# Tag Info

42

No. All parachutes, whether they are drag-only (round) or airfoil (rectangular) will sink. Some airflow is needed to stay inflated, and that airflow comes from the steady descent. Whether your net descent rate is positive or negative is a different question. It is quite easy to be under a parachute and end up rising (I have done it myself), you just need an ...

24

It would be possible in theory, but only in a very side-thinking way: if you make a parachute so large it encapsulates the whole Earth, it will in effect act as a balloon and not fall down, due to the internal pressure of the atmosphere. This wouldn't work in practice for obvious reasons, but maybe in Kerbal you might be able to do something like it..

11

A parachute is a device specifically designed to create viscous friction. Viscous friction generates a force that: is oriented opposite to the velocity; is proportional to (a certain power of [*]) the velocity. So the falling velocity will increase until the drag force (pointing upwards) becomes equal to the weight of the falling object (pointing ...

10

You can't calculate the numerical value of Newton's constant from the first principle because it is a dimensionful constant – it has units – so the numerical value depends on the magnitude of the units. And because e.g. the kilogram is defined as the mass of a platinum prototype hosted by a French chateau (the kilogram has the "least objective" definition so ...

9

Every galaxy has to rotate so that a centrifugal force acts. Without the centrifugal force, all matter contained in the galaxy will collapse into the center of the galaxy due to gravitation. For there to be rotation however, there needs to be an axis, a line about which all matter revolves in the galaxy. Now, the manner in which all the matter revolves ...

7

Actually, there are parts of a galaxy that extend beyond the galactic plane: Galactic halo: This is actually the primary part of a galaxy that is not in the main galactic disk. It's made up of multiple sections, and is composed or an array of objects. Dark matter halo: This is a section of the galaxy's dark matter that exists in a semi-spherical shape. ...

7

No, as commented on above. Worse, we don't know its value very well. There are efforts underway to measure $G$ more accurately, as reported in Nature earlier this month: It is one of nature’s most fundamental numbers, but humanity still doesn’t have an accurate value for the gravitational constant. And, bafflingly, scientists’ ability to pinpoint G ...

7

In the weak field limit, which applies to all the cases you've described, the difference between the time rates for two observers with a Newtonian gravitational potential energy difference of $\Delta\Phi$ is given by: $$\frac{\Delta t_1}{\Delta t_2} = \sqrt{1 - \frac{2\Delta\Phi}{c^2}} \tag{1}$$ Note that the time dilation is related to the gravitational ...

5

Let's look at the forces in nature, there are four of them as far as we know (note that I am not very precise in the numbers I give, but for the comparison I make this is enough): the strong force is very strong, it's coupling constant (which is a measure for its strength) is about 0.1 the weak force is not actually all that weak. It can be unified with ...

5

It doesn't make sense to say "am I experiencing time dialation?" It only makes sense to compare two different observers, and ask whether one of them observes the others' clock to be ticking more slowly, say, when they are looking at minimum distance light rays coming from the other observer. With this in mind, the answers to most of your questions are ...

5

It could be possible if the parachute was very large, rigid, shaped like a floating object, and you started descending from the vacuum of space. In this case the parachute would float on top of the atmosphere. It's easier to visualize if you imagine the parachute being a boat and you fell into some water; the boat would float on top of the water and reduce ...

5

Here's a simple demonstration: Consider flat space (i.e. Minkowski), viewed in a rotating frame (in e.g. cylindrical coordinates one just replaces $\phi$ by $\phi'=\phi+\omega t$). One can calculate (without too much trouble) that, in these coordinates, a spatial line element can be expressed in terms of the canonical cylindrical coordinates as $$... 5 If you consider your photon as a point object, it cannot bend its own path. It will always travel on the ridge it creates, speaking in terms of curvature of space. The other idea is possible. Two photons having a momentum, attract each other, trapping each other, like a positronium (typical example for this behavior). In the model of relativity this is ... 4 The closest you are going to get is a parachute large enough to slow your descent to the point where you can find lift in rising air and climb away. They exist and are called paragliders! Strictly speaking they are still falling at 1 to 2 metres per second but rely on rising air ( thermals, ridge etc ) to 'fall' slower than a parcel of surrounding air. ... 4 Be careful to compare the same quantity. You ask if gravity (a force) affects the voltage (a potential energy). These are related but not identical. For a first round comparison, check out the forces due to gravity and a standard voltage (1 Volt over a distance of 1 cm): First, the gravitational force on an electron at the surface of the Earth:$$F_g = mg = ...

4

The basic idea of general relativity is that a freely moving object follows a path through spacetime called a geodesic. By freely moving I mean the object experiences no force i.e. if you were that object you would be weightless just as if you were floating in space. In flat spacetime geodesics are straight lines i.e. a freely moving object moves in a ...

4

Scales that measure to better than 1% need calibrating to local gravity which depends on latitude, and to a lesser extent, local geology. Due to the equatorial bulge, objects near the equator weigh 0.5% less than those at the poles Electronic scales normally come with 2 masses, typically their full and half range, and a calibrate mode.

3

No, gravity can not be explained by linking it to electromagnetism. One technical reason for this is that the source of the electromagnetic field is a vector, the current density 4-vector $J^\mu$. On the other hand, the source of gravity is mass-energy, which can not be described by a mere vector and must be included in a tensor of rank 2.

3

The two-dimensional Polyakov action for a string with worldsheet $\Sigma$ and worldsheet metric $h_{ab}$ $$\frac{T}{2}\int_\Sigma \sqrt{-h}h^{ab}g_{\mu\nu}\partial_aX^\mu\partial_bX^\nu$$ has full conformal symmetry under the Virasoro algebra and under Weyl transformations1 , which can be seen as gauge degrees of freedom. It follows that we can always ...

3

It's intuitive that while accelerating in a locally constant gravitational field, there is no perception of acceleration, since the body accelerates uniformly. The reason you can't perceive it is not that it's uniform, the reason is that there's nothing to compare with. If there's something to compare with, then you can see the difference. For instance, ...

3

The answer would appear to be "Yes", at least in theory. A "kugelblitz" is a concentration of light so intense that it forms an event horizon and becomes a Black Hole according to general relativity. It would be a BH whose original mass-energy had been in the form of light rather than matter.

3

Without the ability to change the shape of the parachute, no. With the ability, yes - briefly. A modern square parachute acts as a wing, producing enough lift to slow the descent of the vehicle, but it relies on forward momentum to do so and to remain inflated. If the trailing edge of the parachute is pulled down quickly, the air moving under the wing ...

3

It is due to the combined effect of rotation and "dissipation". A rotating cloud of gas consists of particles which interact strongly with each other (colliding physically) on relatively short timescales can radiate away some of their energy and momentum by emitting photons. For both of these reasons, a dense cloud of rotating gas will collapse to form a ...

3

This effect is called Capillary Action. Yes we do in fact observe it in nature in a large scale: How do you think plants are able to "suck up"1 water through its roots and send it to the leaves? One of the major forces responsible for it is capillary action. Here, have a quote from the article mentioned above: Wicking is the absorption of a liquid by a ...

3

The answer is it depends on which observer we are talking about - an observer "with" the collapsing mass sees it and them crushed to a singularity; an external observer "sees" (though see below) the mass frozen just at the event horizon. In GR and a standard black hole, there is only one future for a mass that finds itself at or inside the event horizon, ...

2

Indeed photons are massless particles, so they follow "quickest paths" during their propagation in space-time; these are called "geodesics". However, general relativity doesn't simply says how the way masses attract each other is modified, it most of all says that mass (and energy density) curve space-time; this also curves the geodesics, which photons (in ...

2

It is said that photons have zero rest mass so how can gravitational force of a black hole affect light? Photons have zero rest mass so when they are at rest they have no mass. They are never at rest so this is a little misleading. And if photons do have some effective mass while traveling at speed of light then only can a black hole's ...

2

There are several reasons. One is that when a cloud of gas and dust collapse into a star forming region, it becomes unstable to gravitational fragmentation and usually forms filamentary structures. The gas that lies outside of the densest regions is often not dense enough to be itself then gravitationally unstable. This behaviour is clearly shown in modern ...

2

I will answer "yes" if you think out of the box for a parachute, which is a way for a person ejected from a plane to fall on the earth safely. Theoretically, one might design a parachute with a layer of helium so as to match the parachute and person downward gravitational force at a certain height, possibly 4 km above ground so as to avoid mountains, with ...

2

The simplest reason for this is the fact that gravitational time dilation is governed, to leading order (in the zero-spin case for simplicity), by the factor $\sqrt{1 -\frac{2GM}{c^{2}r}}$. Now, just to make our measurements easier, let's rewrite the mass in terms of the radius of the event horizon: $$r_{0} = \frac{2GM}{c^{2}}$$ Now, our time dilation ...

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