Transcript Conceptual Physics

Chapter Fourteen Notes:
Satellite Motion
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Circular Motion Principles for Satellites
A satellite is any object which is orbiting the earth, sun or other
massive body. Satellites can be categorized as natural satellites or
man-made satellites. The moon, the planets and comets are
examples of natural satellites. Accompanying the orbit of natural
satellites are a host of satellites launched from earth for purposes of
communication,
scientific
research,
weather
forecasting,
intelligence, etc. Whether a moon, a planet, or some man-made
satellite, every satellite's motion is governed by the same physics
principles and described by the same mathematical equations.
The fundamental principle to be understood concerning satellites is
that a satellite is a projectile. That is to say, a satellite is an object
upon which the only force is gravity. Once launched into orbit, the
only force governing the motion of a satellite is the force of gravity.
Newton was the first to theorize that a projectile launched with
sufficient speed would actually orbit the earth. Consider a projectile
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launched horizontally from the top of the legendary Newton's
Mountain - at a location high above the influence of air drag.
As the projectile moves horizontally in a direction tangent to the
earth, the force of gravity would pull it downward. And, if the launch
speed was too small, it would eventually fall to earth. The diagram at
the right resembles that found in Newton's original writings. Paths A
and B illustrate the path of a projectile with insufficient launch speed
for orbital motion. But if launched with sufficient speed, the
projectile would fall towards the earth at the same rate that the
earth curves. This would cause the projectile to stay the same height
above the earth and to orbit in a circular path (such as path C). And
at even greater launch speeds, a cannonball would once more orbit
the earth, but now in an elliptical path (as in path D). At every point
along its trajectory, a satellite is falling toward the earth. Yet
because the earth curves, it never reaches the earth.
So what launch speed does a satellite need in order to orbit
the earth? The answer emerges from a basic fact about the
curvature of the earth. For every 8000 meters measured along
the horizon of the earth, the earth's surface curves downward
by approximately 5 meters. So if you were to look out
horizontally along the horizon of the Earth for 8000 meters,
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you would observe that the Earth curves downwards below
this straight-line path a distance of 5 meters. For a projectile
to orbit the earth, it must travel horizontally a distance of
8000 meters for every 5 meters of vertical fall. It so happens
that the vertical distance which a horizontally launched
projectile would fall in its first second is approximately 5
meters (0.5*g*t2). For this reason, a projectile launched
horizontally with a speed of about 8000 m/s will be capable
of orbiting the earth in a circular path. This assumes that it is
launched above the surface of the earth and encounters
negligible atmospheric drag. As the projectile travels
tangentially a distance of 8000 meters in 1 second, it will
drop approximately 5 meters towards the earth. Yet, the
projectile will remain the same distance above the earth due
to the fact that the earth curves at the same rate that the
projectile falls. If shot with a speed greater than 8000 m/s, it
would orbit the earth in an elliptical path.
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Velocity, Acceleration and Force Vectors
The motion of an orbiting satellite can be described by the same
motion characteristics as any object in circular motion. The velocity
of the satellite would be directed tangent to the circle at every point
along its path. The acceleration of the satellite would be directed
towards the center of the circle - towards the central body which it
is orbiting. And this acceleration is caused by a net force which is
directed inwards in the same direction as the acceleration.
This centripetal force is supplied by gravity - the
force which universally acts at a distance between
any two objects which have mass. Were it not for
this force, the satellite in motion would continue in
motion at the same speed and in the same
direction. It would follow its inertial, straight-line
path. Like any projectile, gravity alone influences
the satellite's trajectory such that it always falls
below its straight-line, inertial path.
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This is depicted in the diagram below. Observe that the
inward net force pushes (or pulls) the satellite (denoted by
blue circle) inwards relative to its straight-line path tangent
to the circle. As a result, after the first interval of time, the
satellite is positioned at position 1 rather than position 1'. In
the next interval of time, the same satellite would travel
tangent to the circle in the absence of gravity and be at
position 2'; but because of the inward force the satellite has
moved to position 2 instead. In the next interval of time, the
same satellite has moved inward to position 3 instead of
tangentially to position 3'. This same reasoning can be
repeated to explain how the inward force causes the satellite
to fall towards the earth without actually falling into it.
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Occasionally satellites will orbit in paths which can be described as
ellipses. In such cases, the central body is located at one of the foci
of the ellipse. Similar motion characteristics apply for satellites
moving in elliptical paths. The velocity of the satellite is directed
tangent to the ellipse. The acceleration of the satellite is directed
towards the focus of the ellipse. And in accord with Newton's second
law of motion, the net force acting upon the satellite is directed in
the same direction as the acceleration - towards the focus of the
ellipse. Once more, this net force is supplied by the force of
gravitational attraction between the central body and the orbiting
satellite. In the case of elliptical paths, there is a component of force
in the same direction as (or opposite direction as) the motion of the
object. As discussed in an earlier Lesson, such a component of force
can cause the satellite to either speed up or slow down in addition to
changing directions. So unlike uniform circular motion, the elliptical
motion of satellites is not characterized by a constant speed.
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In summary, satellites are projectiles which orbit around a
central massive body instead of falling into it. Being
projectiles, they are acted upon by the force of gravity - a
universal force which acts over even large distances between
any two masses. The motion of satellites, like any projectile,
are governed by Newton's laws of motion. For this reason, the
mathematics of these satellites emerges from an application
of Newton's universal law of gravitation to the mathematics of
circular motion. The mathematical equations governing the
motion of satellites will be discussed in a later section.
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The orbits of satellites about a central massive body can be
described as either circular or elliptical. As mentioned earlier in the
chapter, a satellite orbiting about the earth in circular motion is
moving with a constant speed and remains at the same height above
the surface of the earth. It accomplishes this feat by moving with a
tangential velocity that allows it to fall at the same rate at which the
earth curves. At all instances during its trajectory, the force of
gravity acts in a direction perpendicular to the direction which the
satellite is moving. Since perpendicular components of motion are
independent of each other, the inward force cannot affect the
magnitude of the tangential velocity. For this reason, there is no
acceleration in the tangential direction and the satellite remains in
circular motion at a constant speed. A satellite orbiting the earth in
elliptical motion will experience a component of force in the same or
the opposite direction as its motion. This force is capable of doing
work upon the satellite. Thus, the force is capable of slowing down
and speeding up the satellite. When the satellite moves away from
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the earth, there is a component of force in the opposite
direction as its motion. During this portion of the satellite's
trajectory, the force does negative work upon the satellite and
slows it down. When the satellite moves towards the earth,
there is a component of force in the same direction as its
motion. During this portion of the satellite's trajectory, the
force does positive work upon the satellite and speeds it up.
Subsequently, the speed of a satellite in elliptical motion is
constantly changing - increasing as it moves closer to the
earth and decreasing as it moves further from the earth.
These principles are depicted in the diagram below.
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In an earlier chapter, motion was analyzed from an energy
perspective. The governing principle which directed our analysis of
motion was the work-energy theorem. Simply put, the theorem
states that the initial amount of total mechanical energy (TMEi) of a
system plus the work done by external forces (Wext) on that system is
equal to the final amount of total mechanical energy (TMEf) of the
system. The mechanical energy can be either in the form of potential
energy (energy of position - usually vertical height) or kinetic energy
(energy of motion). The work-energy theorem is expressed in
equation form as
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KEi + PEi + Wext = KEf + PEf
The Wext term in this equation is representative of the amount of
work done by external forces. For satellites, the only force is gravity.
Since gravity is considered an internal (conservative) force, the Wext
term is zero. The equation can then be simplified to the following
form.
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KEi + PEi = KEf + PEf
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In such a situation as this, we often say that the total
mechanical energy of the system is conserved. That is, the
sum of kinetic and potential energies is unchanging. While
energy can be transformed from kinetic energy into potential
energy, the total amount remains the same - mechanical
energy is conserved. As a satellite orbits earth, its total
mechanical energy remains the same. Whether in circular or
elliptical motion, there are no external forces capable of
altering its total energy.
Energy Analysis of Circular Orbits
Let's consider the circular motion of a satellite first. When in
circular motion, a satellite remains the same distance above
the surface of the earth; that is, its radius of orbit is fixed.
Furthermore, its speed remains constant. The speed at
positions A, B, C and D are the same. The heights above the
earth's surface at A, B, C and D are also the same. Since
kinetic energy is dependent upon the speed of an object, the
amount of kinetic energy will be constant throughout the
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satellite's motion. And since potential energy is dependent
upon the height of an object, the amount of potential energy
will be constant throughout the satellite's motion. So if the KE
and the PE remain constant, it is quite reasonable to believe
that the TME remains constant.
One means of representing the amount and the type of
energy possessed by an object is a work-energy bar chart. A
work-energy bar chart represents the energy of an object by
means of a vertical bar. The length of the bar is
representative of the amount of energy present - a longer bar
representing a greater amount of energy. In a work-energy
bar chart, a bar is constructed for each form of energy.
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A work-energy bar chart is presented below for a satellite in
uniform circular motion about the earth. Observe that the bar
chart depicts that the potential and kinetic energy of the
satellite are the same at all four labeled positions of its
trajectory (the diagram on the previous page shows the
trajectory).
Energy Analysis of Elliptical Orbits
Like the case of circular motion, the total amount of
mechanical energy of a satellite in elliptical motion also
remains constant. Since the only force doing work upon the
satellite is an internal (conservative) force, the Wext term is
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zero and mechanical energy is conserved. Unlike the case of
circular motion, the energy of a satellite in elliptical motion
will change forms. As mentioned above, the force of gravity
does work upon a satellite to slow it down as it moves away
from the earth and to speed it up as it moves towards the
earth. So if the speed is changing, the kinetic energy will also
be changing. The elliptical trajectory of a satellite is shown
below.
The speed of this satellite is greatest at location A (when the
satellite is closest to the earth - perigee) and least at location
C (when the satellite is furthest from the earth - apogee). So
as the satellite moves from A to B to C, it loses kinetic energy
and gains potential energy. The gain of potential energy as it
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moves from A to B to C is consistent with the fact that the
satellite moves further from the surface of the earth. As the
satellite moves from C to D to E and back to A, it gains speed
and loses height; subsequently there is a gain of kinetic
energy and a loss of potential energy. Yet throughout the
entire elliptical trajectory, the total mechanical energy of the
satellite remains constant. The work-energy bar chart below
depicts these very principles.
An energy analysis of satellite motion yields the same
conclusions as any analysis guided by Newton's laws of
motion.
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Summary:
A satellite orbiting in circular motion maintains a constant
radius of orbit and therefore a constant speed and a constant
height above the earth. A satellite orbiting in elliptical motion
will speed up as its height (or distance from the earth) is
decreasing and slow down as its height (or distance from the
earth) is increasing. The same principles of motion which
apply to objects on earth - Newton's laws and the workenergy theorem - also govern the motion of satellites in the
heavens.
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In the early 1600s, Johannes Kepler proposed three laws of
planetary motion. Kepler was able to summarize the carefully
collected data of his mentor - Tycho Brahe - with three
statements which described the motion of planets in a suncentered solar system. Kepler's efforts to explain the
underlying reasons for such motions are no longer accepted;
nonetheless, the actual laws themselves are still considered
an accurate description of the motion of any planet and any
satellite.
Kepler's three laws of planetary motion can
be described as follows:
1.
2.
3.
The path of the planets about the sun are
elliptical in shape, with the center of the
sun being located at one focus. (The Law
of Ellipses)
An imaginary line drawn from the center
of the sun to the center of the planet will
sweep out equal areas in equal intervals of
time. (The Law of Equal Areas)
The ratio of the squares of the periods of
any two planets is equal to the ratio of the
cubes of their average distances from the
sun. (The Law of Harmonies)
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Kepler's first law - sometimes referred to as the law of ellipses explains that planets are orbiting the sun in a path described as an
ellipse. An ellipse can easily be constructed using a pencil, two
tacks, a string, a sheet of paper and a piece of cardboard. Tack the
sheet of paper to the cardboard using the two tacks. Then tie the
string into a loop and wrap the loop around the two tacks. Take your
pencil and pull the string until the pencil and two tacks make a
triangle (see diagram at the right). Then begin to trace out a path
with the pencil, keeping the string wrapped tightly around the tacks.
The resulting shape will be an ellipse. An ellipse is a special curve in
which the sum of the distances from every point on the curve to two
other points is a constant. The two other points (represented here by
the tack locations) are known as the foci of the ellipse. The closer
together which these points are, the more closely that the ellipse
resembles the shape of a circle. In fact, a circle is the special case of
an ellipse in which the two foci are at the same location. Kepler's
first law is rather simple - all planets orbit the sun
in a path which resembles an ellipse, with the sun
being located at one of the foci of that ellipse.
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Kepler's second law - sometimes referred to as the law of equal
areas - describes the speed at which any given planet will move
while orbiting the sun. The speed at which any planet moves
through space is constantly changing. A planet moves fastest when
it is closest to the sun and slowest when it is furthest from the sun.
Yet, if an imaginary line were drawn from the center of the planet to
the center of the sun, that line would sweep out the same area in
equal periods of time. For instance, if an imaginary line were drawn
from the earth to the sun, then the area swept out by the line in
every 31-day month would be the same. This is depicted in the
diagram below. As can be observed in the diagram, the areas
formed when the earth is closest to the sun can be approximated as
a wide but short triangle; whereas the areas formed when the earth
is farthest from the sun can be approximated as a narrow but long
triangle. These areas are the same size. Since the base of these
triangles are longer when the earth is furthest from the sun, the
earth would have to be moving more slowly in order for this
imaginary area to be the same size as when the earth is closest to
the sun.
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Kepler's third law - sometimes referred to as the law of harmonies compares the orbital period and radius of orbit of a planet to those
of other planets. Unlike Kepler's first and second laws which
describe the motion characteristics of a single planet, the third law
makes a comparison between the motion characteristics of different
planets. The comparison being made is that the ratio of the squares
of the periods to the cubes of their average distances from the sun
is the same for every one of the planets. As an illustration, consider
the orbital period and average distance from sun (orbital radius) for
Earth and mars as given in the table below.
Planet
Period
(s)
Average
Dist. (m)
T2/R3
(s2/m3)
Earth
3.156 x 107 s
1.4957 x 1011
2.977 x 10-19
Mars
5.93 x 107 s
2.278 x 1011
2.975 x 10-19
Observe that the
T2/R3 ratio is the
same for Earth as it
is for mars. In fact, if
the same T2/R3 ratio
is computed for the
other planets, it can
be found that this
ratio is nearly the
same value for all
the planets (see
table
below).
Amazingly,
every
planet has the same
T2/R3 ratio.
Planet
Period
(yr)
Ave.
Dist. (au)
T2/R3
(yr2/au3)
Mercury
0.241
0.39
0.98
Venus
.615
0.72
1.01
Earth
1.00
1.00
1.00
Mars
1.88
1.52
1.01
Jupiter
11.8
5.20
0.99
Saturn
29.5
9.54
1.00
Uranus
84.0
19.18
1.00
Neptune
165
30.06
1.00
Pluto
248
39.44
1.00
(NOTE: The average distance value is given in astronomical units
where 1 a.u. is equal to the distance from the earth to the sun 1.4957 x 1011 m. The orbital period is given in units of earth-years
where 1 earth year is the time required for the earth to orbit the sun
- 3.156 x 107 seconds. )
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In physics, escape velocity is the speed at which the kinetic energy
of an object is equal to its gravitational potential energy. It is
commonly described as the speed needed to "break free" from a
gravitational field, for example for a satellite or rocket to leave
earth. The term escape velocity is actually a misnomer, as the
concept refers to a scalar speed which is independent of direction.
In practice the escape velocity sets the bar for any rocket aiming to
bring a satellite into earth orbit or beyond. It gives a minimum
delta-v budget (See next slide) for rockets when no benefit can be
obtained from the speeds of other bodies, for example on
interplanetary missions where a gravitational slingshot may be
applied.
When escape velocity is calculated by the gravitational potential
energy (Ug) equation
atmospheric friction or air drag is neglected.
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Set
Delta-v budget (or velocity change budget) is an astrogation term
used in astrodynamics and aerospace industry for velocity change
(or delta-v) requirements for the various propulsive tasks and orbital
maneuvers over phases of a space mission.
Sample delta-v budget will enumerate various classes of maneuvers,
delta-v per maneuver, number of maneuvers required over the time
of the mission.
In the absence of an atmosphere, the delta-v is typically the same
for changes in orbit in either direction; in particular, gaining and
losing speed cost an equal effort.
Because the delta-v needed to achieve the mission usually varies
with the relative position of the gravitating bodies, launch windows
are often calculated from porkchop plots that show delta-v plotted
against the launch time.
equal to Kinetic Energy
and solve for velocity, yields 11190 m/s, or 11.2 km/s. Fire anything at a speed
greater than this, and it will leave Earth, going more and more slowly, but never
stopping!
Table 14.1: Escape Speeds at the Surface of Bodies in the Solar System
Astronomical
Body
Mass
(Earth Mass)
Radius
(Earth Radii)
Escape Speed
(km/S)
Sun
333,000
109
620
Sun (at a
distance of
Earth’s orbit
333,000
23,500
42.2
Jupiter
318
11
60.2
Saturn
95.2
9.2
36.0
Neptune
17.3
3.47
24.9
Uranus
14.5
3.7
22.3
Earth
1.00
1.00
11.2
Venus
0.82
0.95
10.4
Mars
0.11
0.53
5.0
0.055
0.38
4.3
0.0123
0.28
2.4
Mercury
Moon