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70
votes
11answers
11k 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 ...
54
votes
8answers
34k 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 ...
33
votes
3answers
2k views

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 ...
18
votes
3answers
5k views

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 ...
15
votes
7answers
6k views

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 ...
14
votes
5answers
1k views

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 ...
10
votes
3answers
3k views

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 ...
9
votes
3answers
8k views

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 ...
9
votes
3answers
1k views

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 ...
8
votes
3answers
340 views

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?
8
votes
5answers
2k 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. ...
7
votes
6answers
10k 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 ...
7
votes
5answers
4k 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, ...
6
votes
7answers
1k 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. ...
6
votes
4answers
2k views

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 ...
6
votes
2answers
271 views

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 ...
5
votes
3answers
2k 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 ...
5
votes
5answers
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. ...
5
votes
4answers
550 views

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 ...
5
votes
5answers
611 views

The Achilles Paradox

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 ...
5
votes
1answer
252 views

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. ...
5
votes
1answer
511 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 ...
4
votes
5answers
556 views

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?
4
votes
3answers
262 views

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 ...
4
votes
1answer
532 views

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 ...
4
votes
2answers
245 views

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 ...
4
votes
6answers
377 views

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. ...
3
votes
2answers
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?
3
votes
4answers
916 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. $A$ ...
3
votes
4answers
287 views

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 ...
3
votes
2answers
128 views

Kinetics energy with different reference frame (a fighter plane shoots a bullet)?

Let say a plane flights at 1km/s, shoots a 2kg projectile at 1km/s, so to the Earth, the projectile flights at 2km/s with 2*2*2/2 = 4MJ. The same gun when fixed on Earth, shoots the same 2kg ...
3
votes
1answer
101 views

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 ...
3
votes
3answers
200 views

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 ...
3
votes
1answer
185 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 ...
3
votes
2answers
614 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 ...
3
votes
1answer
295 views

Equation of Motion in a Non-Inertial (Rotating) Frame

Let me preface my question by informing you this is for an assignment, so I would rather not have explicit answers but rather be given guidance in arriving at the correct solution. The question is ...
2
votes
5answers
18k views

Will a ball thrown straight up in a train land in same spot (in real world)?

I have a question that came up in a discussion with friends. If I throw a ball straight up in an enclosed train car moving with constant velocity, I believe the basic physics books say it will land ...
2
votes
4answers
2k views

Does being suspended in air allow you to not be affected by Earth's rotation?

Let's assume that there was some mechanism by which we could remain suspended in air. By this I mean that our feet is not in contact with the ground. One possible way of doing this would be by means ...
2
votes
3answers
284 views

A question on Galilean Relativity

In a test I took recently in my beginning Physics class there was a question that really irked me: A man is swimming upstream at 10 mph. The water is flowing downstream at 5 mph. How fast is ...
2
votes
2answers
81 views

Distance versus drop

Why if the earth is spinning at 1000mph a person falls from 10 stories his body didn't land about 1.4 miles away but right down as if the earth didn't spin at all
2
votes
1answer
134 views

Exeeding the speed of light by adding velocities [duplicate]

Well you know how it's said that things can't travel at or past the speed of light? However, can't they move at speeds greater than the speed of light relative to another object? For example: What ...
2
votes
1answer
2k 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 ...
2
votes
4answers
105 views

Do these cars rotate on themselves?

I was reading about the moon rotation around earth and the tidal lock related. I found some interesting information already here and on astronomy.stackexchange.com as well. The moon is known to have a ...
2
votes
2answers
231 views

Time variation of the distance between two falling objects [closed]

I have solved this question below from my textbook. My answer differs from the one given in the book. I have tried my best to see where I am wrong. I need your help to find the truth of this question. ...
2
votes
1answer
465 views

Einstein Relative Motion and Time Order of Events

According to Einstein, do observers in relative motion agree on the time order of all events? I don't think they would agree on the timing of events, but I am having trouble figuring out why they ...
2
votes
4answers
677 views

How is this classical “paradox” resolved in electromagnetism?

A magnet and a coil move relative to each other. In the frame of reference of the magnet, there is a magnetic field and consequently a force acting on the charges in the coil according to the Lorentz ...
2
votes
1answer
46 views

Relative Velocity Help

I am struggling with relative velocity and so decided to do some extra problems in my textbook. I have 2 problems that seem to contradict, and its annoying things like this that throws me off, leaving ...
2
votes
2answers
765 views

Galilean transformation in relativity

Assume flat spacetime in a general relativistic framework (or special relativity for that matter) and two observers $A$ and $B$, with non-vanishing velocity relative to each other. We know that they ...
2
votes
2answers
170 views

Relative velocities of boats and a stone thrown by the boat [closed]

Question: A police boat is chasing a boat with criminals along a straight river by moving against the stream. The speed of the river stream is 3 miles per hour, the speed of the boat with ...
2
votes
2answers
1k views

I don't understand the idea of perpendicular vectors being independent of each other?

So problem 71-75 is about vectors. The problem says: A river has a current flowing with a velocity of 2.0 meters per second due east. A boat is 75 meters from the north riverbank. It travels at ...