# Tagged Questions

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15k views

### If all motion is relative, how does light have a finite speed?

I've often heard that Einstein shattered the notion of absolute motion (i.e. all things move relative to one another) and that he established the speed of light as being absolute. That sounds ...
48k views

### Why does the atmosphere rotate along with the earth?

I was reading somewhere about a really cheap way of travelling: using balloons to get ourselves away from the surface of the earth. The idea held that because the earth rotates, we should be able to ...
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### What are galactic speeds measured against?

The Earth moves through space at 67,000 MPH. The Milky Way travels through a local group at 2,237,000 MPH. Wouldn't you need a fixed point to be able to measure velocity against? After all, compared ...
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### Is it possible to stay up while riding a bike on a moving sidewalk without actually moving?

If I ride a bicycle on a moving sidewalk so that I am not in effect moving at all relative to the ground, will I fall over?
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### Why doesn't the speed of Earth (moving through space) kill us?

I have read different speeds of Earth in different sources. $382\;{\rm km}/{\rm s}$, $12\;{\rm m}/{\rm s}$ and even $108,000\;{\rm km}/{\rm h}$. Basically, it's moving too fast around the Sun. And the ...
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### How can kinetic energy be proportional to the square of velocity, when velocity is relative?

Let's start with kinetic energy (from los Wikipedias) The kinetic energy of an object is the energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a ...
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### Can we find actual rest mass of things on Earth

Earth moves around the Sun and the Sun moves around the galaxy and the galaxy moves with unknown speed and direction. We have speed so the mass of us all altered. Can we know the real rest mass? If ...
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### Why do we launch rockets during the day?

The clips that I have seen of rockets launching all seem to be carried out during daytime. However, we learnt at school that rockets are fired closer to the equator and towards the east to take ...
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### How to derive addition of velocities without the Lorentz transformation?

Lorentz contraction and time dilatation can be deduced without Lorentz transformation. Can you deduce also the theorem of addition of velocities $$w~=~\dfrac{u+v}{1+uv/c^2}$$ without Lorentz ...
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### A fly in an accelerating car

A fly is flying around in a car, the fly never touches any surface in the car only fly’s around in the air inside the car. The car accelerates. does the fly slam in to the rear window. or does the fly ...
7k views

### Earth moves how much under my feet when I jump?

If I'm standing at the equator, jump, and land 1 second later, the Earth does NOT move 1000mph (or .28 miles per second) relative to me, since my velocity while jumping is also 1000mph. However, ...
3k views

### Is acceleration relative?

A while back in my Dynamics & Relativity lectures my lecturer mentioned that an object need not be accelerating relative to anything - he said it makes sense for an object to just be accelerating. ...
2k views

### If the Earth is in constant motion then why do we say that an object is in a state of rest?

I got this question as my physics class homework for tomorrow. Anyone please help me out. If Earth constantly rotates and revolves, then how can we call an object in a state of rest?
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### Airplane with banner in a windy day

Will the banner of this airplane be always in the proper direction if the airplane flies in any direction on a windy day?
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### Do you encounter more photons (per unit time) when moving forwards at a constant velocity?

Let's say you have rain hitting you evenly on all sides (not very realistic, I know). If you were to move forwards at a constant speed, there would be more droplets of rain hitting you per second on ...
19k views

### How fast is Earth moving through the universe?

As the galaxy is moving and the solar system orbiting the galaxy and the Earth orbiting the sun. So how fast is each object moving and what is the fastest we move at? Do we even know how fast the ...
2k views

### Measure absolute speed

Currently I'm 17 years old, going to secondary school. So, my ideas might be totally wrong... I know that everything is relative. In the example of speed, the earth moves, and the galaxy moves, etc. ...
3k views

### Ball thrown from a moving train

This is not a question about throwing a ball vertically in a moving train. I am asking what would happen if I throw a ball in a horizontally in a moving train. Assume I am facing an exit door of a ...
2k views

### What happens when the spacecraft velocity equals the velocity at it's exhaust?

So there I was resting me eyes thinking about rocket drives, and what-not. The thought struck me that, perhaps, even before Mr. Einstein interferes with the increasing velocity of the spacecraft Mr. ...
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### A Basic Question about Gravity, Inertia or Momentum or something along those lines

Why is it that if I'm sitting on a seat on a bus or train and its moving quite fast, I am able to throw something in the air and easily catch it? Why is it that I haven't moved 'past' the thing during ...
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### Why absoluteness of time implies galilean transformations?

In Landau course, vol.1 Mechanics, one finds the statement: ...the absoluteness of time necessarily implies that the ordinary law of composition of velocities is applicable to all phenomena. I ...
723 views

### Why a type of motion is relative and another is not? [closed]

Two scientists are completely isolated in two different boxes: First box moves uniformly straight (in a perfect space). Second box rotates uniformly (also in a perfect space). The scientists have ...
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I know that it is a very old question but still I don't find any satisfactory solution for Achilles Paradox. Please explain me the fundamentals of Achilles paradox in terms of stage wise distance ...
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### Is there a closed form solution to the Esdale river problem?

This is probably not well known problem but it looks like open problem. What kind of methods there are to find a closed form solution to the physical situation? Can you solve this problem? You're ...
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### Walking along a moving train, we make 18 steps. Opposite: 11. How long is the train?

A man walks in the same direction as a slowly moving train ($v_{man} > v_{train}$). He counts the train to be 18 steps long. Then he turns around and counts the train to be 11 steps long. (...
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### Kinetic energy question

Thought experiment: I am starting to walk on a corridor. From the corridor's perspective I have a certain increase of kinetic energy. From my perspective the whole universe is gaining kinetic energy ...
600 views

### Lorentz force law in Newtonian relativity

I know that in special relativity Electric and Magnetic fields mix together in different reference frames, but my question is about classical mechanics. It seems weird to me is that the Lorentz Force ...
1k views

### How does a spacecraft enter Earth's atmosphere?

If Earth whirls around the center of our galaxy at some 220 kilometers per second, how can a spacecraft, which is much slower, enter Earth's atmosphere when returning from Mars for example?
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### Space Expansion vs. Relative Motion

Given 2 objects moving at some velocity $v$ relative to one another, is it possible to determine whether they are moving or whether the space between them is expanding?
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### Kinetics energy with different reference frame (a fighter plane shoots a bullet)?

Suppose a plane flies at 1km/s and shoots a 2kg projectile at 1km/s, so to the Earth the projectile flies at 2km/s with 2*2*2/2 = 4MJ. The same gun, when fixed on Earth, shoots the same 2kg ...
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### Do bullet/gun physics change if the gun is traveling very fast?

If I'm running at say $400\: \mathrm{m/s}$ and a bullet travels at $400\: \mathrm{m/s}$ and I fire the gun, will I see the bullet leave the barrel? I either see it stay in the barrel floating ...
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### Does a fly get brought up to speed with a bus?

Consider a scenario where a bus is moving at a constant speed and a fly enters through a window,the fly is also flying at a constant speed. Since the bus is not in contact with the fly and neither is ...
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### If Speed is Relative can Absolute Acceleration be Real

This is , I suppose not a good question , but I think I am missing something which confuses me in this question. So my question is - Velocity is relative. So suppose a car is moving at a speed 200 ...
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### Find the relation between length of pulley and strings [closed]

A pulley is pulled with external force $F$. $x$ and $y$ denote the displacement of two ends of strings of the pulley and $z$ is the displacement of the pulley. Prove That $$z = \dfrac{x+y}{2}$$...
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### Detecting absolute motion inside a box

This is not a contradiction and I know it is impossible but still consider a thought experiment by me and point out if something is wrong. See the following picture and then the explanation follows. ...
1k views

### Puzzling : Relative motion of two points on a rotating disc

Consider two points on a radial line for a rotating disc. One point, $A$, is at the circumference and the other, $B$, is at distance $R/2$ from disc's centre. Relative velocity of $B$ w.r.t. $A$ ...
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### The Earth is spinning, so why don't we jump and land on a different location?

I know there are similar questions in stackexchange but i think it's different and detailed. The earth is spinning 465 meters/second so why don't we jump and land on a different location ? I have ...
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### Chosing a reference frame in which the earth is at rest and doesn't rotate

We may choose a non-rotating earth as our reference frame and ask ourselves: how about the planetary and stellar motions. A star at a distance of 10 million light years would turn around the earth in ...
1k views

### Einstein's first postulate implies the second?

Einstein's two postulates of special relativity are as follows, 1. The principle of relativity: The laws of physics are the same in all inertial systems. There is no way to detect absolute ...
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### Is General Relativity compatible with relative speeds bigger than $c$ between two inertial frames?

My question is motivated by a remark done by Tegmark in his book "Our Mathematical Universe". He says that GRT does not prove that relative speeds between material points are always smaller than c. It ...
8k views

### What does relative to something mean?

I just started learning about vector components and relative motion. I don't understand what relative to something means. I looked online but none of the explanations are helpful. If someone could ...
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### Is this one of the reason we can't travel in time? [closed]

I'd like to ask a question that is not in my field but it's bothering me. Based on that, I'm sorry if I make dumb mistake in my assumptions. The idea of travelling in time means "going" at some place,...
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### What is the relative velocity of this plane relative to the helicopter? [closed]

There is a plane moving 100 m/s due east relative to the ground without vertical motion. There is a helicopter facing north moving straight up at 20 m/s. From the perspective of the helicopter, is the ...
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### energy of the system increasing after acceleration (one reference frame)

The system consists of small mass $m$, large mass $M$, and chemical potential energy $E_{chem}$, as shown in below image, the reference frame is following the small mass throughout. The total energy ...
296 views

### Is the Doppler effect for sound symmetrical for observer or source movement?

It makes intuitive sense to me for the apparent frequency of a sound as modified by the Doppler effect to be based entirely on the speed at which the observer and the source get closer or farther ...
830 views

### Origin of motion and relative speed of bodies in the universe

Charged particles can hit the earth at relativistic speeds. But it seems that all large bodies have fairly low relative speed. Of course, speed can increase considerably when a body orbits close to a ...
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### Is there a stationary frame of reference?

My understanding of relativity is at the beginner level, so please bear with me to help a beginner to understand. Something which I don't understand is consider if you travel close to the speed of ...
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### Disappearing galaxies [duplicate]

According to the cosmological model of the expanding universe, the distant (diametrally opposite) galaxies have bigger and bigger relative comoving speed, and this speed (which is not a relative speed ...
Suppose one needs to travel to Somalia from Indonesia(which are approximately at a distance of $4000$ miles from each other on the line of equator). To accomplish this task, I suggest the following ...