Transcript Energy - vevansphysics

Objective 1: Relate the
Conservation of Energy to
energy transformations
Describe how energy--mechanical, electrical,
chemical, light, sound, and heat--can be
transformed from one form to another
Show understanding that energy
transformations result in no net gain or loss
of energy, but that in energy conversions
less energy is available due to heat loss.
Apply Conservation of
Energy
Apply the concept of conservation and
transformation of energy within and
between organisms and the environment-such as food chains, food webs, and
energy pyramids
Apply the concept of conservation and
transformation of energy to other
everyday phenomena.
Objective 2: Relate waves
to the transfer of energy
Relate wavelength to energy
Relate frequency to energy
Relate wavelength to frequency
Describe how waves travel through
different kinds of media
Mechanical waves
Water, Sound, Slinky, etc.
Electromagnetic waves
Describe how waves can be
destructive &/or beneficial
Describe how waves--earthquake waves,
water waves, and electromagnetic waves-can be destructive (harmful) or beneficial
(good) due to the transfer of energy
Destruction (cons)
Benefits (pros)
ALL WAVES
Transfer energy from one place to another
Energy transferred does NOT have mass.
Actual particles of the wave (such as water
waves) are NOT transferred, but stay in the
same place
Have wavelength, frequency, amplitude
SHORTER WAVELENGTH means HIGHER
FREQUENCY AND ENERGY!
Mechanical vs. Electromagnetic
Require a medium, or
Require NO medium, can
something to travel
travel through outer
through (cannot travel
space!
through space)
Examples of
Water waves, waves in a ELECTROMAGNETIC
rope or slinky, and sound WAVES:
waves are examples
Electromagnetic waves,
from low to high frequency:
RADIO WAVES
LONG wavelength, LOW
frequency AND energy
MICROWAVES
INFRARED (HEAT)
VISIBLE LIGHT
ULTRAVIOLET
HIGH wavelength, HIGH
X-RAYS,
frequency AND energy
GAMMA RAYS
VISIBLE LIGHT
Only a SMALL band of the EM spectrum
Regular “white” light can be separated into all
the different colors with a prism
This was discovered by Isaac Newton!
RED (longest wavelength, lowest energy) to
VIOLET (shortest wavelength, highest energy)
ROY G BIV stands for Red, Orange, Yellow,
Green, Blue, Indigo, Violet
Longitudinal vs. Transverse
Compression waves
where one part of a
medium smashes into
another
Wave particles travel
parallel to the energy
SOUND WAVES are
longitudinal. They
cannot travel in space
because there is no
medium
Up and down waves,
like wiggling a rope
back and forth
Wave particles travel
perpendicular (at
right angles) to the
energy being
transferred
Electromagnetic
waves are transverse!
Electromagnetic vs Sound
ALL travel at the
Travels MANY MANY
“speed of light”,
times slower, only about
186,000 miles/second 370 m/s (almost a million
or 300,000,000 m/s
times slower!)
Transverse
Longitudinal
Don’t need a medium Require a medium
Can travel through
Cannot travel through
outer space, all
outer space
across the universe Energy gets
dispersed(spreads out)
quickly
ENERGY
The ability to “do work” or make a
CHANGE in something
Energy has many forms, and all can be
transformed from one to another
There is a CONSTANT amount of energy
in any given closed system, even in the
universe as a whole!
ENERGY AND WORK
An “ideal system” means NO friction, and no
energy “lost” as heat
Energy is NEVER destroyed. It is only “lost” if
it becomes unusable
In an ideal system (or machine), you get ALL
the energy OUT that you had to put IN. This is
called 100% efficiency.
There are NO “ideal systems” in real life!
In REAL LIFE some energy is ALWAYS changed
to “lost” heat because of friction!
ENERGY AND WORK UNITS
MASS is measured in kilograms [kg]
WEIGHT and other FORCES are measured
in NEWTONS [N]
ENERGY is usually measured in Joules [J]
WORK is usually measured in Newtonmeters [N m]
SINCE ENERGY AND WORK ARE EQUAL, A
JOULE IS EQUAL TO A NEWTON-METER!
POTENTIAL ENERGY
This is STORED ENERGY
Most commonly means GRAVITATIONAL
potential energy, or energy stored because
of a position HIGHER than some reference
point (like the ground)
Potential energy continued
Can be ELASTIC OR SPRING potential
energy, like the energy stored in a
stretched rubber band, the spring in a
wind up toy, or a drawn bow before
shooting an arrow
Potential energy is also stored in
batteries, as CHEMICAL potential energy!
POTENTIAL ENERGY continued
POTENTIAL ENERGY can also be stored up in
chemical bonds, such as in food or fat
There is a tremendous amount of potential
energy in MASS ITSELF, as Einstein showed
with E = mc2
MASS itself is like VERY concentrated,
congealed energy!
Gravitational potential energy is described by
PE = mgh
or mass x gravity x height
KINETIC ENERGY
Energy of motion (kine- means MOTION, like
cinema means moving picture or movie!)
KE = 1/2 mass times velocity squared
If something is NOT MOVING, is has ZERO
KINETIC ENERGY!
In an “ideal system,” ALL (or 100%) of the
WORK you put IN can be changed to KE
In real machines, most energy is changed to
heat. A car is only about 30% efficient! Only
30% of the gas gets changed to KE!
POTENTIAL energy example
If a rock has a mass of 5 kg, and it is on a hill 2
m high, how much PE does it have?
PE = mgh [or mass in kg x height in meters]
g = 10 m/s/s
Weight in Newtons = mass in kg x gravity!
PE = weight in Newtons x height in meters
so PE = 5 kg (10 m/s/s) (2m) = 100 Joules or
100 J !
PE Problems
1. If a rock has 5 kg of mass and is lifted
up a 1 m hill, how much PE does it have?
2. If a rock has has 20 N of weight and
sits on top of a 2 meter tall box, how
much potential energy does it have?
PE Solutions
1. PE = mgh
so PE = 5 kg x 10 m/s/s x 1 m = 50 J
2. PE = weight x height
so PE = 20 N x 2 meters =40 J
Reminder: Weight = mass x gravity. For
example, 2 kg of mass X 10 m/s/s = 20 N!
Kinetic energy example
KE = 1/2 mass x velocity squared
If a 3,000 kg car is traveling at 10 m/s,
how much KE does it have?
KE = 1/2 (3,000 kg) (10 m/s)2

= 1,500 (100) = 150,000 J!
Kinetic energy problems
1. If a 60 kg girl is running at 5 m/s, how
much Kinetic energy does she have?
2. If the girl stops and sits on a bench to
rest, how much kinetic energy does she
have?
Kinetic energy solutions
1. KE = 1/2 mv2
so KE = 1/2 (60 kg) (5 m/s)2
so KE = 30 (25) = 750 J
2. KE = 1/2 (60 kg) (0 m/s)2
so KE = 0
She is not moving, so she has no Kinetic
energy while sitting on the bench!
transferred back and forth
If you do 20 J of WORK lifting a rock onto a
table, how much PE does the rock have?
If the rock then falls off the table, how much
KINETIC ENERGY does it have just before it
hits the floor?
WHERE did the rock GET the kinetic energy?
When the rock smashes into the floor, where
does the energy go?
KE if you double mass
If you had instead lifted a rock with
TWICE as much mass, how much more
WORK would you have put in to lift it?
How much more PE would it have had at
the top?
How much more KE at the bottom?
If you lifted more distance
If, instead of lifting a rock with twice the
mass, you lifted the SAME rock twice the
height, how much more WORK would you
have done?
What if you lifted a rock with TWICE the
MASS a distance TWICE AS FAR? How
much more Work would you have to do?
Roller Coaster
ON a roller coaster, at what point is your
POTENTIAL ENERGY greatest?
Where is your KINETIC ENERGY greatest?
Ignoring energy “lost” as heat due to
friction, what can you say about the
TOTAL amount of energy for the whole
ride on the roller coaster?
Roller Coaster continued
As you go DOWN a hill on the ride,
what kind of energy is being transferred
to what other kind?
As you go UP a hill on the ride,
what kind of energy is being transferred
to what other kind?
Energy transferred
How is energy transferred from the SUN,
to you walking down the sidewalk?
If you hit a baseball, describe the energy
changes occurring.
You throw a ball into the air. What energy
transformations are taking place?
MOMENTUM
Momentum is mass X
velocity
p = mv
The unit is kg m/s
The Law of Conservation
of Momentum
Momentum in a closed system is ALWAYS
CONSERVED
Momentum “before” an event is equal to
momentum “after” an event in the system
Classic examples are explosions, car
crashes, pool balls, & shooting a gun.
Momentum conserved
In a collision, if one pool ball collides into
another one that is at rest, pool ball 1
“shares” some momentum with pool ball 2
The TOTAL momentum of both pool balls
added together is THE SAME before and
after the collision
p = p’
m1v1 = m2v2
The Impulse Momentum
Theorem
CHANGE in momentum is EQUAL to
Impulse
IMPULSE is equal to IMPACT (or force)
times the TIME INTERVAL of the impact
p = Ft
or (mv) = F t
Applications
Why is it better to bend your knees when
you jump off a table?
Why do you move your hand backward
when catching a fast pitch?
Why do air bags help?
Why does a karate expert often try to
have a SHORT time of impact?
More applications
If you only want maximum velocity, such
as trying to achieve maximum range of a
golf ball, you should hit the ball with
a) a short time of impact
b) a long time of impact
c) it makes no difference
Applications continued
If a building is on fire and you want to
minimize the force of impact on your bones
when you jump from the 2nd story window,
you should
a) land with straight legs
b) land on your feet but bend knees
c) drop and roll to maximize time of impact