Skip to main content

New answers tagged

2 votes

Where am I wrong in proving that force perpendicular to motion increases speed and kinetic energy

First, I am using $z$ for the vertical direction. The eq. of motion are: $$ \frac{d^2\vec r}{dt^2} = \frac 1 m \vec F = -g\hat z $$ or: $$ \ddot x = 0 $$ $$ \ddot y = 0 $$ $$ \ddot z = -g $$ which is ...
JEB's user avatar
  • 34.9k
11 votes

Bound states between neutrinos using Schrödinger's equation?

A quick back-of-the envelope estimate, in the style of Fermi: For a neutrino mass of m ~ 1 eV, and a Planck mass M ~ $10^{27}$ eV, and supplanting the newtonian potential $(m/M)^2/r$ for the Coulomb ...
Cosmas Zachos's user avatar
3 votes

Does gravitational redshift conserve energy?

Gravitational redshift is considered conservative in the field of General Relativity. If you have a photon that climbs up and out of a well it loses energy, and manifests as a redshift. If it falls, ...
Statico's user avatar
  • 152
3 votes

Why is nonzero net charge density incompatible with the cosmological principle?

I don't agree with the assumption that the electric and gravitational fields must be proportional to ${\bf r} - {\bf r}_0$ for some central point ${\bf r}_0$. As discussed at Gauss's law in a ...
tparker's user avatar
  • 48.1k
5 votes

How does gravity overpower a vacuum?

I think you are making a common error when thinking about vacuums. We tend to intuitively to think about vacuums as 'pulling' but that's not really the right way to think about it and while it seems ...
JimmyJames's user avatar
10 votes

Why is nonzero net charge density incompatible with the cosmological principle?

The relevant difference between gravitational and electrostatic forces is that a gravitational field accelerates everything equally, whereas an electric field produces different accelerations on ...
Sten's user avatar
  • 6,057
5 votes

Why is the universe charge-neutral?

Charge conservation says that a universe which began charge-neutral would have remained charge-neutral. But why assume that it started out charge-neutral? A better way to ask the question is to ...
rob's user avatar
  • 91.1k
6 votes

Why is the universe charge-neutral?

A nonzero net charge density is incompatible with the cosmological principle. Homogeneity implies a constant electromagnetic field, which implies via Gauss's law that the contained charge is zero. The ...
benrg's user avatar
  • 27.4k
14 votes

How does gravity overpower a vacuum?

In addition, there is no point in the atmosphere where there is sudden "pressure to vacuum", like your question suggests. Pressure is the highest at sea level, and drops as we move higher, ...
joseph h's user avatar
  • 30k
28 votes
Accepted

How does gravity overpower a vacuum?

The escape velocity of the Earth is 11.2 km/s. In other words, you need to move faster than 11.2 km/s to leave the Earth permanently. The Earth's gravity is strong enough to attract everything moving ...
Allure's user avatar
  • 21.3k
0 votes
Accepted

Why is the universe charge-neutral?

There is as of yet insufficient data to produce a meaningful answer. I would certainly love to tell you that the universe is charge-neutral, because that would imply I know enough about the Universe ...
controlgroup's user avatar
2 votes

When is it appropriate to say Newtonian gravity is a force? When is it not appropiate?

Newtonian gravity is always a force. Einsteinian - general relativity - gravity is never a force. The reason gravity isn't always called a "force" is because in general relativity, the most ...
controlgroup's user avatar
0 votes

Thought experiment regarding gravity

I still think the mass of the sun would be different based on gravitational field of the Sun detected at say Mercury vs Pluto. This because the gravitational field has negative energy. So as one ...
Ajay's user avatar
  • 617
11 votes

Can gravity radiate?

Yes, that is indeed what roughly happens. In linearized gravity, the Lorenz gauge can be chosen such that the linearized Einstein field equations take the form $$\Box^2 \bar{h}_{\mu\nu} = -16\pi G T_{\...
Vincent Thacker's user avatar
4 votes

Thought experiment regarding gravity

First of all, your gedankenexperiment is actually fairly realistic (if we consider spheres instead of shells). Imagine for example a star in the center of which occurs nuclear fusion. This is ...
WillHallas's user avatar
2 votes

Thought experiment regarding gravity

Let's look at this mathematically. Let $\vec{g} \equiv \vec{g}(r)$ be the gravitational field. Then Gauss' law in its differential form states $$\operatorname{div} \vec{g} \enspace = \enspace - 4\pi G ...
Octavius's user avatar
  • 715
12 votes

Thought experiment regarding gravity

Now if this mass were distributed to R' where R'>R some work will have to be done so we can use some part of this mass M itself,convert to energy and be left with M' where M' < M. The key error ...
Dale's user avatar
  • 102k
1 vote

Why does a ball bounce back even when it is released and not thrown?

when we thrown a ball towards ground we apply an additional amount of force...When this ball reaches the ground,...the ground experts a similar amount of force on the ball. That's not quite right. ...
Solomon Slow's user avatar
  • 14.8k
2 votes
Accepted

Why does a ball bounce back even when it is released and not thrown?

When the ball reaches the ground with some momentum, it becomes compressed due to inertia (think of a spring for example). Its kinetic energy is then converted to potential energy (spring-like energy) ...
Jesse's user avatar
  • 161
2 votes
Accepted

How does a mass that's dropped when strung by an inelastic, slack string continue its motion?

There are multiple systems that could be intended by this setup. If you intend a system that conserves mechanical energy then the mass will "bounce" when the string becomes taut. It will not ...
Dale's user avatar
  • 102k
2 votes

Definition of the gravitational constant in 1+1 gravity

The $R = 8\pi G_2 T$ equation Mann writes down is nothing but Nordstrom's theory which predates general relativity. The big observational problem with it was that $T = 0$ for electromagnetic fields. ...
Connor Behan's user avatar
  • 8,042
0 votes

Is the Planck mass the "lower limit" for gravity?

$m_p$ is not a lower limit for gravity. It's just the value you get by algebraically manipulating $\hbar$, $G$ and $c$ until you get mass units. If you need some numerical high explosives to blow up ...
g s's user avatar
  • 13.9k
0 votes

Dual of Newtonian gravitational field

No. In Newtonian theory, the only field present is the Newtonian potential. You don't have a dual field. This is no longer true in general relativity. In relativity, the full gravitational field ...
Níckolas Alves's user avatar
4 votes
Accepted

Aren't there only 3 fundamental forces?

Regardless of the underlying mechanisms, gravity provides a channel for an object to realize the presence of other objects around. In other words, it mediates an $interaction$, one way or another. So ...
John's user avatar
  • 3,772
0 votes

Is spacetime flat inside a spherical shell?

Recently, it has been shown that an alternative set of solutions exist for the Einstein field equations inside a thin spherical shell. The solutions were initially found by Jun Ni and have since been ...
MattEdwards's user avatar
4 votes

How could I calculate the time it will take for light and mass to go towards a black hole and come back, to and from constant radial distances?

Since everything is time reversible in your scenario with a stationary mirror or trampoline where we assume ideal conditions, the way down takes the same time as the way up. To keep the equations ...
Yukterez's user avatar
  • 12.2k
1 vote

How could I calculate the time it will take for light and mass to go towards a black hole and come back, to and from constant radial distances?

This is actually a rather simple calculation: you calculate the geodesic on which your particle will travel using the standard geodesic equation and your black hole metric of choice, and then choose ...
paulina's user avatar
  • 1,490
0 votes

Stars that have fairly high gravitational redshift and calculation of their surface temperature by Planck emition spectra?

So for regular stars, not dead remnants, you can estimate this by setting the potential energy at the star's surface equal to the kinetic energy of an in-falling mass: $$ m\frac{GM} R = \frac 1 2 m v^...
JEB's user avatar
  • 34.9k
2 votes

Stars that have fairly high gravitational redshift and calculation of their surface temperature by Planck emition spectra?

The gravitational redshift inferred by a distant observer, when measuring the spectrum of radiation from the surface of a star of mass $M$ and radius $R$ is given by $$\frac{\lambda}{\lambda_0} = \...
ProfRob's user avatar
  • 132k
0 votes

Can Force be measured directly?

Your question, whether forces can be measured directly, is perfectly justified but requires some elaboration (In short, the answer is no). First, the basic equation of mechanics (Newtons Second Law) $$...
BlenderBender's user avatar
0 votes

How are objects inside a black hole affected by the gravity of objects outside the black hole?

They are not affected. Any black hole behaves like if all its mass was located in its center or on the shell of its surface. Thus, it is impossible to transfer information from inside the black hole ...
Anixx's user avatar
  • 11.2k
2 votes

Can a particle moving vertically upwards in space with constant velocity escape the earths orbit?

The only particles that have a constant velocity where no forces of any kind have an effect on its velocity are massless particles such as a photon. Those particles move at the speed of light. An ...
mmesser314's user avatar
  • 40.1k
1 vote

Can a particle moving vertically upwards in space with constant velocity escape the earths orbit?

Replace the particle with a rocket that continually adjusts its thrust to compensate for the force of gravity and maintain constant velocity. Given infinite time it will obviously escape the Earth's ...
KDP's user avatar
  • 5,327
2 votes

Can a particle moving vertically upwards in space with constant velocity escape the earths orbit?

Let's make the question slightly less fantastical by proposing a mechanism. Suppose there is a spiral staircase that extends vertically away from the Earth forever. You could put the axis of the ...
rob's user avatar
  • 91.1k
-4 votes

Can a particle moving vertically upwards in space with constant velocity escape the earths orbit?

When we define the field of attraction due to Earth, it comes from the Potential that we calculate of the Earth, which in turn has an assumption that the potential at infinity is ZERO. Following this ...
doge's user avatar
  • 3
-1 votes

Can a particle moving vertically upwards in space with constant velocity escape the earths orbit?

==[ THE ANSWER YOU MIGHT EXPECT: Yes, if its (constant) velocity exceeds the escape velocity of Earth, right from the start. Otherwise, after an exponentially long amount of time, it will fall back to ...
Miss Understands's user avatar
-1 votes

How hot is the core of a star just before it collapses to form a black hole?

No, they wouldn't be "on the colder side" since the collapsing star would be putting so much pressure on the hydrogen molecules inside of it that the hydrogen molecules would begin to bond ...
Aiden S.'s user avatar
1 vote

Gravitational Constant with ENM Units?

It is certainly true that $1 \space C\cdot V = 1\space kg\cdot m^2\cdot s^{-2}$ and thus one could choose to express $G$ in units of $C\cdot V\cdot m \cdot kg^{-2}$. Does this mean anything ...
Sturrum's user avatar
  • 459
1 vote
Accepted

Gravitational Constant with ENM Units?

Gravity is not an electrostatic phenomenon and the paper is doomed to be incorrect. However, the dimensional analysis is fine. I think maybe what you are confused by is that "Coulomb is a unit of ...
Andrew's user avatar
  • 50.2k
-1 votes

Questions about speed of gravity

@Prahar asked, "What do you mean by saying that the sun is travelling forward?" That's easy. "Forward" is an arbitrary name for the direction in which the Sun is moving in the ...
Solomon Slow's user avatar
  • 14.8k
2 votes
Accepted

Spinfoams/LQG Inequivalence

Why would these models be equivalent in the first place? They are defined through completely different steps. The starting point for both models is the same: this is the unique, well understood “...
Prof. Legolasov's user avatar
0 votes

Entropy and gravity

I found a good example to clarify the gravity-entropy issues. We on earth get radiation from the sun(mostly light)- use the blue part in photo-synthesis and do all sorts of work with the products, ...
Riad's user avatar
  • 506
2 votes

Acceleration at peak of a gravitational wave

The best I can do is a crude estimate. Wikipedia's LIGO article shows this graph. From this, you can say LIGO is most sensitive at about 100 Hz, and the limit of detection is when $\Delta l/l \approx ...
mmesser314's user avatar
  • 40.1k
5 votes
Accepted

How is it that energy of matter yields gravity if the amount of energy in a system is frame dependent while the force caused by gravity is not?

The curvature of spacetime does not depend only on energy, but rather on a frame independent object called the stress energy tensor. This includes not only energy but also momentum and pressure. The ...
Eric Smith's user avatar
  • 9,256
3 votes

How is it that energy of matter yields gravity if the amount of energy in a system is frame dependent while the force caused by gravity is not?

I've been told that the gravitational field arises due to the energy density terms in the stress-energy tensor This is the misconception. Gravity is not sourced exclusively by the energy density ...
Níckolas Alves's user avatar
3 votes

Which of Kepler's laws would remain true if the force of gravity were proportional to the product of squares of each masses?

The first law would still be valid. The Binet equation can be used to show that any central force of the form $\vec{F} = -k/r^2 \hat{r}$ (with $k > 0$) will lead to orbits that are conic sections. ...
Michael Seifert's user avatar
1 vote

What exactly does Einstein's gravitational constant $8\pi G/c^4$ mean?

There are some good answers here, but I think it can be expressed more simply. The Einstein Field equation is: $$G_{\alpha \beta} = \kappa ~T_{\alpha\beta}$$ The $T_{\alpha\beta}$ represents all the ...
RC_23's user avatar
  • 9,291

Top 50 recent answers are included