# Questions tagged [relative-motion]

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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 ...
337 views

### 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 ...
152 views

### Could a gyroscope demonstrate absolute motion?

If a spaceship were moving very fast, and a rocket motor mounted perpendicular to the direction of motion/"main thruster" were turned on, the change in direction to an observer would be proportional ...
64 views

### question from the idea of relative acceleration [closed]

the accelerations of a particle as seen from two frames $S_1$ and $S_2$ have equal magnitude $4 m/s^2$ so my teacher said that the acceleration of $S_2$ with respect to $S_1$ may be anything between ...
46 views

### Direction of motion 1 [closed]

A man on a bike travelling east on straight road at 8km/h sees a car driving north at 60km/h. What is the apparent speed and diretion of motion of car to the cyclist?
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### One dimensional motion - Why does a ball fall back in my hands when I throw it up in a moving bus? [duplicate]

Suppose you are moving in a bus and you throw a ball upwards. Why does it always land in your hand and does not fall behind you? The bus is moving with a uniform velocity in a straight direction. Now ...
196 views

### While jumping in a high speed train why we fall on same place? [duplicate]

while we jump inside a high speed train why do we fall on the exact place? as train is in high speed and we are jumping so we should fall backside. but this doesn't happen why?
91 views

### An ambiguous question on 2 balls [closed]

Two balls A & B are at rest at the bottom of a hill.Sometime after ball A is given a kick up the hill, ball B is also given a kick up the hill. When ball B is going up the hill, ball A is already ...
212 views

### Can absolute speed be determined by an object's mass?

It's been said that nobody can actually know how fast the Earth is moving through space, because a reference point is needed to determine absolute velocity. But if mass increases as speed increases, ...
168 views

### Is light relative? [duplicate]

I was wondering, if two cars (A and B) are going at different velocities let's say A=90km/h, B=80km/h then if we add the two speeds we get 170km/h meaning they are relative to each other. But if one ...
37 views

### Explain why the friction force and the component of weight down the plane are the same value [closed]

can someone expain me why is this. Thanks.
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### Faster alternative to travel to any location in the west? [closed]

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 ...
59 views

### Force impact when lifting objects traveling at a relative velocity of 0

During a break my colleagues and I came up with the following Gedankenexperiment I would appreciate if you guys helped us find the solution for: Truck (weight 5t) traveling at 80 kph with a tow-truck ...
475 views

### Centrifugal Force vs Motion (inertia)

Could centrifugal force be appropriately described as motion (inertia)--rather than an (imaginary) force? Could such a change of perspective eliminate much of the common confusion?
2k views

### Two Trains Relative Motion [on hold]

I am confused about finding the solution to this relative motion problem: You are on a train traveling east at speed of 20 m/s with respect to the ground. Your friend is sitting on another train ...
3k views

### Portal 2 physics - one moving portal [closed]

A problem is posted and solved so many times on 9gag - till yet no good answer. The problem: you have two portals (one moving and one still standing). The one moving reach a still standing cube: what ...
536 views

### What is the effect of constant wind on a hot air balloon?

If a hot air balloon is being carried by a continuous wind current in a particular direction, in which direction will the flags on its basket wave?
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### Acceleration of log moving without slipping with/under a plank

So I have this image: I know how to solve the accompanying physics problem once I have determined the relative accelerations of the logs and the plank. According to my physics textbook, the cm of ...
493 views

### Simple Rotating frame of reference

I have a car moving in straight line with certain time-varying acceleration and there is another car moving in a curvilinear way along some curved path with some time-varying acceleration. The ...
103 views

### Reference frame and conservation of energy [duplicate]

Say spaceship $\alpha$ burns a portion of its fuel to leave planet A and is cruising through space at 10 m/s relative to the surface from which it launched. Spaceship $\alpha$ is being observed by ...
92 views

### Can the use of a magnetic compass be inconsistent with Galileo's Relativity Hypothesis?

I just read Galileo's Relativity Hypothesis from this website: http://physics.ucr.edu/~wudka/Physics7/Notes_www/node47.html It states that - "any two observers moving at constant speed and direction ...
275 views

### Given the 1st Postulate of SR, doesn't the 2nd Postulate go without saying? [duplicate]

i've been looking over possible duplicates of this question and haven't found one yet. so, using Wikipedia as the textual source: The Principle of Relativity – The laws by which the states of ...
396 views

### circular motion relative velocity question

You are standing in the middle of a disk of radius $2\:\mathrm{m}$ rotating at $\pi\:\mathrm{rad s^{-1}}$, and there's a smooth puck at $1.5\:\mathrm{m}$ away from the centre. If the puck was released,...
50 views

### Calculating offset when crossing a river with speed profile [closed]

I am having some issues putting the theory I learned into practise. We have an exercise, that goes as following: A swimmer wants to cross a river with width b. The swimmer heads straight to the other ...
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### Where does the energy required to speed the object in this question come from if the object actually slows down from another perspective?

I was thinking about mechanics and energy and stumped myself with this scenario I made up: Suppose you are in a room and you are motionless. In the same room, a 1 Kg ball is moving to your right with ...
188 views

### Effect of redshift on energy conservation [duplicate]

Light coming from galaxies that are going away from us is redshifted. Since the energy of a photon is purely dependent on its frequency one may conclude that the energy of these photons decreases. The ...
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### Why wasn't the theory that light's velocity depended on its source very popular?

Before special relativity, it was believed that light having a specific velocity meant it had to travel through something. My question is, why wasn't the idea that light depended on its source very ...
310 views

### Feynman lectures on physics: The Michelson-Morley experiment

I have a problem understanding something. I'm reading FLP and here I don't understand why when the system is travelling, the light doen't make the same angle in the mirror C... if B and C are moving ...
2k views

### Stone released from balloon [closed]

Q: If a stone is released from a balloon rising with acceleration a at the instant when its velocity is v. Then immediately after release the acceleration and velocity of the stone are ? I tried to ...
123 views

### Why the point right below the center of a rolling ball on the ball has zero instantaneous velocity [duplicate]

Suppose there is a snooker ball rolling on a table. The velocity of the center of the ball is Rω and its direction is horizontally right. I don't understand why the point right below the center on the ...
105 views

### Will Coulomb law work if both the charges are moving with same speed and same direction?

I read it on wikipidia that Coulomb law will work if both the charges are at rest relatively. Will there be an effect of magnetic field also?
766 views

### How fast are we moving when standing still? [duplicate]

The Earth rotates around itself and revolves around the Sun. Our solar system revolves around the center of our galaxy and our galaxy is moving in some way throughout the universe. If you took into ...
1k views

### What is the velocity relative to earth?

I'm having trouble distinguishing between different relative velocities. Example: Say I'm riding my bike at a constant speed of 5 m/s in a straight line. While riding my bike I throw a baseball with ...
5k views

### How does the angle between two objects change relative velocity?

I apologize in advance if my question isn't clear. If two objects start from the same position, but leave in opposite directions--say $40^0$ west of north, and $50^0$ east of north--how would an ...
110 views

### Does it take less time to drop a ball than fire one horizontally (with $90^{\circ}$) [closed]

So I was arguing about this with my friend. If we take two balls and drop one from a certain height H and then fire another one with horizontally with some initial speed from the same height H, which ...
12k views

### Stone dropped from a moving train

This may look like a stupid question, but it is really getting to me. Imagine a train moving with an acceleration $a$, and a person drops a stone from the window. To an observer on the ground, the ...
4k views

### 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 ...
7k 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?
2k 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 ...
505 views

### Fundamental paradox with Newton's Law of Gravity?

This is my first post here, but I've been struggling with this problem in my head since I studied physics at school when I was 14 (30 years ago!). There seems to be a fundamental paradox with Newton'...
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### 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 ...
266 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 ...
355 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 ...
1k 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 ...
113 views

### Is our galaxy colliding with Andromeda, or is Andromeda colliding with our galaxy? (i.e. Which is moving less / staying still?)

...Or does this question not make sense because of Relativity?
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### 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, ...
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### 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 H-...
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 ...
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 ...