# Tag Info

### 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 ...
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### 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 ...
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### 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, ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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, ...
• 30k
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 ...
• 21.3k
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 ...
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### 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 ...
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### 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 ...
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### 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 ...
• 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. ...
• 14.8k
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) ...
• 161
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 ...
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### 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. ...
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### 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 ...
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### 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 ...
• 21.8k
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 ...
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### 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 ...

### 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 ...
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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 ...
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