Forces Forces Standard: S8P3Standard: S8P3

b. Demonstrate the effect of b. Demonstrate the effect of balanced and unbalanced balanced and unbalanced forces on an object in terms forces on an object in terms of gravity, inertia, and of gravity, inertia, and friction.friction.

ForcesForcesWhat are forces?What are forces?

InertiaInertia

GravityGravity

Newton’s LawsNewton’s Laws

Projectile motionProjectile motion

What is a force?What is a force?

A push or a pullA push or a pull May be unbalanced or balancedMay be unbalanced or balanced

What does a force do?What does a force do?

Gives energy Gives energy to an objectto an object

Can set an Can set an object in object in motion, make motion, make it stop, or it stop, or change its change its motionmotion

Unbalanced forces change Unbalanced forces change motionmotion

Unbalanced forces change Unbalanced forces change motionmotion

Unbalanced forces change Unbalanced forces change motionmotion

What are forces represented What are forces represented by?by?

Forces are represented by Forces are represented by vectorsvectors

Forces are added with Forces are added with vectorsvectors

What is friction?What is friction? A force which opposes motion, A force which opposes motion,

bringing objects to restbringing objects to rest Types:Types:

InertiaInertia

What is inertia?What is inertia?

The tendency The tendency of an object to of an object to stay at rest or stay at rest or in motion.in motion.

Objects with Objects with greater mass greater mass have greater have greater inertiainertia

Inertia keeps astronauts in Inertia keeps astronauts in motionmotion

Seat belts protect against Seat belts protect against inertiainertia

GravityGravity

GravityGravity

The attractiveforce between all objects in theuniverse

F = F = MM11 M M22

d d22

More mass means greater gravityMore mass means greater gravity More distance means less gravityMore distance means less gravity

My gravitationalattraction is smallno matter what the distance!

Massive objects have lots of gravity

The earth exerts a force on the studentand the student exerts a force on the earth becauseof GRAVITY!

Newton’s Laws of Newton’s Laws of MotionMotion

1. Inertia1. Inertia

2. F=ma2. F=ma

3. Action-reaction3. Action-reaction

Newton’s First Law of Newton’s First Law of MotionMotion

The law of The law of inertia:inertia:

An object at An object at rest stays at rest stays at restrest

an object in an object in motion stays motion stays in motionin motion

unless acted unless acted upon by an upon by an outside force.outside force.

Inertia keeps astronauts in Inertia keeps astronauts in motionmotion

Inertia keeps satellites in Inertia keeps satellites in orbitorbit

Inertia keeps you at restInertia keeps you at rest

Newton’s Second Law of Newton’s Second Law of MotionMotion

Force = Mass x accelerationForce = Mass x acceleration A force is needed for accelerationA force is needed for acceleration Acceleration will occur in the Acceleration will occur in the

direction of the force.direction of the force.

Which cars are Which cars are accelerating?accelerating?

If you watch carefully, the green and If you watch carefully, the green and blue cars are accelerating. The red blue cars are accelerating. The red car is traveling at constant speed.car is traveling at constant speed.

A force is needed for A force is needed for accelerationacceleration

Will these accelerate?Will these accelerate? A skier going downhill?A skier going downhill?

– What force is acting onWhat force is acting on

the skier?the skier? An astronaut on a An astronaut on a

spacewalk?spacewalk?– Are any forces acting Are any forces acting

on the astronaut?on the astronaut?

Which of the following Which of the following will accelerate?will accelerate?

A bullet shot from A bullet shot from a gun?a gun?

– What forces are What forces are acting on the acting on the bullet?bullet?

A hammer and A hammer and feather dropped on feather dropped on the moon?the moon?

– What force is acting?What force is acting?

Newton’s 2Newton’s 2ndnd Law Law

F_____F_____

MM A A

Force is measured in Force is measured in newtonsnewtons. A newton . A newton is a is a kg mkg m

ss22

Example: A car with mass 1000kg hits Example: A car with mass 1000kg hits a tree and decelerates at 50m/s/s. a tree and decelerates at 50m/s/s. What force does it exert on the tree?What force does it exert on the tree?

Force = mass x acceleration

Answer:Answer:

F_____F_____

MM AA

1000kg * 50m/s/s = 1000kg * 50m/s/s =

50,000 50,000 kgmkgm or or 50,000N50,000N

ss22

F = maF = ma

A car is traveling at constant speed. A car is traveling at constant speed. Is a force acting on it?Is a force acting on it?

Answer: although there are forces Answer: although there are forces acting on the car (gravity, friction, air acting on the car (gravity, friction, air resistance,etc…) since there is no resistance,etc…) since there is no acceleration, the net force in the acceleration, the net force in the direction of motion must be zero.direction of motion must be zero.

More 2More 2ndnd Law Problems: Law Problems:

1. What is the acceleration on a 1. What is the acceleration on a mass of 50kg if a force of 10N is mass of 50kg if a force of 10N is applied?applied?

2. An object accelerates due to 2. An object accelerates due to gravity at a rate of 10m/s/s. If its gravity at a rate of 10m/s/s. If its mass is 15kg, what force is acting on mass is 15kg, what force is acting on the mass?the mass?

Answers:Answers:

__F_F_

M AM A

1. _1. _10N_10N_ 0.2 n/kg= 0.2 kg m = 0.2 m/s2

50kg A 50kg A s2/kg/kg

2. 2. F______ F______

15kg 10m/s15kg 10m/s22

150 kgm/s150 kgm/s22 or or 150N150N

Gravity is a force that causes Gravity is a force that causes an accelerationan acceleration

On earth, ALL objects accelerate at On earth, ALL objects accelerate at 9.8m/s9.8m/s22 (ignoring air resistance) (ignoring air resistance) because of gravity.because of gravity.

No matter what the mass, ALL No matter what the mass, ALL objects on earth accelerate at 9.8 objects on earth accelerate at 9.8 m/sm/s22

Newton’s 2Newton’s 2ndnd Law Law

F__ F__ m am a

3.3. An object of mass 3kg falls off a An object of mass 3kg falls off a cliff. How much force is acting cliff. How much force is acting on the object?on the object?

AnswerAnswer: F_______: F_______ 3kg 9.8 m/s3kg 9.8 m/s22

29.4N29.4N

Free fall without air Free fall without air resistanceresistance

Both the elephant and the feather land at the same time!

Free fall with air Free fall with air resistanceresistanceThe feather takeslots longer to hitthe ground!

Newton’s Third LawNewton’s Third Law

For every action, there is an equal For every action, there is an equal and opposite reactionand opposite reaction

For every action, there is an For every action, there is an equal and opposite reactionequal and opposite reaction

In rocketry……In rocketry……The gases being expelledis the action. The rocket propelling forward is the reaction.

How do Newton’s laws describe How do Newton’s laws describe the motorcycle rider?the motorcycle rider?

11stst: Inertia keeps the rider moving : Inertia keeps the rider moving forward.forward.

22ndnd: The force with which the rider hits : The force with which the rider hits the ground will be equal to his mass the ground will be equal to his mass times his deceleration.times his deceleration.

33rdrd: The action of hitting the ground : The action of hitting the ground will equal the reaction of the dirt.will equal the reaction of the dirt.

Projectile motionProjectile motion

Projectile motionProjectile motion

An object moves forward due An object moves forward due to inertia and accelerates to inertia and accelerates downward due to gravity.downward due to gravity.

In this video, the ball is In this video, the ball is dropped straight down and dropped straight down and

moves forward due to inertiamoves forward due to inertia

Projectile motion of a ball fired Projectile motion of a ball fired verticallyvertically

The motionof the truckcauses theball to projectforward.

What would happen ifthe truckstopped?

Package dropped from Package dropped from planeplane

The forward motionof the plane causes the package to land in front of where it’s dropped.

SatellitesSatellites