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What is motion?
what do scientists define as motion ?
Why do the planets move the way they do?
Bellringer
Compare and explain in complete sentences and
formulas what is the Newton’s third law of motion.
Motion
What is motion?
what do scientists define as motion ?
Why do the planets move the way they do?
Keep in mind the way scientists like to work:
Observe
Define
Hypothesize/Predict
Test
repeat
Note:the study of motion was motivated by
the motion of celestial objects.
Galileo …
… studied motion;
… was the first to use a
telescope for astronomical
observations;
… saw that there were
ojects that did not move
around the Earth !
Galileo Galilei (1564-1642) “the astronomer”
Believed in the Copernican model.
Demonstrated that Kepler’s and Copernicus’ ideas
were right by making observations with his
telescope.
1. The heavens are not “perfect”:
• geological features on the moon
• sunspots on the surface of the Sun
2. The moons of Jupiter obey Kepler’s laws
3. Phases of Venus, supporting heliocentric theory
4. Observed that Mars, Jupiter and Saturn had no
phases.
5. Observed individual stars in the Milky Way,
thereby showing stellar parallax measuring is
possible.
Galileo Galilei (1564-1642)
Believed in the Copernican model.
Demonstrated that Kepler’s and Copernicus’ ideas
were right by making observations with his
telescope.
1. The heavens are not “perfect”:
• geological features on the moon
• sunspots on the surface of the Sun
Galileo Galilei (1564-1642)
2. The moons of Jupiter obey
Kepler’s laws
Galileo Galilei (1564-1642)
3. Phases of Venus, supporting heliocentric theory
4. Observed that Mars, Jupiter and Saturn had no
phases.
geocentric
heliocentric
3. Phases of Venus, supporting heliocentric theory
4. Observed that Mars, Jupiter and Saturn had no phases.
Galileo Galilei (1564-1642)
Believed in the Copernican model.
Demonstrated that Kepler’s and Copernicus’ ideas
were right by making observations with his
telescope.
2. The moons of Jupiter obey Kepler’s laws
3. Phases of Venus, supporting heliocentric theory
4. Observed that Mars, Jupiter and Saturn had no
phases.
5. Observed individual stars in the Milky Way,
thereby showing stellar parallax measuring is
possible.
Galileo Galilei (1564-1642) “the physicist”
Galileo also experimented with falling
and moving objects and crafted a
theory of motion.
Galileo’s workshop
at the Deutches
Museum
in Munich,
Germany
Galileo Galilei (1564-1642) “the physicist”
Galileo also experimented with falling
and moving objects and crafted a
theory of motion.
1.
An object in motion will continue moving along
a straight line with a constant speed until an
unbalanced force acts on it.
2.
dropped objects
move down at
10 m/s /s
WHY?
t=3 s
v=30 m/s
Gravity makes things accelerate at 10 m/s2
Iron ball
Wood ball
Acceleration of gravity is
independent of the mass
of the falling object!
Fourth manned lunar landing with
David R. Scott, Alfred M. Worden, and James B. Irwin.
Landed at Hadley rilleon July 30, 1971.
Observation
Define __________ ?
How do we describe motion?
How do we describe motion?
(define it)
100 m/s
displacement per time
10 m/s South-East
… & direction
60 km/hr --> 30 km/hr --> 0
changing displacement per time
How do we describe motion?
(define it)
Speed:
Rate at which
object moves
Velocity:
Speed and direction
Acceleration:
Any change
in speed or direction
Thank you, Galileo
Motion: speed, velocity, & acceleration
What about the mass?
Yes, motion depends on mass, too.
Motion with Mass
Momentum
Linear Momentum = mass  velocity
Angular momentum is
rotational momentum of a spinning or
orbiting object
So far…..
Describing Motion:
(basic ingredients)
1.
2.
3.
4.
change in position (displacement)
time
mass
direction
speed
velocity
acceleration
momentum
Isaac Newton (1642-1727)
• Born the year Galileo
died
• Contemporary of Bach
• Derived laws of gravity
and other laws of physics
“If I have seen further, it is by standing on the
shoulders of giants.”
--Isaac Newton
Isaac Newton (1642 - 1727)
• Building on the results of Galileo and Kepler
• Adding physics interpretations to the mathematical
descriptions of astronomy by Copernicus, Galileo
and Kepler
Major achievements:
1. Invented Calculus as a necessary tool to solve
mathematical problems related to motion
2. Formulated the three laws of motion
3. Formulated the universal law of mutual gravitation
Newton’s 3 Laws of Motion
science.discovery.com/interactives/literacy/newton/newton.html
1
Inertia
2
F = ma
3
action = reaction
1. Newton’s Laws of Motion
Also known as :
The Law of Inertia
A body continues at rest or
in uniform motion in a straight line
unless acted upon by some net force.
Newton’s 1st:
object will stay
at rest (or in
uniform motion)
until acted on by
a FORCE
1. Newton’s Laws of Motion
An astronaut floating in
space will continue to
float forever in a straight
line unless some
external force is
accelerating him/her.
2. Newton’s Laws of Motion
The acceleration, a, of a
body is
directly proportional to
the net force F,
in the same direction
as the net force F,
inversely proportional
to its mass, m.
a=F  F=ma
m
Newton’s 2nd:
F = ma
(unbalanced forces
cause changes in
motion.)
3. Newton’s Laws of Motion
To every action,
there is an equal and
opposite reaction.
M = 70 kg
V=?
The same force that is
accelerating the boy
forward, is accelerating
the skateboard
backward.
m = 1 kg
v = 7 m/s
Newton’s 3rd: action - reaction
Newton’s 3rd:
action - reaction
The Universal Law of Gravity
Any two bodies are attracting each
other through gravitation, with a
force proportional to the product of
their masses and inversely
proportional to the square of their
distance:
F=-G
Mm ^
d
d2
(G is the Universal constant of gravity.)
What determines the strength of gravity?
The Universal Law of Gravitation:
1. Every mass attracts every other mass.
2. Attraction is directly proportional to the product
of their masses.
3. Attraction is inversely proportional to the
square of the distance between their centers.
F=ma
All objects on Earth fall with the same
acceleration, g.
g = 9.8 m/s2 (about 10 m/s2 --- Galileo)
The acceleration:
a=g
Fweight  m  g
Your weight is the force of Earth on YOU
Summary … so far:
• How do we describe motion?
– Speed = distance / time
– Speed & direction => velocity
– Change in velocity => acceleration
– Mass effect motion
– Momentum = mass x velocity
– Force causes change in momentum,
producing acceleration
Motion
• described by:
speed, velocity, and acceleration
• determined by:
Newton’s 3 Laws
• has:
energy
Energy
• makes change
Energy makes matter move,too.
Moving matter has energy.
Energy and matter changes form.
Energy cannot be destroyed.
There are 2 forms of ENERGY
Kinetic energy
is motion––
of waves,
electrons,
atoms,
molecules,
substances,
and objects.
Potential energy
is stored energy
and the
energy of position
of objects,
nucleus,
chemical
There are 2 forms of ENERGY
Kinetic energy
is motion––
Electrical --- charges
Radiant --- EM energy
Thermal --- heat
Motion --- Newton’s Laws
Sound --- waves through substances
There are 2 forms of ENERGY
Potential energy
is stored energy
and the
energy of position
Chemical --- stored in bonds (atom/molecule)
Stored mechanical --- springs, rubber band
Nuclear --- stored in nucleus (fusion/fission)
Gravitational --- stored in position
Gravitational Potential Energy
On Earth, GPE depends on:
– object’s mass (m)
– strength of gravity (g)
– distance object could
potentially fall
Gravitational Potential Energy
In space, an object or gas cloud has more
gravitational energy when it is spread out than
when it contracts.
 A contracting cloud converts gravitational
potential energy to thermal energy.
Mass-Energy
• Mass itself is a form of potential
energy
E =
• A small amount of mass
can release a great deal of
energy
• Concentrated energy can
spontaneously turn into
particles (for example, in
particle accelerators)
2
mc
Energy
• Energy can be neither created nor
destroyed.
• It can change form or be exchanged
between objects.
• The total energy content of the Universe
was determined in the Big Bang and
remains the same today.
Energy is Conserved
Conservation of energy
(energybefore = energyafter )
Anything else conserved?
Conservation of momentum
Conservation of Momentum
BEFORE
• The total momentum
of interacting objects
cannot change
unless an external
force acts on them
• Interacting objects
exchange momentum
through equal and
opposite forces
AFTER
Conservation of angular momentum
Angular momentum
conservation also
explains why objects
rotate faster as they
shrink in radius
MASS, WEIGHT… does it matter?
YES!
MASS is the amount of matter of the object.
Weight is the amount of force on the object.
Let’s apply EVERYTHING we know about
motion to orbital motion
… and so on and on ….
Orbital Motion
The Sun
exerts a
force on the
planets
(and vice
versa!)
How do gravity and energy together allow us
to understand orbits?
Total orbital energy
(gravitational+kinetic)
stays constant if there
is no external force

Orbits cannot
change
spontaneously.
More kinetic energy;
Less gravitational
energy
Less kinetic energy;
More gravitational energy.
How Can an Orbit Change ?
An object gains or lose orbital energy.
HOW does that happen?
• Friction: atmospheric
drag, or tidal flexing
of a “fluid” object
• A gravitational
encounter.
Conservation of Angular Momentum
angular momentum = (mass x velocity) x radius of orbit
• The angular momentum of an object cannot
change unless an external force (torque) is
acting on it
• Earth’s rotation and orbit will continue
forever because it can’t “get rid of “
angular momentum
So far …
• Why do objects move at constant velocity
if no force acts on them?
– Conservation of momentum
• Where do objects get their energy?
– Conservation of energy: energy cannot
be created or destroyed but only
transformed from one type to another.
– Energy comes in three basic types:
kinetic, potential, radiative.
In order to stay on a closed orbit, an object
has to be within a certain range of
velocities:
Too slow =>
Object falls
back down
to Earth
In order to stay on a closed orbit, an
object has to be within a certain range of
velocities:
Too fast =>
Object
escapes
Earth’s
gravity
Escape Velocity
• If an object gains
enough orbital energy, it
may escape (change
from a bound to
unbound orbit)
• Escape velocity from
Earth ≈ 11 km/s from
sea level (about 40,000
km/hr)
AstroTour
Velocity, Acceleration, Inertia
AstroTour
Newton’s Laws and Universal Gravitation
AstroTour
Elliptical Orbits
Now you know !