FORCESFORCES

Write 5 examples of an object Write 5 examples of an object acceleratingaccelerating

In each example, what causes the In each example, what causes the object to accelerate?object to accelerate?

FORCESFORCES

Newton’s Three LawsNewton’s Three Laws

Newton’s First Law of MotionNewton’s First Law of Motion

Law of InertiaLaw of Inertia• Inertia is a property of matter.Inertia is a property of matter.• Inertia is an object’s tendency to resist a Inertia is an object’s tendency to resist a

change in motion.change in motion. The law states:The law states:

• An object at rest will stay at rest unless a net An object at rest will stay at rest unless a net force acts upon it.force acts upon it.

• An object in motion will stay in motion unless a An object in motion will stay in motion unless a net force acts upon it.net force acts upon it.

Egg Drop ExperimentEgg Drop Experiment

What will happen to the egg when What will happen to the egg when the pie tin is hit?the pie tin is hit?

Why would it do that?Why would it do that?

Newton’s Second Law of MotionNewton’s Second Law of Motion

Force = mass x accelerationForce = mass x acceleration• F=maF=ma

Force and acceleration are directly Force and acceleration are directly proportional. More Force means more proportional. More Force means more acceleration.acceleration.

Mass and acceleration are inversely Mass and acceleration are inversely related. With the same amount of force, an related. With the same amount of force, an object with greater mass will have less object with greater mass will have less accelerationacceleration

Ball Bearing RaceBall Bearing Race

Which ball moved farther?Which ball moved farther?

Why did it move farther?Why did it move farther?

Newton’s Third Law of MotionNewton’s Third Law of Motion

States that:States that:• For every force there is an equal and For every force there is an equal and

opposite reaction force.opposite reaction force.

When you push against something, it When you push against something, it pushes back!pushes back!

Third Law ExamplesThird Law Examples

Space Shuttle LaunchesSpace Shuttle Launches

Jumping, Swimming, RunningJumping, Swimming, Running

GravityGravity

SwingsSwings

TYPES OF FORCESTYPES OF FORCES

FFg g = Gravity!= Gravity! FFa a = Applied Force= Applied Force FFff = = FrictionFriction FFN N = Normal Force= Normal Force FFt t = Tension Force= Tension Force FFss = Force of a spring = Force of a spring FFairair= Air Resistance= Air Resistance FFnetnet = The sum of all forces acting on = The sum of all forces acting on

an object.an object.

About ForcesAbout Forces

All forces:All forces:• Obey the three lawsObey the three laws• Are measured in Newtons (N)Are measured in Newtons (N)

1 Newton = 1 kg * m/s1 Newton = 1 kg * m/s22

Note: mass is in kilogramsNote: mass is in kilograms

• Have a magnitude and directionHave a magnitude and direction They’re vectors!They’re vectors! Are represented with arrows.Are represented with arrows.

2s

mkgN

Force of GravityForce of Gravity

Always present in a downward Always present in a downward direction.direction.

Also known as WeightAlso known as Weight

Can be calculated using Newton’s Can be calculated using Newton’s Second Law: F=maSecond Law: F=ma• FFgg=m*a=m*ag g (a(agg=9.8m/s=9.8m/s22))

Gravity ExampleGravity Example

Your cat has a mass of 1.6 kg. What Your cat has a mass of 1.6 kg. What is it’s weight?is it’s weight?

FFgg = ma = ma FFgg = 1.6kg * 9.8m/s2 = 1.6kg * 9.8m/s2

FFgg = 15.68 kg*m/s2 = 15.68 N = 15.68 kg*m/s2 = 15.68 N

Normal ForceNormal Force

Always perpendicular to the applied Always perpendicular to the applied surface.surface.

Equal to the total force applied to Equal to the total force applied to that surfacethat surface

Normal Force CalculationNormal Force Calculation

You headbutt someone with an You headbutt someone with an applied force of 10 N. What is the applied force of 10 N. What is the normal force applied to your head?normal force applied to your head?

FFNN = 10N = 10N

Tension ForceTension Force

Always in the direction of an object Always in the direction of an object being pulled.being pulled.

Used with ropes, strings, cables, etc. Used with ropes, strings, cables, etc.

Can either be used when pulling or Can either be used when pulling or hanging an object. hanging an object. • When hanging, FWhen hanging, Ftt is equal to F is equal to Fgg • FFt t has an upper limit, if that is exceeded, has an upper limit, if that is exceeded,

the cable/string will snap!the cable/string will snap!

Tension Force CalculationTension Force Calculation

A 4000N block is hanging from a A 4000N block is hanging from a crane. What is the Tension Force crane. What is the Tension Force applied by the cable?applied by the cable?

FFtt = 4000N = 4000N

FrictionFriction Always acts in the opposite direction of the Always acts in the opposite direction of the

intended motion.intended motion.

Friction must be overcome for an object to Friction must be overcome for an object to accelerate across a surface.accelerate across a surface.

Two types of Friction:Two types of Friction:• Static Friction – opposes the start of motion Static Friction – opposes the start of motion

(before an object is moving)(before an object is moving)• Kinetic Friction – opposes the current motion of Kinetic Friction – opposes the current motion of

an objectan object Static Friction is larger than Kinetic FrictionStatic Friction is larger than Kinetic Friction

Calculating FrictionCalculating Friction

FFff==μμFFNN

μμ – (pronounced “Mew”) - Coefficient – (pronounced “Mew”) - Coefficient of Frictionof Friction• Doesn’t have unitsDoesn’t have units• Value is different for different surfacesValue is different for different surfaces

Rough surfaces have a higher Rough surfaces have a higher μμ

Friction CalculationFriction Calculation

The coefficient of friction for a object The coefficient of friction for a object sliding on a horizontal concrete floor sliding on a horizontal concrete floor is .62, if the object weighs 20N, what is .62, if the object weighs 20N, what is Fis Fff? ?

FFff==μμFFNN

FFff= .62 * 20N= .62 * 20N FFff= 12.4N= 12.4N

Air ResistanceAir Resistance

When an object moves through air, When an object moves through air, the air pushes back on the object.the air pushes back on the object.• Acts in the direction opposite the objects Acts in the direction opposite the objects

motionmotion

FFairair is normally very small, so we can is normally very small, so we can

ignore it, except when an object is ignore it, except when an object is falling.falling.

Free Body DiagramsFree Body Diagrams

Used to show the forces acting on a Used to show the forces acting on a body.body.

Use a box (or dot) to represent an Use a box (or dot) to represent an object.object.• Draw the forces that effect the objectDraw the forces that effect the object• Remember that if the object isn’t Remember that if the object isn’t

accelerating, Faccelerating, Fnetnet=0.=0.

Free Body Diagram PracticeFree Body Diagram Practice

A book is sitting on a deskA book is sitting on a desk

Free Body Diagram PracticeFree Body Diagram Practice

A book is being pushed along a desk.A book is being pushed along a desk.

Free Body Diagram PracticeFree Body Diagram Practice

A ball is hanging from a stringA ball is hanging from a string

Free Body Diagram PracticeFree Body Diagram Practice

The same ball, but the string has The same ball, but the string has been cut.been cut.