Questions tagged [relative-motion]

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3answers
98 views

How can an accelerated frame and a gravitational field be equal?

How can two things that are different be equal? In a gravitational field there is actually gravity that push us down, but in the accelerated frame there is nothing that push us down.
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Relative velocity related to acceleration

Let's say we have a car moving on the horizontal ground with a velocity of $10 \hat{i} \frac{m}{s}$. A person inside throws a ball vertically upward with a velocity of $3 \hat{j} \frac{m}{s}$. Now I ...
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A question about relative motion and how to make sense of it

So, here is the question: Assuming the 2D Cartesian system and the basis vectors $\hat{i}$ and $\hat{j}$, we have two cars A and B located at (0,6) and (-30,0) respectively. Car A starts moving with a ...
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5answers
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Thought experiment on relative motion

Imagine two persons (A and B) in a vacuum in outer space that are in front of each other and are not in motion. Say that suddenly A starts moving away with a constant velocity from B. In this case, ...
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2answers
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Why is light speed constant even in respect of a moving body? [duplicate]

My teacher told that speed of light is constant. The speed of moving object appears 0 when seen by another moving object( of same speed as that of first) but she told that light speed is constant ...
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2answers
33 views

Sign convention for relative displacment

If an object A is $X\ \mathrm m$ behind B,than the relative displacemnt of A wrt B would be $-X$. However is object A was moving towards B(at rest) at a rate of $Y\ \mathrm{m/s}$, then $$V_{ab}=+Y-0=+...
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3answers
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Kinetic energy "paradox" -- where am I wrong here? [duplicate]

Body $A$ is at rest and has mass $2$, so the energy required in order to accelerate it to a speed of $v$ is $v^2$, and so the energy required in order to accelerate it to a speed of $2$ is $4$. On the ...
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1answer
41 views

For an airplane moving north from some latitude (say $30^{\circ}N$), why does rotation of the Earth cause an increase in apparent drag?

I'm not able to understand the answer to this example: Example 10 from the Curtis Orbital Mechanics text book : An airplane of mass $70 000\ \mathrm{ kg}$ is traveling due north at latitude $30^\circ$...
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11answers
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What is the essential difference between constant speed and acceleration?

I do know what the difference is but what I am trying to understand is how an object knows its speed is changing and/or how space knows an object is accelerating. The particular thing I am interested ...
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1answer
25 views

How do you find relative motion of two objects not travelling in a straight path?

I understand relative motion linearly, but how about more complex motion, like travelling in a circle? Would you just find the objects velocities at some point using the formula $v=\omega r$ and then ...
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0answers
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Relativistic addition of velocities

Suppose I have two frames of reference $S$ and $S'$ moving relative to each other. Suppose an object has a velocity of $v_s = a\hat{i}+b\hat{j}\space$ in the frame $S$. Similarly, it has a velocity of ...
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5answers
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If motion relative to a frame of reference is purely relative, how do we account for the work done to move relative to the frame of reference?

I get the idea that everything is in motion, and there's no absolute reference frame for everything. But when we consider local events, like a train passing through a town, I have trouble accepting ...
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3answers
598 views

Greatest distance between a particle moving with constant velocity and a particle moving with constant acceleration

Two particles start moving along the same straight line starting at the same moment from the same point in the same direction. The first moves with constant velocity $u$ and the second with constant ...
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2answers
74 views

How do we know if a particle is moving or not? [duplicate]

if there is only one particle in the universe, how do we know if it is moving or not? We don't have any other object to track the distance between the two, then is it possible to determine the ...
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1answer
40 views

Does the rotation of Earth make two electrostatically charged bodies interact magnetically (with each other)?

Does the rotation of Earth make two electrostaticaly charged bodies fixed on its surface interact magnetically (with each other)? If yes, is this interaction different as geographical latitude changes?...
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0answers
49 views

Should the velocity increase or decrease?

Suppose some projectile is traveling along the positive $x$ direction at a velocity $v$. While moving it is emitting particles, and thus losing mass. If the particles are being emitted in the same ...
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0answers
39 views

Doubt regarding derivation of rocket equation

I'm having quite some issues deriving the ideal rocket equation, and I suspect it is due to lack of identifying scalar terms and vector terms. Here is how I derive it : Let $v$ be the velocity of the ...
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1answer
53 views

Can a bubble of vacuum move through space?

If there are really regions in space where a vacuum exists, I have learned there can still exist electric and magnetic fields within. That makes this region then differentiable from the regions around ...
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10answers
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Why do bodies traveling at constant velocity experience the same physics?

I am new to special relativity. I don't have a background in physics. I was reading Brian Green's The Elegant Universe and got hung up on an example. In fact, Einstein realized that the principle of ...
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1answer
52 views

Distance between two airplanes [closed]

I need help with a vector problem where my solution is different from that of my teacher. The translation of the problem reads as follows: An air traffic controller observes two aircraft on his radar ...
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3answers
391 views

Interesting answer as a range of tension in pulley-block-plane system

I want some intuitive understanding on why there will be a range in tension in the below question. (On solving we will get that the system is at rest ($a=0$) and since its starts from rest the blocks ...
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1answer
49 views

Calculating the velocity of a tennis ball after impacting a moving racket

Suppose I have a tennis ball moving with $v_1$ (blue arrow in first image below) impacting a moving tennis racket with velocity $v_r$ (black arrow in first image below) In order to calculate the ...
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2answers
41 views

Power Demand for Motion of Vehicle on a Road and Plain in Air with Air Resistance

Let's say we have an ideal motorcycle without frictional losses (no tire slip, ideal machine without losses). But we want to consider wind force which is proportional to the square of speed $F = k \...
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3answers
204 views

Charges and relative motion

We know that coulomb charge can't be applied on charges in motion. Why? What if they are moving in same velocity? Aren't they actually at rest for each other? ( yeah, it depends upon medium, so I'm ...
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5answers
210 views

Is rotational motion relative, as is linear motion?

In special relativity only relative velocity is what matters. A rocket and the universe can be in relative linear motion. In the rocket the universe is in motion. The universe got this motion when it ...
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1answer
60 views

What is the velocity of a photon with respect to another one? [duplicate]

When light comes from a source, there are numerous photons associated with the energy packets. Now, what is the velocity of a single photon with respect to another photon coming form the same source i....
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2answers
90 views

What would an outside observer see if I throw a ball in the opposite direction of the train's movement towards a catcher? [duplicate]

Without taking air resistance into consideration, what would a stationary observer see if I am inside a moving train traveling at 60mph and throw a ball at 60mph opposite the direction of the train? ...
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1answer
138 views

Relative Circular Motion [duplicate]

Four particles $P_1, P_2,P_3,P_4$ are moving in a plane. At $t=0$, they are at the four corners of a square $ABCD$ of edge length $l$. Each of the particles has a constant speed $v$. The velocity of $...
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0answers
105 views

Work done on a moving cart in wind

Let us suppose a cart is moving in a rough ground with a propeller attached to the roof of the cart. Let the velocity of the cart at the given point of analysis be V and velocity of wind is -W. The ...
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1answer
31 views

CoM-and-relative velocity

In our scrip we are considering the elastic collision between two particles, one with inital velocity $\vec v$ and the other $\vec w$. We also consider that the particles have the same mass. ...
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2answers
61 views

Can we use relative motion if acceleration of two bodies are different?

Suppose, on the $x$-axis, a body $A$ is moving with velocity $v_1$ and acceleration $a_1$, and a second body $B$ is moving with velocity $v_2$ and acceleration $a_2$. $B$ is at a distance of $S$ from $...
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2answers
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Relative Velocity (Kinematics) [closed]

I was studying the concept of relative velocity and after solving a few problems I am running into this confusion. a) Let’s say two cars are moving perpendicular to each other (no collision) with ...
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2answers
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(Misunderstood) special relativity and equivalence of motion [duplicate]

Let $A$ be the person who is standing on the Earth (ignoring gravitational field, etc.). Let $B$ be the person who is moving to the Proxima Centaura and back. By the time dilation equation we get: $$...
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1answer
109 views

How to calculate relative velocity in curved spacetime?

Is there a nice geometric way to calculate relative velocity in curved space time? I'm looking from something similar to how the separation vector $n$ between $2$ neighboring geodesics obey: $$ \...
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3answers
149 views

I am moving right and rain is falling vertically down.Why should I hold umbrella at an angle?

This is related to relative velocity.I get that, from my moving frame of reference rain is making an angle.But still... it doesn't make sense to hold umbrella at an angle when rain is falling ...
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0answers
38 views

If motion is relative, can we say that the Earth doesn't revolve around the Sun? [duplicate]

Considering motion to be relative, consider these two cases as follows An observer is standing on the sun and observing the earth and the earth has no background (no stars and stuff in the background)...
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5answers
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How should I understand the idea of relativity of space?

In the book General Physics by L.D.Landau, he mentioned the following in the first and second paragraph The fundamental concept of mechanics is that of motion of a body with respect to other bodies. ...
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0answers
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Sliding Friction Force

Sliding friction force is not dependent on velocity. Is wear dependent on velocity? For example, it takes 10 Newtons to overcome sliding friction, no matter if the relative velocity is 1m/s or 1000m/s....
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1answer
29 views

Velocity of a swimmer relative to the Earth [closed]

Julia can swim 0.34 m/s in still water. She swims directly east across a river that is 180 m wide and lands at a point that is 81 m north downstream. Determine Julia's velocity while swimming relative ...
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1answer
63 views

Velocity of an object with respect to the same object

I know that velocity of an object with respect to the same object is a null vector but I'm not able to get a grasp of it. Whatever I had thought till now is that if I'm sitting in a train (which may ...
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3answers
43 views

Relative Motion and Motion on ground

When I walk on the ground I know that it's me who is walking and not that I am just moving my legs forward and backward and the ground is moving.But according to relative motion I should not be able ...
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2answers
229 views

What will be the minimum drift possible when a boat crosses a river? [closed]

Imagine a boat with a constant speed $v$ that has to cross a river of width $d$. Let the river flow with a constant velocity $u$. I want to know the angle at which the boat should sail in order to ...
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1answer
43 views

Measuring the speed of an object

If I understand correctly someone said (Einstein?) that all motion is relative. However, if it were possible; given that light always travels in a vacuum at a fixed speed, could all other motion be ...
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1answer
57 views

How would we know whether a object is moving in deep space?

In deep space, there isn't anything we can compare our position with (According to my knowledge). In other words, we won't be able to define an origin so how would we know whether we are in motion or ...
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4answers
73 views

How can we tell if a charge is moving in a Magnetic Field?

In the equation $\vec F=q\vec v×\vec B$ ($\vec B$ is the magnetic field), what is $\vec v$ with respect to? If we were to imagine $\vec B$ to be infinitely large. How could we tell if the charge is ...
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2answers
90 views

Why does these two masses not have same acceleration? [closed]

Assumptions: No friction , $m_2$=5kg and $z_1$=2kg. $z_2$ is fixed at its place. Here, there is a mass of $5kg$ on a movable wedge of mass $2kg$. So, my question is that can we say that acceleration ...
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1answer
84 views

Acceleration and geodesics in General Relativity

Consider this situation: A) An aeroplane travels a distance from point A to point B which are located on the opposite sides of the Earth. The aeroplane has taken the shortest possible distance to ...
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1answer
366 views

Is work done by the internal forces independent of the frame of reference?

This question is about work done being dependent on the frame of reference, which is, obviously the net work done. I ask what is the reason for the work done by internal forces to be independent of ...
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1answer
20 views

Using rate and per unit time together for describing effects of motion

I have observed recently 3 different elementary definitions of velocity Rate of change of position, change in position per unit time ,and rate of change in position with respect to time,so I am not ...
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1answer
83 views

A question on time taken in different frames of references

Let us say , a swimmer has to swim upstream and downstream a river , between two points which are at distance "L" from each other . The swimmer can swim in still-water at velocity "Vs&...

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