Download Energy - Indiana University Astronomy

Vesta, the second
largest object in
the asteroid belt,
was recently
imaged for the first
time by the robotic
Dawn satellite that
arrived last month.
Homework #2
short answers due Wednesday, 2:30 pm
multiple choice deadline Thursday, 11:00 pm
The first exam has been shifted from Monday,
September 26 to Monday, October 3.
Scalars and Vectors
Scalar: a quantity described solely by its
size (and units)
Vector: a quantity described by its size
AND direction
• speed – rate at which an
object moves [e.g., m/s].
A scalar quantity.
• velocity – an object’s speed
AND direction, [e.g.,10 m/s
east]. A vector
quantity.
• acceleration – a change in an
object’s velocity, i.e., a
change in speed OR direction
[m/s2]. A vector quantity.
Momentum (p) – the mass of an object times its
velocity (p=mv)
Force (f) – anything that can cause a change in an
object’s momentum
As long as the object’s mass does not change, a force
causes a change in velocity, or an acceleration (a)
Force, momentum, and
acceleration are all vectors
Newton’s First Law of Motion
A body in motion remains in motion and a
body at rest remains at rest unless acted
upon by an outside force.
OR
If the net force acting on an object is zero,
then there is no change in the object’s motion.
Newton’s Second Law of Motion
The change in a body’s
velocity due to an applied
force is in the same
direction as the force, and
is proportional to the force,
but is inversely
proportional to the body’s
mass.
F = ma
Or
F = rate of change of momentum
Forces only affect motion in the direction of the force.
Gravity & Orbits
A planet is always changing
its direction of motion.
Newton’s second law
therefore states that a force
must be acting on the planet.
Gravity provides
this force.
Motion without
an applied force
gravity
F = ma can be rewritten to show that for a given force,
the acceleration is inversely proportional to the mass:
a=F/m
Least mass
a=F/m
Most mass
Intermediate
mass
Do not confuse mass and density
Mass = amount of matter
Density = amount of matter per volume
Higher density means more matter packed
into same volume
Law of Conservation of Momentum
• If the net force (sum of all external
forces) acting on an object is zero,
then the total momentum of a system
remains constant.
Momentum: p = mv
Newton’s Third Law of Motion
“For every applied
force, a force of equal
size but opposite
direction arises”
or
For every action there is an
equal and opposite reaction
A Universe of Matter and Energy
What is matter?
What is energy?
Matter – material such as rocks,
water, air; “stuff” composed of
atoms
Energy – makes or has the
potential to make matter move!
The history of the universe, including biological
organisms, is based upon the interplay between
matter and energy.
Three Basic Types of Energy
• kinetic
– energy of motion
• potential
– stored energy; e.g., chemical,
gravitational, electrical, etc.
• radiative
– energy transported by light
(electromagetic radiation)
Conservation of
Energy
Fundamental law of
nature
Energy can be neither
created nor destroyed
It can change form or be
exchanged between objects.
K.E.
The total energy content of
the Universe was determined
in the Big Bang and remains
the same today.
P.E.
R.E.
Kinetic Energy (K.E.):
energy of motion
K.E. = 1/2 mv2
(m is mass, v is velocity)
On the microscopic level:
temperature
is a measure of the
AVERAGE kinetic energy
of particles within a substance
Temperature Scales
Temperature vs. Heat
●
●
lower T
higher T
same T
Temperature is the average
kinetic energy.
Heat (thermal energy) is the
total kinetic energy.
less heat
more heat
Sound waves are a form of kinetic energy on
a microscopic level
(organized vibration of molecules)
Applying what we’ve learned -
pizza vs. soup
caution in the kitchen
Potential Energy:
Energy that is “stored” within an
object and that has the potential of
being released in a different form
Gravitational Potential Energy
●
●
gravitational potential energy is the
energy which an object stores due to
its ability to fall
It depends on:
– the object’s mass (m)
– the strength of gravity (g)
– the distance which it can fall (d)
●
P.E. = mgd
m
g
d
●
gravitational potential energy
●
P.E. = mgd
g
Mass-Energy Potential Energy
●
●
mass-energy: energy is stored in
matter itself
this mass-energy is what would
be released if an amount of mass,
m, were converted into energy
E = mc2
[ c = 3 x 108 m/s
is the speed of light]
Chemical Potential Energy
●
Chemical potential energy:
energy stored chemical bounds
There are many additional
examples of potential energy. e.g.,
stretched springs, …
Energy, while
conserved, can be
transformed from one
type of energy to
another
Potential
Kinetic
Potential
Kinetic
Kinetic
Potential
Orbits & Energy
Uphill
Maximum
Potential
Energy
Maximum
Kinetic
Energy
Downhill
From the
perspective of
energy, which
planet is more
difficult to reach
from the Earth?
Mars
Venus
From the
perspective of
energy, which
planet is more
difficult to reach
from the Earth?
Mars
It is “uphill”
ConceptTest
The gravitational potential energy of an interstellar
cloud of gas that is slowly shrinking in size …
(yellow) stays the same
(red) slowing changes into other forms of energy
(green) slowly increases
(blue) interstellar clouds do not have gravitational
potential energy
ConceptTest
The gravitational potential energy of an interstellar
cloud of gas that is slowly shrinking in size …
(yellow) stays the same
(red) slowing changes into other forms of energy
(green) slowly increases
(blue) interstellar clouds do not have gravitational
potential energy
ConceptTest
In the not too distant future scientists will develop an
engine that produces more energy than it uses. This
statement is …
(yellow) likely to be true
(red) false
(green) difficult to determine if this will happen or not.
(blue) this has already happened, but the government is
suppressing this discovery.
ConceptTest
In the not too distant future scientists will develop an
engine that produces more energy than it uses. This
statement is …
(yellow) likely to be true
(red) false
(green) difficult to determine if this will happen or not.
(blue) this has already happened, but the government is
suppressing this discovery.