Section 1: The Nature of ForceSection 1: The Nature of ForceWhat is a force?What is a force?

ForceForceA A pushpush or or pullpull on an object on an objectHas both Has both SizeSize & & DirectionDirection

Size:Size:Measured in SI units called Measured in SI units called newtonsnewtons (N) (N)

Spring ScaleSpring Scale

How do you combine forces?How do you combine forces?

Direction:Direction:Same direction: Same direction: AddAdd (+) (+)Diff. direction: Diff. direction: SubtractSubtract (-) (-)Net forceNet force = combination of = combination of all forces acting on objectall forces acting on object

Unbalanced ForcesUnbalanced ForcesResult in Result in motionmotion

What does balanced forces mean?What does balanced forces mean?

Balanced Forces Balanced Forces Net force = Net force = 00No No motionmotion

Lab: The Nail Challenge!Lab: The Nail Challenge!Objective:Objective:

Balance nails Balance nails on single nail on single nail headhead

Work in pairsWork in pairs

Section 2: Friction and GravitySection 2: Friction and GravityWhat is friction?What is friction?

FrictionFrictionResistance to Resistance to motionmotion OppositeOpposite direction of travel direction of travelCaused when 2 surfaces Caused when 2 surfaces rubrub togethertogether

resistive force (resistive force (slowsslows down down objects)objects)

What does Friction depend on?What does Friction depend on?Friction depends on…Friction depends on…

TypesTypes of surfaces of surfacesHow How hardhard surfaces push surfaces push togethertogether

What are sliding and rolling friction?What are sliding and rolling friction?Types of FrictionTypes of Friction

SlidingSliding Friction: Friction: solid surfaces slide solid surfaces slide over each otherover each other

RollingRolling Friction: Friction: object rolls over object rolls over surfacesurface

What are fluid and static friction?What are fluid and static friction?Types of FrictionTypes of Friction

FluidFluid Friction: Friction: object moves object moves through fluid through fluid (or air)(or air)

StaticStatic Friction: Friction: objects not objects not movingmoving

What are some uses for friction?What are some uses for friction?Is Friction Is Friction

harmful or harmful or helpful?helpful?

Ways to Ways to reducereduce friction friction

Ways to Ways to increaseincrease frictionfriction

What is a gravitational What is a gravitational force?force?

Gravitational ForceGravitational ForceForce of Force of attractionattraction between between 2 objects2 objects

Pulls things toward each otherPulls things toward each otherDepends on: Depends on:

MassMass DistanceDistance

What is the difference between What is the difference between mass and weight?mass and weight?MassMass

Amount of Amount of mattermatterSameSame no matter where you are no matter where you areSI units = kilograms (kg)SI units = kilograms (kg)

1 kg = 1000 grams (g)1 kg = 1000 grams (g)Weight Weight

Force of Force of gravitygravitySI units = SI units = newtonsnewtons (N) (N)Depends on where you areDepends on where you are

What is Free fall?What is Free fall?

Free FallFree FallOnly force acting on an Only force acting on an object is object is gravitygravity

Objects in free fall Objects in free fall accelerateaccelerate as they fall as they fall

All objects free fall at the All objects free fall at the same rate (same rate (9.8 9.8 m/sm/s22))

Gravity & Freefall Gravity & Freefall

What is Air resistance?What is Air resistance?Air resistance Air resistance

Type of Type of fluidfluid friction frictionOpposes Opposes motionmotion of objects of objects through airthrough air

Depends on: Depends on: Size, Size, ShapeShape, Speed, Speed

What is Terminal Velocity?What is Terminal Velocity?Terminal VelocityTerminal Velocity

As an object falls it picks up As an object falls it picks up speedspeed

Increased speed Increased speed increased air increased air resistanceresistance

Eventually force of air Eventually force of air resistance = force of resistance = force of gravitygravity TERMINAL VELOCITY TERMINAL VELOCITYObject stops Object stops acceleratingaccelerating!!

Terminal VelocityTerminal Velocity

Section 3: Newton’s First and Second LawsSection 3: Newton’s First and Second LawsWhat is Inertia?What is Inertia?

InertiaInertiaTendency Tendency of object to of object to resistresist a a change in change in it’s it’s motionmotion

What does inertia depend on?What does inertia depend on?Inertia Inertia

depends on depends on MassMass““Amount” Amount” of of inertiainertia depends on depends on objects’ objects’ massmass

What is Newton’s 1What is Newton’s 1stst Law? Law?Newton’s 1Newton’s 1stst Law of Motion Law of Motion

Object at Object at restrest will remain at will remain at restrest

Object in Object in motionmotion will remain will remain in in motionmotion unless acted on by unless acted on by an unbalanced an unbalanced force. force.

Practice Problem 1Practice Problem 1 Imagine a place in the cosmos Imagine a place in the cosmos far from all gravitational and far from all gravitational and frictional influences. Suppose frictional influences. Suppose an astronaut in that place an astronaut in that place throws a rock. The rock will:throws a rock. The rock will:a) gradually stop.a) gradually stop.b) continue in motion in the b) continue in motion in the same direction at constant same direction at constant speed. speed.

Practice Problem 2Practice Problem 2 An 2-kg object is moving An 2-kg object is moving horizontally with a speed of 4 horizontally with a speed of 4 m/s. How much net force is m/s. How much net force is required to keep the object required to keep the object moving with the same speed moving with the same speed and in the same direction? and in the same direction?

0 N (no force)0 N (no force)

Practice Problem 3Practice Problem 3 Ben Tooclose is being chased Ben Tooclose is being chased

through the woods by a bull moose through the woods by a bull moose which he was attempting to which he was attempting to photograph. The enormous mass of photograph. The enormous mass of the bull moose is extremely the bull moose is extremely intimidating. Yet, if Ben makes a intimidating. Yet, if Ben makes a zigzag pattern through the woods, he zigzag pattern through the woods, he will be able to use the large mass of will be able to use the large mass of the moose to his own advantage. the moose to his own advantage. Explain this in terms of inertia and Explain this in terms of inertia and Newton's first law of motion. Newton's first law of motion.

Newton’s 1Newton’s 1stst Law Review Law ReviewUnbalanced Unbalanced

force from force from another car another car changeschanges your your CAR’s motionCAR’s motion

You continue You continue as before until as before until your seatbelt your seatbelt changes changes YOUR YOUR motionmotion

What is Newton’s 2What is Newton’s 2ndnd law of motion? law of motion? Newton’s 2Newton’s 2ndnd law of Motion law of Motion

Force, Force, MassMass & Acceleration are & Acceleration are relatedrelated

Force = Mass X Force = Mass X AccelerationAcceleration OROR Acceleration = Force Acceleration = Force ÷Mass÷Mass

FYI, 1 FYI, 1 NN = 1kg X 1 m/s = 1kg X 1 m/s22

Force, Mass & AccelerationForce, Mass & Acceleration

Force, Mass & AccelerationForce, Mass & AccelerationA 52 kg water skier is being A 52 kg water skier is being

pulled by a speedboat. The pulled by a speedboat. The force causes her to accelerate force causes her to accelerate @ 2 m/s@ 2 m/s22. Calculate the FORCE . Calculate the FORCE that causes this acceleration.that causes this acceleration.F = 52 kg x 2 m/sF = 52 kg x 2 m/s22

= 104 kg x m/s= 104 kg x m/s22 = 104 = 104 kg*m/skg*m/s22

= 104 N= 104 N

Force, Mass & AccelerationForce, Mass & AccelerationWhat is the force on a 1000 kg What is the force on a 1000 kg

elevator accelerating at 2 m/selevator accelerating at 2 m/s22??1000 kg X 2 m/s1000 kg X 2 m/s22 = 2000 N = 2000 N

How much force is needed to How much force is needed to accelerate a 55 kg cart at 15 accelerate a 55 kg cart at 15 m/sm/s22??55 kg X 15m/s55 kg X 15m/s22 = 825 N = 825 N

Section 4: Newton’s Third LawSection 4: Newton’s Third LawWhat is Newton’s 3What is Newton’s 3rdrd Law? Law?

Newton’s 3Newton’s 3rdrd law of Motion law of MotionFor every For every action, there is an , there is an equal and and opposite reactionreaction

““Action” & “Reaction” are Action” & “Reaction” are names of names of forces

How do forces always occur?How do forces always occur?Forces Forces ALWAYS occur in occur in

pairs.pairs.Single forces Single forces NEVER happenhappen

2 objects are involved in objects are involved in every forceevery forceAction force: “A pushes Action force: “A pushes B””Reaction force: “Reaction force: “B pushes pushes A”A”

What do equal and opposite mean?What do equal and opposite mean?In Newton’s Third In Newton’s Third

Law, “equal” means: Law, “equal” means: Equal in Equal in sizeEqual in Equal in time

““Opposite” Means:Opposite” Means:Opposite in Opposite in direction

Don’t Action & Reaction forces cancel each other?

Action & Reaction forces Action & Reaction forces act on act on DIFFERENT objects objects

In “In “Net force” problems, force” problems, we are talking about we are talking about opposing forces acting on forces acting on the the SAME object object

Try These!!

If forces are equal and opposite why don't they cancel each other out?

They occur on two different objects.  Forces can only cancel out when the forces are acting on the same object.

If the forces are equal and opposite how do two different objects obtain different accelerations in the same interaction? (Remember F=ma)

Different accelerations are obtained when the objects have different masses.