# Tag Info

58

You do, but it's too small to really notice First, it's not correct to say that we don't feel Earth's rotation because it's rotating at a constant speed. Think about driving a car, or riding in an airplane. Whether you're cruising down the road at 90 kph, or soaring through the air at 900 kph, you don't really "feel the speed". However, When you take a ...

48

In a similar way, we could ask... No beams can be exactly 1 meter long. No beams can be exactly straight. The material making up a beam cannot be truly isotropic. So why should we bother calculating the stress in a 1 meter straight beam having isotropic material? Because knowing how to perform this calculation is a building block for doing more ...

47

All models are approximations, we judge a model on how useful it is. Understanding the collapse of a non-rotating star to a black hole gives insight into the nature of gravitational collapse. Much of the physics of collapse does not depend on spin. The formation of an event horizon, for example. Models can be refined, and in this case, considering ...

24

Firstly the speeds are massively different (about 1000 mph (1610 kph) on the equator for Earth's rotation and 70,000 mph (112,654 kph) for the revolution), so the change is not large. Secondly, the green line is far straighter than it appears in your picture (because the orbit is so large) so Earth's motion around the Sun is pretty close to motion at ...

18

Of course you would need to specify who the person is - an Olympic athlete? Let us assume so and then you can scale downwards accordingly. So an Olympic high jumper can jump hard enough to raise their centre of gravity about 2m off the ground. Let us assume this is a ballistic problem. The athlete actually gives themselves sufficient upward speed to get ...

17

Another consideration is that the physics that describe a rotating black hole was much harder to develop. The maths describing the Schwarzschild (uncharged, non-spinning) black hole was developed in 1916. This was expanded to charged, non-spinning black holes in 1918 (The Reissner–Nordström metric) It wasn't until 1963 that the Kerr metric for uncharged ...

12

The problem with how you're looking at it is that velocities don't cause or result from forces, but accelerations do. Think of Newton's 2nd law, $F = m a$. Circular motion is motion at constant speed but changing direction, this changing direction is a type of acceleration because velocity is a vector (has direction) and acceleration is change in velocity. ...

11

"Harvest moon" just means the full moon at around the time of the equinox on September 21. The full moon is low in the sky during summer (June and July in the Northern Hemisphere) and high during the winter. So September is a month when the full moon is much higher in the sky than the moon in August. In the past this allowed harvests to continue after dark....

9

I will give a biologically motivated answer to this question: Feeling the Earth rotation carries no meaning to us. It is always approximately the same and we will blend out such background information and concentrate on news that are really important to us: Is there a danger approaching? Is the some food to gain? What are our peers doing? Because there was ...

9

In the currently dominant theories, the Universe is basically the same everywhere, if you look on a large enough scale. There may not be a furthest star from Earth at all (the universe may be infinite) or it may be like "the furthest point on Earth from London" which exists, but is not a specially interesting place. The distinction between those two ...

9

Applying a numerical density to a black hole isn't possible. The material inside the event horizon will fall to a "singularity" (or some other ultrahigh density state that we currently have no adequate theory to describe) on a relatively short timescale. What you can do, is exactly what you have done, which is divide the gravitational mass of a black hole ...

7

In general, the closer you are to the primary, the shorter the orbital period, except of course that you can't orbit inside the primary. So, taking the primary to be a sphere of radius $R$ and density $\rho$ we find that the mass is $$\frac{4}{3}\pi R^3\rho$$ so that the acceleration due to gravity at the surface is $$G\frac{4}{3}\pi R\rho$$ For a surface-...

7

So if I read your question correctly, you're asking why as stars get dimmer, they are given higher magnitudes? The reason is purely historical. The ancient greeks assigned stars with 6 brightness levels or magnitudes. The brightest stars were of first magnitude and the least bright stars (to the naked eye) were of 6th magnitude. In the 1800's this system ...

6

Because the Moon phase changes relatively slowly (especially at the Full Moon), a similar Moon phase can be seen from all places around the world. For example, the Full Moon in September 2019 occurs at 4:33 UT. If the Moon is visible from your location at that time, then you can see the Moon when it is exactly "Full". If Moon is not visible until 5 hours ...

6

The statement "They're only emitting infrared light" is wrong, or at least poorly phrased. The Ly$\alpha$ forest The Ly$\alpha$ forest (LAF) is caused by the spectrum of the quasar being redshifted along its way; wherever a cloud of neutral hydrogen is located, the part of the spectrum that at that at that position has been redshifted to $\lambda_0 = 1216\,... 5 Coronal mass ejections consist of a very hot, but thin, plasma. Their very weak intrinsic emission would be dominated by ultraviolet and X-ray lines and bremsstrahlung continuum. There is very little optical radiation. However, CMEs can and are monitored at optical wavelengths using the light that they scatter from the Sun. The process is Thomson scattering ... 5 This is Venus, high in the morning sky, amid the faint pillar of light called the Zodiacal Light. The glow is sunlight reflected off cometary dust in the inner solar system. Above is the centre of the Galaxy area of Sagittarius. Alan Dyer Since the Zodiacal Light lies in the ecliptic plane, the orientation of the Solar system in the Galaxy is something ... 5 Collisions between a rogue brown dwarf and any other star would be very rare because the space between them is so vast. I don't want to say it'd never happen, but it would be a rare event. It's much more common for two stars that are already in the same system to collide by spiraling into each other, usually by tidal decay. A collision with a brown ... 5 The redshift you're referring to is a cosmological redshift, which is fundamentally different from a Doppler redshift. At the heart of it, it has nothing to do with neither time, nor distance. It is a consequence of light traveling through an expanding space, no more and no less. Because space expands, the emitting source is carried away to a certain ... 5 Nice thought, but as the saying goes, space is really big. Stars have a tiny angular size when viewed from even their nearest neighbours (except perhaps in clusters & galaxy cores), so not much starlight actually gets intercepted by other stars, relative to the amount of light that gets absorbed by interstellar (or intergalactic) gas and dust. And even ... 5 To simplify things, I will assume that we have a spherical body and that we are considering Newtonian Physics. If we have two bodies of the same mass but different radius, as long as we are outside of both bodies, and at the same distance from their centres, the force of gravity will be the same: $$F = G \frac{m M_i}{r_i^2}$$ where$m$is "our" mass,$M_i$... 5 I agree with @uhoh that you don't have to an expert, but above-average knowledge of coding is definitely useful, bordering on "a must". Not for writing huge programs with 1000s of lines, but for writing smaller pieces of code that help you in everyday tasks. As uhoh says, you can very well find your place in a group where other people are in charge of ... 4 Much depends on what you mean by thing. But first lets think about space, or space-time. If you have a flat sheet of paper, and you draw a triangle on it, you will find that the angles add up to 180 degrees (I hope you are familiar with this fact). However, if you have curved surface and draw lines that are "straight" in the sense that they are the ... 4 How LIGO, LISA, etc. Detect Gravitational Waves The point of instruments like LIGO and LISA is to measure time-varying changes in the distance within different arms of the instrument. In the case of an arm oriented in the direction of an incoming gravitational wave (GW), the length of the arm will increase and decrease, while an arm oriented perpendicular ... 4 The non-thermal S-Z effect is caused by inverse Compton scattering of the CMB photons from a non-thermal population of electrons - i.e. electrons that have high energies not because they are hot, but because they have been accelerated non-thermally. The usual mechanisms are accelerating by electromagnetic fields and the Lorentz force. The rest-mass energy ... 4 When quoted like this they would usually be the most likely value (peak of the probability distribution) or the median value (where half the distribution is above or below), and then the superscripts and subscripts would be increments that would contain$\sim 34\$% of the probability distribution. In the M87 papers, the numbers are the 50th percentile (median)...

4

I am no expert and I am sure there will be a more rigorous answer later, but here my 2 cents: It appears to me that the term "energy" and energy conservation becomes a bit more complicated in general relativity and cosmology, taking into account more abstract forms as the energy of the gravitational field or space curvature. As you seem to be more ...

4

There is no tilting cycle. The Earth continues to tilt the same way all year round. The axis points towards the pole star. But as the Earth goes round the sun, in December the northern hemisphere points away from the sun. In June the Northern hemisphere points towards the sun, as the Ascii art below shows (not to scale) N N \ ...

4

This is considered very unlikely. It is actually very difficult for things to be captured into orbits. They pick up speed as they fall in towards the larger object, and that's automatically enough speed to carry them back out. Capture either requires dust and gas to slow the body down, or very lucky gravitational interaction with a third object. This has ...

4

A blog called Science Olympiad Blog claims to have the problem sheets for 2011-2014.

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