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.