Class Starter1. If a car travels west 75 kilometers takes a u-

turn and travels back east 25 kilometers what is the car’s final displacement?

2. If a car at rest, traveled north 5.5 s and reached a final velocity of 22.0 m/s, what was the car’s acceleration?

3. If a force propels a car to move 300km in 4hrs what is the average speed (do not need to convert)?

50 km west

4.0 m/s2

75 km/h

Ch 12Ch 12

http://www1.teachertube.com/viewVideo.php?video_id=540&title=ForceMan_

WHAT IS A FORCE?– a push or a pull

– one body exerts a force on another

– SI units are Newton (N), 1N=1kgm/s2

– = ( m )( a )

F

m a

Effects of forces on objects

– Balanced forces – forces are Balanced forces – forces are

equal in size & opposite in equal in size & opposite in

directiondirection

–Net force – force that changes Net force – force that changes the velocity of the objectthe velocity of the object

•Object accelerates in the Object accelerates in the direction of the greater forcedirection of the greater force

• A group of animals are playing tug-of-war. The elephants pull the rope with a force of 100N to the left. The donkeys pull the rope with a force of 105N to the right. Which way will the rope accelerate?

• To the RIGHT

Friction– A force that opposes motion

between 2 surfaces that are touching each other

Friction–Amount of friction depends on 2 factors

1. the kinds of surfaces (texture)

2. the force pressing the surfaces together (pressure)

Four types of Friction

1. Static-friction force acts on 1. Static-friction force acts on objects that are objects that are NOTNOT moving, moving, acts in direction acts in direction OPPOSITEOPPOSITE to to the applied forcethe applied force

2. Sliding-force opposes the 2. Sliding-force opposes the direction of motion of object as direction of motion of object as it slides over a surface it slides over a surface (less (less than static)than static)

Four types of Friction

3. Rolling-force acts on rolling Rolling-force acts on rolling objects (less than static or objects (less than static or sliding)sliding)

4. Fluid-opposes motion of 4. Fluid-opposes motion of

an object through an object through

a liquid/gasa liquid/gas

Gravitational Force

• Gravity – the attracting force exerted by every object on every other object

• Amount of gravitational force between objects depends on 2 things...

Gravitational Force

1) mass

a) more mass=more g force

b) less mass=less g force

2) distance

a) greater distance=less g force

b) lesser distance=more g force

ACCELERATION DUE TO GRAVITY

Falling objects accelerate at 9.8 m/s2

*same for ALL

(in a vacuum w/

no air resistance)This means that for each s the body is falling, its velocity increases by 9.8 m per s

Air Resistance

• force air exerts on a moving object• Opposes gravity• Amount depends

on:1. Speed 2. Size3. Shape 4.Density

Terminal Velocity

• The highest velocity that will be reached by a falling object

• Downward Fg equals the upward Fair resistance

Projectiles • Anything shot/thrown through the air• A thrown ball has constant horizontal

velocity• Best angle to get farthest distance = 45O

• Gravity pulls the ball downward

• Vertical acceleration increases changing the direction of the projectile to forward and downward, making a curved path

Projectiles

Circles Centripetal Acceleration towards the center of a curved or circular path

Circles

Centripetal Force exerted toward the center of a curved path,

w/out this force object will shoot off in a straight

line in the direction it was

traveling

STAR Questions

• List all 4 things that affect the amount of air resistance on an object.

• Speed, size, shape and density

• List the 2 things that affect the amount of friction btw 2 objects.

• Texture and pressure

• List the 2 things that affect the amt of gravitational force btw 2 objects.

• Mass and distance

Newton’s 1st Law

• States an object moving at a constant velocity keeps moving at that velocity unless a net force acts on it.

• If an object is at rest, it stays in rest unless a net force acts on it.

• “If I push it, it goes”

• Also called the law of inertia

Inertia

• Tendency of an object to resist any change in its motion

• If an object is moving, it keeps moving at that speed & direction unless a force acts on it

Inertia• The more mass an object has, the

greater its inertia

• More force would be needed to change the velocity of the object w/ greater inertia (which also means greater mass)

Newton’s 2nd Law

• The acceleration of an object is equal The acceleration of an object is equal to the net force acting on it divided by to the net force acting on it divided by the object’s massthe object’s mass

• a=F/m or simply F=maa=F/m or simply F=ma

• amt of acceleration is affected byamt of acceleration is affected by

1. size of the force1. size of the force

2. mass of the object2. mass of the objectF

m a

Thanks Cambridge Physics Outlet for amazing graphics!

Examples:

• What is the force required to push a 25kg girl in a 100kg wagon with an acceleration of 3m/s2?

• Find the acceleration of an object with a mass of 20kg and a Force of 50N being applied to it?

Mass• amount of matter in an object• constant independent of location• measured in g & Kg

Weight

• measure of force of gravity

on an object Fg=W=mg

• measured in Newtons (N)

*g = 9.8m/s2

*Mass has to be in

kg to calculate Weight

Fg

m g

• What is the weight of a person who has a mass 50,000 g?

• If a person has a mass of 50kg on Earth, would mass be less than 50kg on moon?

• Would they weigh the same on the moon?

Newton’s third law of motion

• Whenever one object exerts a force on a second object, the second object exerts an equal & opposite force on the first object (action / reaction)

• They do NOT cancel out because they act on different objects

Thanks Cambridge Physics Outlet for amazing graphics!

Momentum

• More mass = More momentum

• More velocity = More momentum

• Momentum is transferred

Law of conservation of Momentum

• In a closed system, the loss of momentum of one objects equals the gain in momentum of another object – momentum is CONSERVED

STAR Questions

• What is the rate of acceleration due to gravity for a car?

• 9.8 m/s2

• Describe Newton’s 1st law.

• If I push it = it goes, if no push = no go

• Describe Newton’s 3rd law.

• Action = reaction