Transcript 15.1 Energy and Its Forms

Chapter 15 – Energy
15.1 Energy and Its Forms
Work is done when a force moves an object through
a distance
Energy – The ability to do work
In other words, energy is
transferred by moving an
object through a distance.
So, work is a transfer of energy
15.1 Energy and Its Forms
Think about the energy needed to
carry your backpack up a flight of
stairs
- You do work by lifting the backpack
against gravity (requires energy)
- The energy to do this comes from
your muscles
- Your muscles get the energy from
the food we eat
- The energy in the plants you eat, or animals you
eat that ate the plants, comes from the sun
15.1 Energy and Its Forms
Recall, work is done when a force moves an
object through a distance
What is Newton’s 2nd Law? F  ma
2
2
v

v
A kinematics equation:
f
i  2ad
v f  vi
2
Solve for a:
a
2d
2
Substitute a:
Rearrange:
Work
1
1
2
2
Fd  mv f  mvi
2
2
Kinetic
Energy
15.1 Energy and Its Forms
What is meant by kinetic energy?
Applying a force to an object through some distance sets that
object into motion
An object in motion can apply a force to something and in turn
do work to that object (it has energy– the ability to do work)
Kinetic Energy – energy an object
has because it is in motion
1 2
KE  mv
2
The moving bowling ball can do
work on the pins
The unit of KE is the joule
15.1 Energy and Its Forms
1 2
Kinetic Energy ( KE )  mv
2
- Doubling the mass Doubles the kinetic energy
- Doubling the speed Quadruples the kinetic energy
15.1 Energy and Its Forms
Page 448
- Read Math Skills
- Do Math Practice
15.1 Energy and Its Forms
Potential Energy – Energy that is stored as a result
of position or shape
15.1 Energy and Its Forms
Positive work must be done to raise the hammer
from the ground.
When raised to a height
and dropped, gravity
can do work on the
hammer
As it falls, the hammer
will gain kinetic energy,
and be able to do work
on the pile
The hammer, when
raised has the potential
to do work
15.1 Energy and Its Forms
15.1 Energy and Its Forms
Gravitational Potential Energy – Potential energy
that depends on an object’s height
- The baby works to get to the
top of the diving board (F x d)
- The work done is dependent on
the mass of the baby and the
height the baby climbed
- That work becomes
Gravitational Potential Energy
15.1 Energy and Its Forms
Gravitational Potential Energy depends on mass,
height, and the acceleration due to gravity (g)
Gravitational Potential Energy ( PE)  mgh
- mass (m) is in kilograms
- g is 9.8 m/s2
- height (h) is in meters
- always measured relative to an arbitrary
reference level
15.1 Energy and Its Forms
Elastic Potential Energy – The potential energy of
an object that is stretched or compressed
- Something is considered
elastic if it springs back to its
original shape after being
stretched or compressed
15.1 Energy and Its Forms
There are 6 major forms of energy
- Mechanical Energy
- Thermal Energy
- Chemical Energy
- Electrical Energy
- Electromagnetic Energy
- Nuclear Energy
15.1 Energy and Its Forms
Mechanical Energy – the energy associated with
motion and the position of everyday objects.
It is the sum of an object’s potential and kinetic
energy
15.1 Energy and Its Forms
Thermal Energy – The total potential and kinetic
energy of all the atoms and molecules in an object
Recall the kinetic theory of matter – atoms and
molecules are always in motion
15.1 Energy and Its Forms
Chemical Energy – The energy stored in chemical
bonds
When bonds are broken, the released energy can
do work
15.1 Energy and Its Forms
Electrical Energy – energy associated with electric
charge
Electric charges can exert forces that do work
15.1 Energy and Its Forms
Electromagnetic Energy – a form of energy that
travels through space in the form of waves
The sun radiates electromagnetic waves
15.1 Energy and Its Forms
Nuclear Energy – The energy stored in atomic
nuclei
Fission and Fusion are two processes that release
nuclear energy
15.2 Energy Conversion and Conservation
Energy conversion – energy can be converted from
one form into another
Gravitational potential
energy to kinetic energy
15.2 Energy Conversion and Conservation
Energy conversion – energy can be converted from
one form into another
Elastic potential energy to
kinetic energy
15.2 Energy Conversion and Conservation
Energy conversion – energy can be converted from
one form into another
Potential energy to electric
energy and
electromagnetic energy
15.2 Energy Conversion and Conservation
Law of Conservation of Energy – energy cannot
be created or destroyed
15.2 Energy Conversion and Conservation
Energy Conversions
Pendulums
15.2 Energy Conversion and Conservation
15.2 Energy Conversion and Conservation
15.2 Energy Conversion and Conservation
15.2 Energy Conversion and Conservation
15.2 Energy Conversion and Conservation
Energy Conversions
Pole Vault
15.2 Energy Conversion and Conservation
15.2 Energy Conversion and Conservation
Energy Conversions
Mechanical Energy – The sum of an object’s total
KE and total PE
Mechanical Energy = KE + PE
15.2 Energy Conversion and Conservation
Energy Conversions Calculations
Law of Conservation of Mechanical Energy
(ignore friction!)
(KE + PE)beginning = (KE + PE)end
15.2 Energy Conversion and Conservation
Example 1
If you throw a baseball straight up with a speed of 5 m/s,
what will be the speed of the ball when it comes back to
your hand. Ignore air resistance?
Example 2
A 1000 kg car is coasting at 10 m/s toward a hill that is 10 m
high. Will the car make it to the top of the hill if the driver does
not step on the gas pedal?
Page 458 Math Skills and Math Practice
Read Page 459 on E = mc2
15.2 Energy Conversion and Conservation
Questions to answer from Roller Coasters video
1. How does the roller coaster get to the top of the first
hill?
2. How does the roller coaster continue to move after
the first hill?
3. When does the roller coaster have the most potential
energy?
4. What happen’s to the roller coaster’s potential energy
as it goes down a hill?
5. Why are the frames of modern roller coasters made
out of steel, instead of the wooden frames that were
once used?
15.2 Energy Resources
15.2 Energy Conversion and Conservation
Energy Conversions
The gravitational potential energy of an object is
converted to kinetic energy of motion as the
object falls (roller coaster)
The elastic potential energy of a spring is
converted to kinetic energy of motion as the
spring is restored (screen door)
HEAT
HEAT
• Conduction- transfer of thermal energy
with no overall transfer of matter
• Occurs within a material or between
materials that are touching
• Due to particle collisions
• NEWTON’S CRADLE
CONDUCTION
Conduction in gases
• Slower than in liquids or solids.
• Why?
• Gas particles are further apart and
therefore collide less frequently
Why are metals good condcutors?
• Some electrons move freely in metals
• These collide with each other and other
atoms to transfer thermal energy
Thermal Conductors
• Conduct thermal energy well
• What are pots and pans made of?
• Why do tile floors feel colder than carpet
or wood?
Thermal Insulators
• Conduct thermal energy poorly
• Air- very good insulator- double paned
windows
• What are coolers made of?
CONVECTION
• Transfer of energy when particles of fluids
move from one place to another
• We all know that hot air……
• RISES
• Why?
• Becomes less dense as particles gain KE
and spread out.
• Therefore, cool air sinks
CONVECTION
Convection Currents
• The cycling of hot air rising and cool air
sinking
• Important in ocean currents, weather
systems, movements of hot rocks in the
Earth
RADIATION
• Transfer of energy by waves moving
through space
• What do you feel when you stand by a hot
stove without touching it?
• What do you feel when you go outside on
a sunny day?
• When temperature increases rate of
radiation increases.