The Newtonian model of gravity in which the force between two objects is given by GMm/r^2.
46
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7answers
3k views
Does juggling balls reduce the total weight of the juggler and balls?
A friend offered me a brain teaser to which the solution involves a $195$ pound man juggling two $3$-pound balls to traverse a bridge having a maximum capacity of only $200$ pounds. He explained that ...
42
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9answers
7k views
Don't heavier objects actually fall faster because they exert their own gravity?
The common understanding is that, setting air resistance aside, all objects dropped to Earth fall at the same rate. This is often demonstrated through the thought experiment of cutting a large object ...
33
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7answers
3k views
Why is the Earth so fat?
I made a naive calculation of the height of Earth's equatorial bulge and found that it should be about 10km. The true height is about 20km. My question is: why is there this discrepancy?
The ...
19
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1answer
704 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 ...
18
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4answers
1k views
Would you be weightless at the center of the Earth?
If you could travel to the center of the Earth (or any planet), would you be weightless there?
16
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4answers
587 views
Staying in orbit - but doesn't any perturbation start a positive feedback?
I am not a physicist; I am a software engineer. While trying to fall asleep recently, I started thinking about the following.
There are many explanations online of how any object stays in orbit. The ...
15
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8answers
12k views
Why doesn't the Moon fall upon Earth?
Why doesn't the Moon, or for that matter anything rotating another larger body, ever fall into the larger body?
15
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9answers
2k views
Why do space crafts take off with rockets instead of just ascending like an aircraft until they reach space?
I guess it's not a very educated question, but I never quite understood why spacecrafts have to shoot up and can't just reach space by simply continuing an upwards ascent like an airplane.
14
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5answers
1k views
Why does the moon face earth with the same side?
I know this is an astronomy question, but no such stackexchange site exists. So here I am, asking about the physics of the solar system.
I know that the rotation period of the moon equals its ...
13
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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 ...
13
votes
4answers
501 views
Why are so many forces explainable using inverse squares when space is three dimensional?
It seems paradoxical that the strength of so many phenomena (Newtonian gravity, Coulomb force) are calculable by the inverse square of distance.
However, since volume is determined by three ...
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 ...
13
votes
4answers
495 views
Anti-gravity in an infinite lattice of point masses
Another interesting infinite lattice problem I found while watching a physics documentary.
Imagine an infinite square lattice of point masses, subject to gravity. The masses involved are all $m$ and ...
12
votes
1answer
466 views
James Webb Space Telescope's halo orbit at Lagrange point L2
The James Webb Space Telescope (JWST) is expected to be launched in 2018 and operate in the L2 vicinity, about 1.5 million km from Earth.
It will be placed in a halo orbit around the unstable L2 ...
11
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3answers
804 views
Significance of the second focus in elliptical orbits
1.In classical mechanics, using Newton's laws, the ellipticity of orbits is derived. It is also said that the center of mass is at one of the foci.
2.Each body will orbit the center of the mass of ...
11
votes
3answers
2k views
Why are Saturn's rings so thin?
Take a look at this picture (from APOD http://apod.nasa.gov/apod/ap110308.html):
I presume that rocks within rings smash each other. Below the picture there is a note which says that Saturn's rings ...
10
votes
7answers
2k views
Would it help if you jump inside a free falling elevator?
Imagine you're trapped inside a free falling elevator. Would you decrease your impact impulse by jumping during the fall? When?
10
votes
5answers
937 views
The square in the Newton's law of universal gravitation is really a square?
When I was in the university (in the late 90s, circa 1995) I was told there had been research investigating the $2$ (the square of distance) in the Newton's law of universal gravitation.
...
10
votes
4answers
568 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 ...
10
votes
4answers
968 views
Acceleration of two falling objects with identical form and air drag but different masses
I have a theoretical question that has been bugging me and my peers for weeks now - and we have yet to settle on a concrete answer.
Imagine two balloons, one is filled with air, one with concrete. ...
9
votes
3answers
456 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 ...
9
votes
4answers
223 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, ...
9
votes
1answer
272 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 ...
9
votes
3answers
429 views
How do we explain accelerated motion in Newtonian physics and in modern physics?
Maybe my question will seem stupid, but I am not a physicist so I have some problems understanding a classic Newtonian experiment: in the bucket experiment, why does he have to introduce the absolute ...
9
votes
3answers
1k views
Why does the moon drift away from earth?
I once saw on TV that the moon is slowly drifting away from the earth, something like an inch a year. In relation to that the day on earth what also increase in time.
I wonder why is that?
8
votes
6answers
1k views
Is Newton's Law of Gravity consistent with General Relativity?
By 'Newton's Law of Gravity', I am referring to
The magnitude of the force of gravity is proportional to the product of the mass of the two objects and inversely proportional to their distance ...
8
votes
3answers
293 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 ...
8
votes
1answer
1k views
What is the maximum efficiency of a trebuchet?
Using purely gravitational potential energy, what is the highest efficiency one can achieve with a trebuchet counter-weight type of machine? Efficiency defined here as transformation of potential ...
7
votes
4answers
467 views
What causes a soccer ball to follow a curved path?
Soccer players kick the ball in a linear kick, though you find it to turn sideways, not even in one direction. Just mid air it changes that curve's direction. Any physical explanation?
Maybe this ...
7
votes
1answer
124 views
Gravitationally bound systems in an expanding universe
This isn't yet a complete question; rather, I'm looking for a qual-level question and answer describing a gravitationally bound system in an expanding universe. Since it's qual level, this needs a ...
6
votes
10answers
956 views
Is the distance between the sun and the earth increasing?
M = mass of the sun
m = mass of the earth
r = distance between the earth and the sun
The sun is converting mass into energy by nuclear fusion.
$F = \frac{GMm}{r^2} = \frac{mv^2}{r} \rightarrow r ...
6
votes
7answers
1k views
How does Newtonian gravitation conflict with special relativity?
In the Wikipedia article Classical Field Theory (Gravitation), it says
After Newtonian gravitation was found to be inconsistent with special relativity, . . .
I don't see how Newtonian ...
6
votes
7answers
1k views
How does the earth move?
My son who is 5 years old is asking me a question about how the earth moves around the sun. What answer should I give him?
6
votes
4answers
326 views
What's the exact gravitational force between spherically symmetric masses?
Consider spherical symmetric$^1$ masses of radii $R_1$ and $R_2$, with spherical symmetric density distributions $\rho_1(r_1)$ and $\rho_2(r_2)$, and with a distance between the centers of the spheres ...
6
votes
6answers
533 views
When driving uphill why can't I reach a velocity that I would have been able to maintain if I started with it?
Consider these two situations when driving on a long straight road uphill:
Starting at a high velocity $v_h$, which the car is able to maintain.
Starting at a lower velocity $v_l$, and then trying ...
6
votes
4answers
120 views
Integrating radial free fall in Newtonian gravity
I thought this would be a simple question, but I'm having trouble figuring it out. Not a homework assignment btw. I am a physics student and am just genuinely interested in physics problems involving ...
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
...
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 ...
6
votes
5answers
290 views
Intuitive explanation of the inverse square power $\frac{1}{r^2}$ in Newton's law of gravity
Is there an intuitive explanation why it is plausible that the gravitational force which acts between two point masses is proportional to the inverse square of the distance $r$ between the masses (and ...
6
votes
2answers
150 views
Is it possible to use a balloon to float so high in the atmosphere that you can be gravitationally pulled towards a satellite?
A recent joke on the comedy panel show 8 out of 10 cats prompted this question. I'm pretty sure the answer's no, but hopefully someone can surprise me.
If you put a person in a balloon, such that ...
6
votes
4answers
612 views
Can gravity be shielded, like electromagnetism?
If I remember well, they said that it can't, but I do not know why.
Yes, I meant if gravity can be shielded using something like a Faraday cage
(or something else?).
Thank you.
6
votes
1answer
142 views
How would a large a mass be stable at the Earth Sun L4 or L5 point?
I've heard about the Trojan asteroids and there is the famous idea of putting a space colony at one of these points, but the explanations I see for how something is stable at those points it they are ...
6
votes
1answer
421 views
The gravitational potential of ellipsoid
In the literature (Kirchhoff G. - Mechanic (1897), Lecture 18 or Lamb, H. - Hydrodynamics (1879)) one can find the following analytical closed form expression for the gravitational potential of ...
5
votes
11answers
1k 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 ...
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?
5
votes
4answers
2k views
The Time That 2 Masses Will Collide Due To Newtonian Gravity
My friend and I have been wracking our heads with this one for the past 3 hours...
We have 2 point masses, $m$ and $M$ in a perfect world separated by radius r. Starting from rest, they both begin to ...
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 ...
5
votes
2answers
642 views
Calculating gravity when taking into account the change of gravitational force
This is a problem that has bothered me for a couple of weeks now, and I can't seem to wrap my head around it and understand it.
Let's say we have a planet with a mass of m. We also have an object of ...
5
votes
3answers
270 views
the collision of Phobos
Mars has two moons: Phobos and Deimos. Both are irregular and are believed to have been captured from the nearby asteroid belt.
Phobos always shows the same face to Mars because of tidal forces ...
5
votes
3answers
230 views
Differences between the gravitational constants $G$ and $g$?
There's a formula (described by Sir Isaac Newton) that gives the force acting between two objects:
$$F = \frac{Gm_1m_2}{r^2}$$
And then there's a formula for weight of an object
$$w = mg$$
My ...
