Force• What is force?

– Push or Pull• It’s what causes “things” to accelerate

1000 kg1000 kg

Balanced v. Unbalanced Forces

• If all forces are balanced there is no acceleration in any direction. – (Either Zero Motion or Constant Velocity)

• If one force is greater than the opposing force we get acceleration in a direction.

Balanced Forces (Balanced Forces = No Acceleration)

5 N 5 N

Equal Pushing

Object

Notice that all the forces are equally pointed in the opposite direction. Hence they balance

each other – or cancel each other.

Forces may cancel each other and produce No Acceleration!

Gravity pulls down on you…

The ground pushes back up…

THIS KEEPS YOU WHERE YOU ARE!If these football players push on each other equally as hard,

will either one move?

Gravity pulls down

Ground pushes up

Balanced Forces (Balanced Forces = No Acceleration)

5 N5 N

5 N 5 N

Equal Pushing

Equal Pulling

Forces may cancel each other and produce No Acceleration!

Balanced Forces (Balanced Forces = No Acceleration)

Unbalanced ForcesCauses Acceleration

Adding Forces Subtracting Forces

Adding Forces

• Two forces can add together to produce a larger net force than either original force.– Positive Acceleration occurs (Speed up)

5 N 5 N = 10 N

Unbalanced ForcesCauses Acceleration

Adding Forces

5 NObject

5 N

Notice that all the forces are pointed in the same direction. Hence they add together.

Subtracting Forces

• Two forces can subtract to produce a net force in the direction of the larger force.– Negative Acceleration occurs (Slow Down)

10 N 5 N= 5 N

Unbalanced ForcesCauses Acceleration

Subtracting Forces

5 N5 NObject

5 N

Notice that all the forces are unequal and pointed in the opposite direction. Hence they

are unbalanced and in opposition to each other – or one partially cancels the other.

Balanced Forces

Balanced

Push

i.e. Pushing a Car

No Acceleration

Balanced

Pull

i.e. Tug-o-war

No Acceleration

10N10N

10N10N

Un-Balanced Forces

AdditiveUn-Balanced

Same Direction

Faster

Acceleration

SubtractiveUn-Balanced

Opposite Direction

Slower

Acceleration

10N10N

10N10N

10N

Balance between Gravity and Friction

The skydiver is colliding with themolecules of gas in the air!

Which slows down the fall of the skydiver!

Q: If the friction of the air and the pull of gravity were in perfect balance,

what would the skydiver be doing?

A: The skydiver would be going a constant velocity (moving with no

acceleration) down towards the ground until it hit the ground.

How it worksNotice that as the skydiver

leaves the plane, the force of force of gravitygravity accelerates him faster towards the earth; however, the moment he jumps out of

the plane, another force begins to oppose gravity – Air ResistanceAir Resistance, (Friction).

This friction with the air begins to slow down his rate of acceleration. Meaning,

he’s still speeding up, just not as fast.

Once the force of air friction force of air friction equals the force of gravityequals the force of gravity,

the skydiver steadily descends towards earth at a

constant velocity.

Gravity

0 m/s30 m/s

Air Resistance

Constant Velocity

Speed:

0 m/s10 m/s20 m/s25 m/s28 m/s

Acc

eler

atin

gC

onst

ant

Vel

ocity

Accelerating

Forces may cancel each other and produce No Acceleration!

Balanced Forces (Balanced Forces = No Acceleration)

5 N 5 N

Equal Pushing

Object

Notice that all the forces are equally pointed in the opposite direction. Hence they balance

each other – or cancel each other.

Wait a minute. This object was moving.That’s Right. As you may have noticed, although I may have balanced forces, I might

still be moving.

Notice that when the forces are balanced, the object might still be moving, but the objects are not accelerating, instead they have a constant velocity. Hence, once in

motion – it’s always in motion unless acted upon by what? Another Force.

Equal Pulling

Constant Velocity

1 2 3 4 5

In other words, you still need to use the gas In other words, you still need to use the gas peddle to balance out all the force of friction to peddle to balance out all the force of friction to

keep you going at a constant speed.keep you going at a constant speed.

Time [Sec]

Dis

tanc

e [m

]

0 1

2

3

4 5

6

5 N

Acceleration

To get an object to move faster, create an To get an object to move faster, create an unbalanced force!unbalanced force!

1 2

5 N

Force• Speed = Distance divided by time

• S= D/T

• What is the speed of a rocket traveling at 4,000 miles in 1.5 hours?

2,667 miles/hr

The Law of Conservation of Energy

• Energy cannot be __________ or ____________; it may be transformed from one form into another, but the total amount of energy _________ changes.

createddestroyed

never

• “the ability to do work”

• The combination of energy and matter make up the universe: – Matter is substance, and energy is the

____________ of substance.

What is energy?

mover

Energy is nature’s way of keeping score.

We sense energy only when it changes,

either a transformation from one form of energy to another,

or a transfer of energy from one point to another.

How is all energy divided?

PotentialEnergy

KineticEnergy

All Energy

GravitationPotentialEnergy

ElasticPotentialEnergy

ChemicalPotentialEnergy

Potential Kinetic

• energy of _________

or energy in ______________.

– Water behind a dam– Hammer over head– Food on the plate

• energy of ________, the form capable of doing work – Flowing water– A falling hammer– Food being converted

into usable energy in the cell.

position

storage

motion

PE = mgh

____ = mass (kg)

____ = height (m)

____ = accel due to gravity

(9.8 m/s2)

m

hg

What is Gravitational Potential Energy?o Potential energy

due to an object’s position

o P.E. = ________x ________ x

_____________Don’t look down,

Rover!Good boy!

massheight

gravity

What is Potential Energy?

oEnergy that is stored and waiting to be used later

What is Elastic Potential Energy?

o Potential energy due compression or expansion of an elastic object.

Notice the ball compressingand expanding

What is Chemical Potential Energy?

o Potential energy stored within the chemical bonds of an object

What is Kinetic Energy?

o Energy an object has due to its motion

o K.E. = .5(mass x speed2)

KE = 1/2 m v2

• 4 kg bowling ball

• at 10 m/s

• = .5 (4kg) (10m/s)2 • = 200 J

• .25 kg baseball

• at 50 m/s

• = .5 (.25kg) (50 m/s)2

• = 312 J

Courtesy Jerry Ohlinger’s Movie Material Store

Work is done when a force is exerted over a distance.

Work

• is equal to the force that is exerted multiplied by the distance over which it is exerted.

• W = _______

• The unit of work combines the unit of force (N) with the unit of distance (m)

• Newton-meter (N-m) aka ________.

F x d

Joule

You carry a 20 kg suitcase upstairs, a distance of 4m. How much work did you

do? • W = F x d

• F = ma

• = (20 kg) (10m/s2) = 200 N

• W = F x d

• = (200 N) (4m)

• = 800 J

6 fundamental forms of energy

1. mechanical energy

2. Thermal (heat) energy

3. electromagnetic energy

4. electrical energy

5. nuclear energy

6. chemical energy

What is the source of our energy?

• The source of practically all our energy is the _______.SUN

What is Mechanical Energy?

o Energy due to a object’s _________

(kinetic) or __________ (potential).

The bowling ball has mechanical energy.

When the ball strikes the pins, mechanical energy is transferred

to the pins!

motionposition

Examples of Mechanical Energy

What is ElectromagneticEnergy?

o _________________

o Includes energy from gamma rays, xrays, ultraviolet rays, visible light,

infrared rays, microwave and

radio bands

Light energy

What is Electrical Energy?

o Energy caused by the movement of _____________

o Easily transported through power

lines and converted into other forms of

energy

electrons

What is Chemical Energy?

o Energy that is available for release

from _____________________.

The chemical bonds in a matchstick store

energy that is transformed into

thermal energy when the match is struck.

Chemical reactions

Examples of Chemical Energy

What is Thermal Energy?o Heat energy

o The heat energy of an object determines how active its atoms are.

A hot object is one whose atoms and molecules are excited and show rapid

movement.

A cooler object's molecules and atoms will show less

movement.

QUIZ TIME!What type of energy

cooks food in a microwave oven?ELECTROMAGNETIC

ENERGY

What type of energy is the spinning plate inside of a microwave oven?

MECHANICAL ENERGY

QUIZ TIME!Electrical energy is

transported to your house through power lines.

When you plug an electric fan to a power outlet, electrical energy is transform into what type of energy?

MECHANICAL ENERGY

QUIZ TIME!

What energy transformation occurs when an electric

lamp is turned on?

ELECTRICAL ENERGY

ELECTROMAGNETIC ENERGY

What types of energy are shown below?

Mechanical and Thermal Energy(Don’t forget friction)

What type of energy is shown below?

Chemical Energy

What types of energy are shown below?

Electrical, Mechanical andElectromagnetic Energy

What type of energy is shown below?

Chemical Energy (yummy)

What type of energy is shown below?

Thermal Energy

What types of energy are shown below?

Mechanical, Electromagnetic, Electrical, Chemical and

Thermal Energy

The Law of Conservation of Energy

• Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes.