Chapter 12 Forces and Motion

Chapter 12 Forces and Chapter 12 Forces and MotionMotion

Section 12.1 ForcesSection 12.1 ForcesSection 12.2 Newton’s First and Second Section 12.2 Newton’s First and Second Laws of MotionLaws of MotionSection 12.3 Newton’s Third Law of Section 12.3 Newton’s Third Law of Motion and MomentumMotion and Momentum

Section 12.1 ForcesSection 12.1 Forces

What is a Force?What is a Force? Def.-a push or a pull that acts on an objectDef.-a push or a pull that acts on an object Key Concept: A force can cause a resting Key Concept: A force can cause a resting

object to move, or it can accelerate a object to move, or it can accelerate a moving object by changing the object’s moving object by changing the object’s speed or directionspeed or direction

Ex. Figure 1 pg. 356; man walking in Ex. Figure 1 pg. 356; man walking in gusty wind- wind (force) can change his gusty wind- wind (force) can change his speed or directionspeed or direction

Section 12.1 ForcesSection 12.1 Forces Measuring ForceMeasuring Force

Most of the time force is easy to measure.Most of the time force is easy to measure. Figure 2; fruit acts as a force (weight) on the Figure 2; fruit acts as a force (weight) on the

spring in a scalespring in a scale Units of ForceUnits of Force

Force is measured in Newtons (N)Force is measured in Newtons (N) 1 Newton-the force that causes a 1 kg mass to 1 Newton-the force that causes a 1 kg mass to

accelerate at a rate of 1 m/saccelerate at a rate of 1 m/s2 2 or is in other words or is in other words 1kg*m/s1kg*m/s22

Newton: after Sir Isaac Newton (scientist) who Newton: after Sir Isaac Newton (scientist) who explained how force, mass, and acceleration are explained how force, mass, and acceleration are relatedrelated


Representing ForceRepresenting Force Use an arrow to represent the direction Use an arrow to represent the direction

and strength of a force.and strength of a force. Direction of arrow=direction of force; the Direction of arrow=direction of force; the

length of arrow=the strength/magnitude length of arrow=the strength/magnitude of the forceof the force


Combining ForcesCombining Forces You can combine force arrows to show the You can combine force arrows to show the

result of how forces combine.result of how forces combine. Forces in the same direction add togetherForces in the same direction add together; ;

forces in opposite directions subtract from forces in opposite directions subtract from one anotherone another..

Net force-the overall force acting on an Net force-the overall force acting on an object after all the forces are combinedobject after all the forces are combined


Balanced ForcesBalanced Forces Key Concept: When the forces on an Key Concept: When the forces on an

object are balanced, the net force is zero object are balanced, the net force is zero and there is no change in the object’s and there is no change in the object’s motion.motion.

Ex. Tug of war and arm wrestlingEx. Tug of war and arm wrestling An unlimited number of individual forces An unlimited number of individual forces

can act on an object and still produce a can act on an object and still produce a net force of zero.net force of zero.


Unbalanced ForcesUnbalanced Forces An unbalanced force is a force that results when An unbalanced force is a force that results when

the net force acting on an object is not equal to 0.the net force acting on an object is not equal to 0. Key Concept: When an unbalanced force acts on Key Concept: When an unbalanced force acts on

an object, the object accelerates.an object, the object accelerates. Forces acting in opposite directions can also Forces acting in opposite directions can also

combine to produce an unbalanced force (winners combine to produce an unbalanced force (winners of tug of war).of tug of war).

The net force equals the size of the larger force The net force equals the size of the larger force minus the size of the smaller force.minus the size of the smaller force.

Combining Combining Forces Acting on Forces Acting on

an Objectan Object

Figure 4


FrictionFriction ****All moving objects are subject to friction.All moving objects are subject to friction. Def.-a force that opposes the motion of Def.-a force that opposes the motion of

objects that touch as they move past each objects that touch as they move past each otherother

**Without friction, surfaces would be more **Without friction, surfaces would be more slippery than ice.slippery than ice.

Friction Friction acts at the surface where objects are acts at the surface where objects are in contact in contact (that includes solid objects that (that includes solid objects that are directly touching each other and also are directly touching each other and also objects moving through a liquid or a gas)objects moving through a liquid or a gas)


FrictionFriction Key Concept: There are four main types of Key Concept: There are four main types of

friction: static friction, sliding friction, friction: static friction, sliding friction, rolling friction, and fluid friction.rolling friction, and fluid friction.

Static FrictionStatic Friction Def.-the friction that acts on objects that Def.-the friction that acts on objects that

are movingare moving **It always acts in the direction opposite to **It always acts in the direction opposite to

that of the applied force (Taking steps, that of the applied force (Taking steps, push off a step, between your shoes and push off a step, between your shoes and the ground)the ground)


Sliding FrictionSliding Friction Def.-A force that opposes the direction of Def.-A force that opposes the direction of

motion of an object as it slides over a motion of an object as it slides over a surfacesurface

**Sliding friction is less than static friction, **Sliding friction is less than static friction, so less force is needed to keep an object so less force is needed to keep an object moving moving


Rolling FrictionRolling Friction ** When a round object rolls across a flat floor, ** When a round object rolls across a flat floor,

both the object and the floor both the object and the floor are bent slightly out are bent slightly out of shapeof shape..

Def.-the friction force that acts on rolling objects ( Def.-the friction force that acts on rolling objects ( is about 100 to 1000 times less than the force of is about 100 to 1000 times less than the force of static or sliding frictionstatic or sliding friction))

Ex. Ball bearings (Figure 6 pg. 360)-used to Ex. Ball bearings (Figure 6 pg. 360)-used to reduce friction (rolling friction replaces sliding reduce friction (rolling friction replaces sliding friction) inline skates, automobiles, bicycles, friction) inline skates, automobiles, bicycles, skateboardsskateboards


Fluid FrictionFluid Friction Water and a mixture of gases such as air are Water and a mixture of gases such as air are

known as known as fluids.fluids. Def.-the force that opposes the motion of an Def.-the force that opposes the motion of an

object through a fluidobject through a fluid Increases as the speed of an object moving Increases as the speed of an object moving

through the fluid increasesthrough the fluid increases Air resistance-fluid friction acting on an object Air resistance-fluid friction acting on an object

moving through the airmoving through the air At higher speeds, air resistance can be a At higher speeds, air resistance can be a

significant force.significant force.

Section 12.1 ForcesSection 12.1 Forces GravityGravity

Def.-a force that acts between any two massesDef.-a force that acts between any two masses Is an attractive force (Is an attractive force (pulls objects togetherpulls objects together)) **Gravity holds us to the ground.****Gravity holds us to the ground.** Does not require objects to be in contact for it Does not require objects to be in contact for it

to act on themto act on them Key Concept: Earth’s gravity acts downward Key Concept: Earth’s gravity acts downward

toward the center of Earth.toward the center of Earth. Upward force usually balances the downward Upward force usually balances the downward

force of gravity. (Book on desk; boulder on force of gravity. (Book on desk; boulder on cliff)cliff)


Falling ObjectsFalling Objects Both gravity and air resistanceBoth gravity and air resistance affect the affect the

motion of a falling object.motion of a falling object. Key Concept: Gravity causes objects to Key Concept: Gravity causes objects to

accelerate downward, whereas air accelerate downward, whereas air resistance acts in the direction opposite to resistance acts in the direction opposite to the motion and reduces acceleration. the motion and reduces acceleration. **Fig. 8 flying squirrel**Fig. 8 flying squirrel

Large surface area maximizes the force of Large surface area maximizes the force of air resistanceair resistance


Falling ObjectsFalling Objects Falling objects accelerate and gain speed=increase in Falling objects accelerate and gain speed=increase in

air resistanceair resistance If an object falls for a long time, the upward force of air If an object falls for a long time, the upward force of air

resistance becomes equal to the downward force of resistance becomes equal to the downward force of gravity.gravity.

Forces acting on the object are Forces acting on the object are balanced, acceleration balanced, acceleration is 0, and the object continues to fall at a constant is 0, and the object continues to fall at a constant velocity.velocity.

Terminal velocity-the constant velocity of a falling Terminal velocity-the constant velocity of a falling object when the force of air resistance equals the force object when the force of air resistance equals the force of gravity (2 objects w/ same mass fall at same rate)of gravity (2 objects w/ same mass fall at same rate)


Projectile MotionProjectile Motion Ex. A ball that is thrown, follows a curved Ex. A ball that is thrown, follows a curved

pathpath Def.-the motion of a falling object (projectile) Def.-the motion of a falling object (projectile)

after it is given an initial forward velocityafter it is given an initial forward velocity Air resistance and gravity are the only forces Air resistance and gravity are the only forces

acting on a projectileacting on a projectile.. Key Concept: The combination of an initial Key Concept: The combination of an initial

forward velocity and the downward vertical forward velocity and the downward vertical force of gravity causes the ball to follow a force of gravity causes the ball to follow a curved path.curved path.

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

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


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 object , an object

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

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

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

change in its motionchange in its motion


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 object Mass-a measure of the inertia of an object

and depends on the amount of matter the and depends on the amount of matter the object containsobject 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.

Newton’s Second LawNewton’s Second LawSection 12.2


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.

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

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

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

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


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)


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.

Conservation of Conservation of MomentumMomentum

Figure 17A and 17B

Conservation of Conservation of MomentumMomentum

Figure 17C

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Chapter 12 Forces and Motion