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5
votes
7answers
258 views

Why a type of motion is relative and another is not? [on hold]

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 ...
0
votes
0answers
28 views

River Boat Problem with a Twist [on hold]

I am trying to solve the following river boat problem . A river of width w is floating such that the stream velocity varies with y as $v_R=v_0(1+(\frac{\sqrt 3 - 1}{w})y)$ where y is the ...
1
vote
1answer
35 views

How can we find velocity, acceleration etc, of a revolving particle with respect to an observer inside the circle(not at center)

A particle is revolving in horizontal a circle of radius $R$ with constant speed of $|\vec{v}|$ and constant angular velocity $\omega$. There is another observer standing inside the circle, at a ...
1
vote
2answers
52 views

Velocity of approach of a beam of light and a moving object

I came across a problem on Brian Greene's course on Special Relativity. (http://www.worldscienceu.com/courses/6/elements/TnRUSU) From George's perspective, how fast does a beam of light approach ...
4
votes
6answers
397 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. ...
0
votes
1answer
94 views

Does Earth get gravity due to it's spinning? [duplicate]

Consider my example below. Example: 1. A man standing on grass(point) on earth surface. 2. He jumped. 3. And returned back, and he could find the same grass(point) under his foots. Right? If earth is ...
0
votes
4answers
115 views

Can you tell your absolute speed in space?

Normally in relativity your speed can only be known relative to another object, given that as one approaches light speed more energy is required to accelerate faster, based on the energy consumption ...
1
vote
7answers
3k views

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 ...
-1
votes
3answers
122 views

Isn't a physical frame of reference useless for calculating speed? [closed]

Please ignore relativistic effects and the effects of the expansion of space-time due to the expanding universe theory for the purposes of this question. Whenever someone asks what is the speed of X, ...
2
votes
0answers
36 views

Would impact angle matter on relativistic impactor?

I'm trying to calculate (for fun) a comparison between a kinetic impactor and an H-bomb. I would assume this to be a fairly straight forward problem involving kinetic energy and a table of various ...
71
votes
11answers
12k 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 ...
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 ...
0
votes
1answer
38 views

Relative motion with two observers

For four particles A, B, C, D , the velocities of one with respect to other are given as $ V_{DC}$ (Velocity of D with respect to A) is 20 m per second towards north, $ V_{BC}$ is 20 m per second ...
1
vote
1answer
522 views

Dropping a ball in a fast moving train

If I drop a ball in a train moving at a constant speed, will it land on the spot I aimed it at or a little away as the train has moved while it was in air? If it lands away, will the observer not know ...
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 ...
1
vote
3answers
52 views

Unable to understand relative motion

If you sit in a fast moving car on a highway, you will see the objects outside the window moving backwards with respect to you. Assume that your motion is uniform. One would think that all static ...
1
vote
1answer
96 views

Since the Earth is moving through space, do we experience time dilation?

What I mean is, the Earth orbits around the sun at around 30km/s, while I understand that this isn't a very high velocity does time dilation still occur to say a person who is completely stationary in ...
0
votes
2answers
142 views

How insects got hits on the wind shield of a car?

There are rare chances when any insects got hit the wind shield of a car while driving in highway but it makes to wonder that how is that happen ? Does insect suffered a greater change in momentum ...
0
votes
2answers
385 views

Relative motion. Setting course of closest approach

Let $r_{P/Q}$ be the position vector of $\overrightarrow P$ relative to vector $\overrightarrow Q$ and $v_{P/Q}$ the velocity vector of $\overrightarrow P$ relative to $\overrightarrow Q$. Suppose ...
1
vote
1answer
44 views

Smooth collar sliding down smooth shaft [closed]

For part (a), we find that the acceleration of the collar is $6.94~\mathrm{m/s^2}$. In part (b), we say that the acceleration of B with respect to A is $6.94~\mathrm{m/s^2}$, which makes sense. ...
2
votes
1answer
48 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 ...
1
vote
1answer
104 views

Trajectory of a swimmer trying to reach the opposite bank [closed]

I came across the following question 2 days ago: $Q$. A swimmer is swimming in a river with a speed $u$ with respect to the speed of water which is $v$. It is assumed that the river has straight ...
3
votes
1answer
106 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 ...
0
votes
1answer
27 views

Relative Velocities/Accelerations

I seem to just not understand how relative velocities and accelerations work. Here is a problem that I will use to ask my question: When the box lands, its moving 3m/s to the right, and the belt ...
1
vote
1answer
32 views

Relative Movements Agreement on Object speed

I am confused about Relative Motion.Lets suppose we have one train and two observer.One of them inside the train and the other one is outside.Lets think the outside observer see the train moves a ...
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?
1
vote
1answer
51 views

Coriolis Effect vs airplane

So, if I build a highway from north pole towards equator and sit in a car, speed pretty fast towards it, I should feel force with the Earths rotation due to the different rotation speeds of this ...
0
votes
2answers
3k views

Speed of a fly inside a car

A couple of weeks ago I was travelling in a car (120 km/h approximately) and I saw a fly flying in front of me (inside the car, near my nose, windows closed). I wonder how was that possible. Does it ...
1
vote
1answer
324 views

What would happen to electronic circuits when traveling near the speed of light?

Imagine a space ship, loaded with all sorts of computer systems, traveling near the speed of light. Electricity itself is very fast, and can reach speeds close the speed of light. (up to 99% ...
0
votes
2answers
106 views

When we are inside a plane and look outside the window why does everything (outside) appear to be at rest?

When we are inside a train and we look outside the roadside trees appear to be moving in the opposite direction but when we are inside a plane and look outside the window everything seems to be at ...
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 ...
0
votes
0answers
24 views

Relative speed and time [duplicate]

There is a notion that travelling at the speed of light is impossible for a matter. But what if the there are 2 objects that are coming towards each other with half the speed of light. Thus relatively ...
0
votes
0answers
39 views

Is it possible to derive the Doppler effect formulas for sound using relative velocity?

Is it possible to derive the Doppler effect formulas for sound using relative velocity? If possible can anyone please explain it to me.
0
votes
1answer
67 views

Does it take more effort to move against earth's rotation?

I know that if we stand still, we are traveling at 0 m/s relative to the Earth. But if we move against the rotation of the Earth we lower our speed, so, wouldn't we have to fight against the ...
0
votes
1answer
115 views

Since Earth spins, would an aircraft travelling opposite to direction of Earth spin take less time? [duplicate]

Suppose we want to reach the point on earth which in relative terms is exactly on the opposite end of the sphere we call earth (I know it is not an exact sphere). We either dig vertically downwards, ...
-2
votes
1answer
90 views

Earth's direction in space [closed]

After watching videos about the rotation of the planets being helical and not heliocentric, I wondered, as Earth travels through space, which part of Earth is the forefront?
0
votes
1answer
29 views

Why distant are stationary?

Why do for an observer looking out of the window through a fast moving train ,the distant objects appear to be stationary?
3
votes
4answers
941 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. $A$ ...
-1
votes
1answer
48 views

Simultaneous observations in relativity

Consider an object which responds to two different signals A and B: it responds to signal A with blue color and signal B with red color. Also, it responds only to the signal that reached first. In ...
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 ...
0
votes
1answer
26 views

Who is moving and with who's relation?

What if we are in a cube of 3×3 m  which is further enclosed in a sphere of radius 50 m? And the sphere start to rotate on its own axis would we experience force? If not then what happens if there is ...
0
votes
0answers
28 views

Faster than light travel at half speed [duplicate]

Peter and Susan are facing each other in outer space. Both of them ignite their their incredibly powerful jet pack thrusters and accelerated away from each other to just over half the speed of light. ...
3
votes
2answers
132 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 ...
1
vote
1answer
44 views

Speed of a river [closed]

I've got a problem (Sorry if it's too basic). It is: Going upstream in a canoe, the passengers throw an empty bottle. They keep rowing for 60 minutes, and move 2 kilometers upstream. They then start ...
-1
votes
1answer
103 views

Problem related in relative motion [closed]

The speed of a boat is 1.5 m/s in still water. One needs to cross a river of width 500 m with this boat. Along the direction of the river a strong wind is blowing with a speed of 0.9 m/s. The boat is ...
1
vote
1answer
207 views

Problem on relative motion involving wind speed .Finding the real velocity of wind?

Here is the question For a person running west at 7km/hr wind appears to blow from north-west .But when he walks towards west at 3km/hr the wind appears to blow from the north. What is the true ...
0
votes
1answer
64 views

Muzzle Velocity confusion

Does muzzle velocity refer to the velocity of the bullet with respect to ground or the velocity with respect to the gun?
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 ...
0
votes
2answers
169 views

Perspective and changes in kinetic energy

Say you have two planes flying next to each other at the same speed and one decides to pick up speed by burning a tank of rocket fuel. If someone on the ground wanted to know that plane's new speed ...