Transcript Slide 1
ENERGY
AND
WORK
Chapter 7
Energy: The ability to do work
Types –
Kinetic – energy of motion
Mechanical – done by machines
Potential – stored energy (gravitational
or elastic/flexed solids)
Chemical – found in the bonds between
atoms
Types cont.
Thermal – heat or internal kinetic
energy
Sound – vibrations in matter carried
on waves
Light – electromagnetic radiation
Electrical – moving and static
electrical charges
Nuclear – found within the nucleus
of atoms
Mechanical Energy and
Work
Work – a force exerted
during a distance. W = F*d
Work and Energy… You
must have energy to do
work.
Unit is Joules (J) .
Mechanical Work
230N
A weightlifter attempting to lift 230N
(about 50 lbs) must pull with a
force equal to the weight of the
barbell to get it off the ground.
At the top of his lift, he has raised the
bars 1 meter.
1m
How much work has he done?
W = F*d
W = 230N * 1m
What unit do we use?
W = 230 J
Joules
230N
Cont.
How much work to raise the
bar over his head to the
height of 2.5 m? *
W= 230N*2.5m
W= 575J
If we doubled the weight to
460N, what happens to the
amount of work?
2.5
m
It would
double as
well! Directly
Proportional
*If his arms were long enough!
Work and Vectors
(direction)
Work is a vector, which means it has a direction.
Work can only be done if the force and distance
directions are the same.
F
NO WORK
DONE!!!
distance
Power
Amount of work done per the
amount of time it takes to do it.
Power =
Work done (J)
Time (s)
Unit of power = J/s = 1 watt (W)
1 kilowatt (kW) = 1000 W
Law of Conservation of
Energy
1st Law of Thermodynamics
Energy can be changed from one form to
another, but it can NOT be destroyed.
When a change of energy occurs, the final
energy is equal to the beginning energy.
The ultimate source of ALL energy on
Earth?
Conservation of Energy:
bow and arrow
The bow stores potential
energy in its bent limbs
when drawn.
This energy is transformed
into kinetic energy as the
arrow is released.
PE of the bow = KE of
arrow
PE & KE = work the archer
had to use to draw the bow
Where does the energy go???
Even though the arrow is now at rest (no
KE) and the bow is not drawn (no PE), NO
ENERGY HAS BEEN DESTROYED!
Where did it go?...
The target did “Work” to the arrow by
bringing it to rest with friction… a force
which was exerted over a distance (the
depth the arrow entered the target)
Whenever energy is “lost” it is ALWAYS
converted to HEAT.
Potential Energy: Stored!
Gravitational
PE g = mass * gravity * height
Higher an object = ____PE
Greater the mass = ____PE
Greater gravity = ____PE
Elastic
PE elastic= distance stretched x weight
Greater the weight = ____PE
Greater the distance stretched = ____PE
Kinetic Energy: Motion!
KE = ½ mass * velocity2
The more massive the object = ____KE
The faster the object = ____KE
Why does velocity have a greater role in
KE than mass?
The mass is cut in half but the velocity is squared!
Other thoughts
What would happen to the depth of
penetration if the arrow were moving faster?
What happens to the speed of the arrow
when shot from a “stiffer” (harder to draw)
bow?
What happens to the speed of the arrow if it
weighs more?
What happens to the speed of the arrow if it
is longer? (bow pushes it for a longer
distance)
Law of Conservation of
Energy Again!!!
Energy can be changed from one form to another, but it
can NOT be destroyed.
This means that when a change of energy occurs, the
final energy is equal to the beginning energy.
Law of Conservation of
Energy – YET AGAIN!!!
KEi + PEi = KEf + PEf
In other words, the sum total of KE +PE before
and after some kind of transformation MUST be
equal.
Conversions
meters
Distance has to be in ______.
Newtons
Force has to be in ______.
How do you change kg to N? x10
seconds
Time has to be in _____.
Mass has to be in kilograms
_____.
m/s
Velocity has to be in _____.
How do you change W to kW? /103
Law of Thermodynamics - Entropy
Entropy – a change of a usuable form of energy
into an unusable form… HEAT (thermal energy)
Whenever energy is “lost” it is ALWAYS
converted in to HEAT.