: team compared to single person; Different: team compared to single person; when there is a net non-zero force for the when there is a net non-zero force for the arm wrestler, the winner’s force will stop arm wrestler, the winner’s force will stop when their hands hit the table; direction of when their hands hit the table; direction of force is straight line for tug of war but angled force is straight line for tug of war but angled for the arm wrestlerfor the arm wrestler

Catching a baseball

In space

30 N to the right

The net force becomes 0 N so it won’t accelerate

5 N to the left

Their strength and direction

Non zero net forces

FrictionFriction

The The forceforce that that 2 surfaces2 surfaces exert on each other exert on each other when they when they rub againstrub against each each otherother

Ex: If you Ex: If you slideslide a a bookbook across a table, the across a table, the surface of the surface of the book rubs againstbook rubs against the surface of the surface of the the tabletable

Factors that Affect FrictionFactors that Affect Friction

2 2 FactorsFactors: :

types of surfacestypes of surfaces involved involved

how how hardhard the surfaces are the surfaces are pushed togetherpushed together

Ex: The Ex: The football playerfootball player is is pushingpushing on a on a blocking blocking sled sled

If his coach wanted to If his coach wanted to make it hardermake it harder to move to move the sled, the coach could the sled, the coach could change the surfacechange the surface of the sled. of the sled. CoveringCovering the the bottombottom of the sled of the sled with rubberwith rubber would would increase frictionincrease friction and make and make the sled the sled harder to move. harder to move.

SmoothSmooth surfaces produce surfaces produce less frictionless friction than than rough surfacesrough surfaces

Ex: if you Ex: if you rubbed your handsrubbed your hands together together forcefullyforcefully, there would be , there would be more frictionmore friction than if than if you you rubbedrubbed your your hands together lightlyhands together lightly

Friction Friction increasesincreases when when surfaces push hardersurfaces push harder against each otheragainst each other

Friction Friction actsacts in a in a direction oppositedirection opposite to the to the direction of the direction of the object’s motionobject’s motion (opposes the (opposes the motion of objects that touch as they move past motion of objects that touch as they move past each other)each other)

WithoutWithout friction, a friction, a movingmoving object object will not stopwill not stop until it until it strikes another objectstrikes another object

How could you make it easier moving heavy How could you make it easier moving heavy objects?objects?

How could you make it easier moving heavy How could you make it easier moving heavy objects?objects?

Put sliders, dolly, or slick surface under the Put sliders, dolly, or slick surface under the objectobject

How could you make it easier moving heavy How could you make it easier moving heavy objects?objects?

Put it on a dolly or put sliders under itPut it on a dolly or put sliders under it

What would happen to the football player if he What would happen to the football player if he switched to a much heavier sled?switched to a much heavier sled?

What would happen to the football player if he What would happen to the football player if he switched to a much heavier sled?switched to a much heavier sled?

He would find it harder to push because it He would find it harder to push because it pushes down harder against the groundpushes down harder against the ground

Types of FrictionTypes of Friction

There are There are 4 4 main types of main types of frictionfriction: :

Static Static friction friction

Sliding Sliding friction friction

Rolling Rolling friction friction

FluidFluid friction friction

Static FrictionStatic FrictionThe friction force that The friction force that acts on objects that are acts on objects that are not movingnot moving

Sliding FrictionSliding Friction

A force that A force that opposesopposes an object’s an object’s directiondirection of of motion as it motion as it slides over a surfaceslides over a surface

Rolling FrictionRolling Friction

The friction force that The friction force that acts on rolling objectsacts on rolling objects

Fluid FrictionFluid Friction

This force This force opposesopposes an object’s an object’s motion through motion through a fluida fluid

Ex: Ex: Air resistanceAir resistance that that acts on an object acts on an object moving through the air moving through the air

Why is it more difficult to walk on ice than on Why is it more difficult to walk on ice than on the floor?the floor?

Why is it more difficult to walk on ice than on Why is it more difficult to walk on ice than on the floor?the floor?

Ex: Because your feet slide on the ice (There is Ex: Because your feet slide on the ice (There is more friction between your shoes and the floor more friction between your shoes and the floor than there is between your shoes and ice)than there is between your shoes and ice)

Which type of friction keeps your shoes from Which type of friction keeps your shoes from slipping on the floor?slipping on the floor?

Which type of friction keeps your shoes from Which type of friction keeps your shoes from slipping on the floor?slipping on the floor?

Static frictionStatic friction

Which type of friction keeps you skating on an Which type of friction keeps you skating on an icy pond?icy pond?

Which type of friction keeps you skating on an Which type of friction keeps you skating on an icy pond?icy pond?

Sliding frictionSliding friction

Question...Question...

How is friction between people like friction How is friction between people like friction between objects?between objects?

Question...Question...How is friction between people like friction How is friction between people like friction between objects?between objects?

The strength of the friction depends on how The strength of the friction depends on how intensely 2 people dislike each other, just as intensely 2 people dislike each other, just as the strength of a frictional force depends on the strength of a frictional force depends on how hard the surfaces are pushed together how hard the surfaces are pushed together

The strength of the friction depends on the The strength of the friction depends on the personalities of the 2 people, just as the personalities of the 2 people, just as the strength of the frictional force depends on the strength of the frictional force depends on the types of surfaces involvedtypes of surfaces involved

Name 4 types of friction and give an example of Name 4 types of friction and give an example of each.each.

Name 4 types of friction and give an example of Name 4 types of friction and give an example of each.each.

Static: pushing a car that is not movingStatic: pushing a car that is not moving

Sliding: a sled sliding down a hillSliding: a sled sliding down a hill

Fluid: a plane flying through the airFluid: a plane flying through the air

Rolling: skateboard wheels on cementRolling: skateboard wheels on cement

What types of friction occur between your bike What types of friction occur between your bike tires and the ground when you ride over tires and the ground when you ride over cement, ride through a puddle, and apply your cement, ride through a puddle, and apply your brakes? brakes?

What types of friction occur between your bike What types of friction occur between your bike tires and the ground when you ride over cement tires and the ground when you ride over cement ((rollingrolling), ride through a puddle (), ride through a puddle (fluidfluid), and apply ), and apply your brakes (your brakes (slidingsliding)? )?

What is friction affected by?What is friction affected by?

What is friction affected by?What is friction affected by?

The surfaces involved and how hard they press The surfaces involved and how hard they press against each otheragainst each other

GravityGravity

AA forceforce that acts that acts between any two massesbetween any two masses

It is an It is an attractive forceattractive force

Earth’s Earth’s gravitygravity acts acts downwarddownward toward the toward the centercenter of Earth of Earth

What forces are acting on the skydiver?What forces are acting on the skydiver?

What forces are acting on the skydiver?What forces are acting on the skydiver?

Gravity and frictionGravity and friction

Which force is greater?Which force is greater?

Which force is greater?Which force is greater?

GravityGravity

How can you tell?How can you tell?

How can you tell?How can you tell?

The net downward force causes them to fall The net downward force causes them to fall toward Earthtoward Earth

How will the skydiver land safely?How will the skydiver land safely?

How will the skydiver land safely?How will the skydiver land safely?

He will deploy his parachute which will slow his He will deploy his parachute which will slow his fallfall

Sir Isaac Sir Isaac NewtonNewton concluded that a concluded that a force pulledforce pulled objects objects straight downstraight down toward the toward the centercenter of of EarthEarth

We are so used to We are so used to objects fallingobjects falling that we may that we may not have thought about not have thought about why they fallwhy they fall

Newton published his Newton published his work on gravitywork on gravity in 1687. in 1687.

What observations might you make today that What observations might you make today that would lead you to the same conclusions about would lead you to the same conclusions about gravity?gravity?

What observations might you make today that What observations might you make today that would lead you to the same conclusions about would lead you to the same conclusions about gravity?gravity?

Rain falling, apples falling from trees, thrown Rain falling, apples falling from trees, thrown baseballs eventually hitting the groundbaseballs eventually hitting the ground

Universal GravitationUniversal Gravitation

Newton realized that Newton realized that gravitygravity acts acts everywhereeverywhere in in the the universeuniverse, not just on Earth, not just on Earth

Ex: makes Ex: makes skydivers fallskydivers fall to the ground to the ground

keeps the keeps the moon orbiting aroundmoon orbiting around Earth Earth

keeps all the keeps all the planetplanets in our solar system s in our solar system

orbiting aroundorbiting around the sun the sun

Law of Universal GravitationLaw of Universal Gravitation

The force of The force of gravitygravity acts acts between all objectsbetween all objects in in the universe that the universe that have masshave mass

This means that This means that any 2 objectsany 2 objects in the universe in the universe that that have mass attract each otherhave mass attract each other

You are You are attracted attracted not only not only to Earthto Earth but also to but also to the the moonmoon, the , the other planetsother planets in the solar system, in the solar system, and and all other objectsall other objects around you around you

EarthEarth and the and the objectsobjects around you are around you are attracted attracted to youto you as well as well

You do You do not noticenot notice the the attractionattraction among among small small objects because these objects because these forcesforces are are extremelyextremely small small compared to the compared to the forceforce of of Earth’s Earth’s attractionattraction

The The gravitationalgravitational force force between somebetween some objects objects is is strongerstronger thanthan the force the force between othersbetween others

You You observeobserve only the only the effectseffects of the of the strongest strongest gravitational gravitational forcesforces

Ex: You Ex: You don’t seedon’t see your your pencil flypencil fly toward the toward the wall wall the way you the way you see it fallsee it fall toward Earth toward Earth

Factors Affecting GravityFactors Affecting Gravity22 factors: factors: MassMass

DistanceDistance

Mass is a Mass is a measuremeasure of the of the amount of matteramount of matter in in an object (SI unit is the an object (SI unit is the kilogramkilogram))

The The more massmore mass an object has, the an object has, the greatergreater the the gravitational forcegravitational force between it and other objects between it and other objects

Ex: Ex: Earth’sEarth’s gravitational force on nearby objects gravitational force on nearby objects is is strong strong because the because the mass of Earth is so largemass of Earth is so large. . The The more massivemore massive planets interact with a planets interact with a greatergreater gravitational gravitational force thanforce than the the less less massivemassive planets planets

Gravitational Gravitational forceforce also also depends ondepends on the the distancedistance between the objects’ between the objects’ centerscenters

Ex: As Ex: As distance increasesdistance increases, gravitational , gravitational forceforce decreasesdecreases. That’s why . That’s why EarthEarth can can exertexert a visible a visible gravitational gravitational force on a pencilforce on a pencil in your in your room room and and notnot on a pencil on a pencil on the moonon the moon..

Ex: Ex: Athletes in spaceAthletes in space would be able to perform would be able to perform featsfeats that are that are impossible on Earthimpossible on Earth......

Gravitational Gravitational forcesforces are are weakerweaker so golf so golf balls balls and baseballs can float or and baseballs can float or fly farther in spacefly farther in space

Give an example of another sport which would Give an example of another sport which would be affected by reduced gravitational forcebe affected by reduced gravitational force

Give an example of another sport which would Give an example of another sport which would be affected by reduced gravitational forcebe affected by reduced gravitational force

Gymnastics, jumping events in trackGymnastics, jumping events in track

What’s that Again?What’s that Again?The The more massmore mass an object has, the an object has, the greatergreater its its gravitational forcegravitational force

The The shorter the distanceshorter the distance is is betweenbetween one one object object and another, the and another, the strongerstronger the the gravitational gravitational forceforce between the objects between the objects

Between which 2 objects is the gravitational Between which 2 objects is the gravitational force stronger, a robin and a worm or a bear force stronger, a robin and a worm or a bear and a cub? Why?and a cub? Why?

Between which 2 objects is the gravitational Between which 2 objects is the gravitational force stronger, a robin and a worm or a bear force stronger, a robin and a worm or a bear and a cub? Why?and a cub? Why?

Assuming the distances are the same, Assuming the distances are the same, gravitational force is stronger between a bear gravitational force is stronger between a bear and cub because they have greater massand cub because they have greater mass

Between which 2 objects is the gravitational Between which 2 objects is the gravitational force stronger, 2 apples in a fruit bowl or 2 force stronger, 2 apples in a fruit bowl or 2 apples on different branches of an apple tree? apples on different branches of an apple tree? Why?Why?

Between which 2 objects is the gravitational Between which 2 objects is the gravitational force stronger, 2 apples in a fruit bowl or 2 force stronger, 2 apples in a fruit bowl or 2 apples on different branches of an apple tree? apples on different branches of an apple tree? Why?Why?

Between 2 apples in a fruit bowl because the Between 2 apples in a fruit bowl because the apples in the bowl are closer, and gravitational apples in the bowl are closer, and gravitational force decreases as distance increasesforce decreases as distance increases

In the solar system, which body exerts the most In the solar system, which body exerts the most gravitational force? Why?gravitational force? Why?

In the solar system, which body exerts the most In the solar system, which body exerts the most gravitational force? Why?gravitational force? Why?

The sun since it has the greatest massThe sun since it has the greatest mass

1. Circle the object in the outermost orbit which 1. Circle the object in the outermost orbit which experiences the greatest gravitational pull from experiences the greatest gravitational pull from the sun. the sun.

2. Planet B’s force arrow from the gravitational 2. Planet B’s force arrow from the gravitational pull of the sun should be (longer/shorter) than pull of the sun should be (longer/shorter) than planet A’s arrow. planet A’s arrow.

3. Draw what a planet would look like it is was 3. Draw what a planet would look like it is was the same distance from the sun as Planet C but the same distance from the sun as Planet C but experienced a smaller gravitational pull from the experienced a smaller gravitational pull from the sun.sun.

Mass and WeightMass and WeightMassMass is sometimes is sometimes confusedconfused with with weightweight

Mass is the Mass is the measuremeasure of the of the amount of matteramount of matter in in an objectan object

Weight Weight is a is a measuremeasure of the of the force of gravityforce of gravity on on an objectan object

Ex: Your Ex: Your bathroom scalebathroom scale displays the displays the gravitational forcegravitational force Earth is Earth is exerting on youexerting on you

What happens to the gravitational force What happens to the gravitational force between 2 objects when their masses are between 2 objects when their masses are increased?increased?

What happens to the gravitational force What happens to the gravitational force between 2 objects when their masses are between 2 objects when their masses are increased?increased?

When mass increases, the force increasesWhen mass increases, the force increases

What happens when the distance between the What happens when the distance between the objects increases?objects increases?

What happens when the distance between the What happens when the distance between the objects increases?objects increases?

When distance increases, the force decreasesWhen distance increases, the force decreases

If the mass of Earth increased, what would If the mass of Earth increased, what would happen to your weight?happen to your weight?

If the mass of Earth increased, what would If the mass of Earth increased, what would happen to your weight?happen to your weight?

My weight would also increase. The My weight would also increase. The gravitational force would be stronger.gravitational force would be stronger.

What about your mass?What about your mass?

What about your mass?What about your mass?

My mass would stay the same.My mass would stay the same.

At any time, your At any time, your massmass is the is the same on Earthsame on Earth as it would be as it would be on any other planeton any other planet

But your But your weight variesweight varies with the with the strengthstrength of the of the gravitational forcegravitational force

The The dogdog has a has a different weightdifferent weight at different at different places in the places in the solar systemsolar system

On the On the moonmoon, he would weigh about , he would weigh about 1/6 1/6 of what of what he he does on Earthdoes on Earth

On On MarsMars, he would weigh just over , he would weigh just over 1/31/3 of what of what he he does on Earthdoes on Earth

The Ups and Downs of It...The Ups and Downs of It...GravityGravity causes causes objectsobjects to to accelerate accelerate downward downward

Air resistanceAir resistance acts in the acts in the direction oppositedirection opposite to to the the downwarddownward motion of motion of gravitygravity and and reduces reduces accelerationacceleration

Terminal VelocityTerminal Velocity

The The constant velocityconstant velocity of a falling object when of a falling object when the the force of air resistance equalsforce of air resistance equals the the force of force of gravitygravity

Projectile MotionProjectile Motion

The The combinationcombination of an of an initial forward velocityinitial forward velocity and the and the downward vertical force of gravitydownward vertical force of gravity causes the ball to causes the ball to follow a curved pathfollow a curved path

Projectile motionProjectile motion is a falling object’s is a falling object’s motion motion afterafter it is given a it is given a forward velocityforward velocity

Projectile MotionProjectile Motion