Tagged Questions
3
votes
1answer
35 views
Why doesn't this equation for orbital motion change with position in the orbit?
The question and answer are on pg.8-10 of this PDF:
At first, I went through it, thinking nothing of it. But then, I wondered: "What if we picked a final state in which the space junk was NOT at ...
0
votes
1answer
66 views
Vector cross product of $\mathbf{r}$ and $\ddot{\mathbf{r}}$ in polar coordinates
I'm struggling with the following question:
Question 6 A planet of mass $m$ moves under the gravitational attraction of a central star of mass $M$. The equation of motion of the planet is
...
1
vote
1answer
53 views
Defining the star as the ellipse focus rather than the barycenter, what does the other focus do? [duplicate]
There are a lot of images and animations on the internet depicting two bodies orbiting around their common barycenter. The barycenter is defined as the (let's say right) focus of the ellipse.
If we ...
3
votes
2answers
65 views
Orbits within a $-\vec{r}$ field
Let's say that we have a cold dark matter theory, so we imagine weakly interacting particles. Now, let's say that one of those dark-matter particles has a rare interaction while traveling through the ...
0
votes
0answers
28 views
Understanding Kepler's $2^{nd}$ law in terms of angular momentum conservation
A) Explain how Kepler's $2^{nd}$ law - "The radius vector from the Sun to a planet sweeps out equal areas in equal time intervals" - can be understood in terms of angular momentum conservation.
I ...
8
votes
3answers
311 views
Is it possible that 5 planets can revolve around a single star in a single orbit?
I'm writing a novel and I'm quite confused if this system could be possible in the real universe. Is it possible that a system exist, where 5 identical planets which could be of same characteristics ...
1
vote
2answers
144 views
Semi-major axis and ellipticity of a binary system?
In the image below (source at bottom), it seems to be suggesting that
\begin{equation}
a = a_{1} + a_{2}, \hspace{8cm}(1)
\end{equation}
where $a_{1}$ and $a_{2}$ are the semi-major axis of the ...
0
votes
1answer
79 views
Has anyone on Earth ever seen the dark side of the moon and if so where are the pictures? [duplicate]
If the Moon rotates then we should see the dark side right? But as far as I know the Moon only shows one side to Earth, how can this be if it is rotating?
4
votes
2answers
116 views
If the moon was rapid enough would it be able to orbit the earth from a close distance?
If the moon was close in orbit that it's surface was like 100 km away from the earth's surface. And it had a large enough angular velocity will it be able to hold orbit?
If this was possible, is ...
0
votes
2answers
54 views
Saturn ring stabilization
The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System.
I'm wondering, what power is primarily responsible for that stability?
© Public Image by NASA ...
5
votes
2answers
80 views
Gravitational potential outside Lagrangian points or Lagrange points
The diagram in Why are L4 and L5 lagrangian points stable? shows that the gravitational potential decreases outside the ring of Lagrange points — this image shows it even more clearly:
If I ...
0
votes
1answer
64 views
What Speed Would an object need to leave the earth at to reach L1? [closed]
Let's say the Earth is an airless sphere. What speed would an object weighing 1 kg need to leave the surface at in order to get to and be motionless at L1, where the Moon's gravity becomes stronger ...
0
votes
0answers
57 views
How much energy would it take to move the asteroid that has been implicated in the dinosaur extinction by a few centimeters? [closed]
One of the greatest mass extinctions occurred about 65 million years ago, when, along with many other life-forms, the dinosaurs went extinct. Most geologists and paleontologists agree that this event ...
10
votes
4answers
571 views
Is Feynman's explanation of how the moon stays in orbit wrong?
Yesterday, I understood what it means to say that the moon is constantly falling (from a lecture by Richard Feynman). In the picture below there is the moon in green which is orbiting the earth in ...
1
vote
1answer
52 views
Simulating an orbit, primary is not at focus
I've been toying around with some -very- simple orbital simulators, mostly using preexisting physics libraries (I took a layman's stab at doing it with vectors too). The thing that is confusing me is ...
2
votes
1answer
186 views
How can a satellite's speed decrease without its orbital angular momentum changing?
I have no idea what the answer is. I'm supposed to answer it within 3-4 sentences.
0
votes
3answers
85 views
Stresses in asteroid during close flyby
The acceleration of an asteroid (such as 2012DA14) as it approaches earth is proportional to the reciprocal of distance $r$ from earth center, squared. the derivative of the acceleration, or jerk, is ...
2
votes
2answers
81 views
Shoot object into the Sun using minimal energy
Say I want to shoot a cannonball into the Sun with minimal energy (minimal initial velocity relative to Earth).
In which direction do I shoot it?
Let's neglect Earth's gravity, if that would make ...
0
votes
1answer
86 views
Lagrange L4 L5 points and perifocal plane
I have 2 satellites at the L4 and L5 points and these are watching an object. Each satellite provides the angle to the object from its own position from a line parallel to the $\text{x-axis}$ of ...
0
votes
1answer
176 views
Energy in orbit of satellites around the earth lost?
If the total mechanical energy in a satellite's orbit (assuming circular) is greater when it is closer to the earth, and hence smaller when it is farther from the earth, then we can say that as the ...
0
votes
1answer
148 views
Two moons of Earth?
Hypothetically, suppose there is a situation where the Earth's moon gets neatly sliced into two equal hemispheres, and the force responsible for this slicing also creates a distance between the two ...
9
votes
3answers
460 views
Gravity in other dimensions than 3 and stable orbits
I have heard from here that stable orbits (ones that require a large amount of force to push it significantly out of it's elliptical path) can only exist in a three spatial dimensions because gravity ...
6
votes
2answers
186 views
Is the gravitational potential of a planet in orbit always equal to minus the squared velocity?
Say a planet (mass $m$) is orbiting a star (mass $M$) in a perfect circle, so it is in circular motion.
$F=ma$ and the gravitational force between two masses $F=\frac{GMm}{r^2}$ so
...
0
votes
2answers
94 views
Definitions of Lagrange points: $L_4$ and $L_5$
We have the the five Lagrange points (let consider Earth and Sun):
$L_1$ - lie between Sun and Earth;
$L_2$ - beyond the Earth;
$L_3$ - beyond the Sun;
And what's the difference between $L_4$ and ...
3
votes
4answers
650 views
Can a balloon float into space? (+orbital velocity)
After watching the recent "space jump" a question arose. Why can a balloon not float into space? Can one be made/designed to do this?
Next, everything in orbit is falling back to earth. It only ...
0
votes
1answer
80 views
What is the orbital motion where both foci are located at one point?
What is the orbital motion where both foci are located at one point? I know that an ellipse orbit is motion with two distinct foci.
1
vote
1answer
170 views
Result of increasing the radius of earth?
How can increasing the radius of earth may cause an impact on the solar system ? Like, would earth may start making a bigger orbit (due to increase in size and wait) or vice versa ? or else ?
PS: The ...
3
votes
1answer
211 views
How long will it take for a bullet to reach a Geostationary orbit?
I'm curious to know this. Neglect air friction and imagine a bullet that were shot normal to the Earth's surface, from the Equator. I will have to consider the Coriolis effect and so I expect the path ...
1
vote
2answers
99 views
A particle of charge $-e$ orbits a particle of charge $Ze$, what is its orbital frequency?
A point particle $P$ of charge $Ze$ is fixed at the origin in 3-dimensions, while a point particle $E$ of mass $m$ and charge $-e$ moves in the electric field of $P$.
I have the Newtonian equation of ...
0
votes
1answer
342 views
Satellite Orbital Period [closed]
I know I can calculate the period of a satellite orbit by Kepler's third law, but somehow it does not work out.
The sattelite is 20200km from surface of the earth.
$r=$orbits radius=earths ...
2
votes
1answer
51 views
Single plane Ring system [duplicate]
Possible Duplicate:
Why are our planets in the solar system all on the same disc/plane/layer?
I've noticed this in many pictures, Planets are shown with a single ring around them (in some ...
0
votes
1answer
66 views
What is the pause called at the apex of an object's trajectory?
My apologies for such a basic question--I am a musician, not a physicist. But I cannot anywhere find the word, if one exists, that describes that elegant pause of an object such as a ball, thrown ...
0
votes
2answers
332 views
Does effective potential for a gravitational force have a maximum below $E=0$?
The relevant figure is below (taken from Goldstein's Classical Mechanics). This figure plots the effective potential for a gravitational force. Does the effective potential $V'$ go flat below $E_2=0$? ...
2
votes
1answer
164 views
Do Lagrangian points actually maintain a fixed distance?
I was reading on up Lagrangian points and the restricted three-body problem.
From what I was able to tell, the Lagrangian points are 5 points in a two-body system such that a third body would be ...
5
votes
2answers
199 views
Why is there this asymmetry between the two foci of an orbital ellipse?
Why does the Earth revolve with the Sun at one of its foci? Does the other focus do nothing? Why is there this asymmetry in our solar system?
2
votes
2answers
491 views
Why is the center-of-mass of 2 bodies at the focus of their elliptical orbits?
Why is the center-of-mass of 2 bodies (which interact only via Newtonian gravity) located at a focus of each of the elliptical orbits?
I know that when there are no external forces, the center of ...
1
vote
1answer
71 views
What's the amount of deviation of cellestial orbits from perfect ellipses
It's well known that the planets don't orbit the sun in perfect circles and the characteristics of the elliptical orbits which serve as better approximations to their motion have been calculated ...
19
votes
1answer
707 views
Why does it take so long to get to the ISS?
I don't understand why when first launched Space X's Dragon capsule had to orbit the Earth many times in order to match up with the ISS? Was this purely to match it's speed, or to get closer (as in ...
9
votes
1answer
273 views
Apollo and orbital mechanics: orbital decay if the Trans Earth Injection (TEI) burn had failed
I'm reading Jim Lovell (Apollo 8 and 13) and Jeffrey Kluger's book Apollo 13, which is a fantastic read about a long past era I only have kindergarten memories of. On page 54 there is a paragraph that ...
0
votes
1answer
106 views
What would be the path of the earth seen by the astronaut in the lunar sky? [closed]
An astronaut camps on the moon for a period of one month as per the earth’s calendar.
What would be the path of the earth seen by the astronaut in the lunar sky?
(A) The earth remains approximately ...
1
vote
0answers
367 views
Calculating semi-major axis of binary stars from velocity, position and mass
I'm trying to calculate the 'instantaneous' semi-major axis of a binary system with two equal (known) mass stars for an $N$-body simulation. I know their velocities and positions at a given time, but ...
0
votes
1answer
110 views
How to find the orbit of a moving point object given 3 past passing positions?
How do I find the orbit of a moving point object given 3 past passing positions and the tangents at these passing positions, and given that the orbit is known to be an ellipse?
2
votes
1answer
262 views
Radial fall in a Newtonian gravitational field [duplicate]
Suppose an object of mass $m$ starts at rest at a radial distance $ r_0$ from a perfectly spherical mass $M$ (where $m << M$), $r_0 > R =$ radius of $M$.
Can we analytically determine when ...
6
votes
7answers
1k views
How does the earth move?
My son who is 5 years old is asking me a question about how the earth moves around the sun. What answer should I give him?
13
votes
5answers
2k views
Why do we always see the same side of the Moon?
I am puzzled why we always see the same side of the Moon even though it is rotating around its own axis apart from revolving around the earth. Shouldn't this only be possible if the Moon is not ...
9
votes
4answers
225 views
Two planets in same orbit - not planets?
Let us pretend for a moment that there are two identical planets that are exactly opposite their star from each other and are the same distance from said star. (This would make them, at all times, ...
5
votes
11answers
2k views
Why do we say that the earth moves around the sun?
In history we are taught that the Catholic Church was wrong, because the Sun does not move around the Earth, instead the Earth moves around the Sun.
But then in physics we learn that movement is ...
13
votes
9answers
3k views
Why are orbits elliptical?
Almost all of the orbits of planets and other celestial bodies are elliptical, not circular.
Is this due to gravitational pull by other nearby massive bodies?
If this was the case a two body system ...
2
votes
0answers
138 views
Calculation of a Gravity Resonance Keyhole
Can anyone describe the mathematics behind the calculation of a resonance keyhole (for a two-body model)? It seems like the size and position of the keyhole should be a function only of mass and ...
6
votes
3answers
199 views
The feasibility of a satellite orbiting at a fixed time
I was speaking with some friends of mine, one of whom was an aerospace engineer. He posited the infeasibility of a hypothetical "Margaritaville Satellite" that orbited earth in such a way that ...
