# Tag Info

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Isn't a physical frame of reference useless for calculating speed? There aren't really any physical frames of reference. You can't step outside and point up to the clear night sky and say "Look, there's a reference frame". You can point to the Moon and the stars, but they are what they are. You can use them in your reference frame, but that reference ...

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Velocity and speed of a body are measured with respect to another body (see other answer). This is the only possible definition. The "actual" speed respect to the "empty space" is not well defined instead. And this is because we are not moving inside an "empty space" or an "aether" which fills the universe. To grasp the concept think about how you will ...

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When physicists use the word velocity it has a precise definition that is meaningful and unambiguous. If I measure the displacement from me to you then the result is a vector i.e. it tells me how far apart we are and in what direction you are. The velocity tells me how this vector is changing in time. The point is that I can do this for any pair of objects: ...

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You are correct time does stop at $c$ speed of light, however we (planet) did not place a timer inside the light but are measuring it from outside so time for us passes normally and nothing weird happens and nothing is frozen. As a result we simply are measuring the gap of time from the 1 point to another point whilst a object (light) goes to it so as ...

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The thing is, in relativity you cannot have a reference frame "chasing" a photon. You'll get singularities if you try to view the world from a photon's perspective. A photon cannot move like you and you cannot move like a photon. As a photon, travelling along a light-like world line, experiences no proper time it's proper velocity is simply undefined. ...

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Light travels at the speed $c$ this speed is finite and with out using any relativity we can calculate the time it takes for something travelling at this speed to reach us: $\text{time} = \frac{\text{Distance}}{\text{speed}}$ or $t= \frac{d}{c} = \text{8 minutes}$ in this case. For a person travelling very close to the speed of light with velocity $v$ from ...

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First, the chemical reaction that takes place in the cartridge of a gun produces high temperature, high-pressure gas, without the need for external oxygen. The gases then push the gun and the bullet apart. Secondly, the bullet will leave the barrel with a certain amount of momentum, found by multiplying the mass of the bullet by the velocity of the bullet. ...

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You release a ball in space. Measure its distance every 1s or C second. if you find $S(t)-S(t+c)$ is not constant , where c is a constant. then you are in a Non inertial system

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Inertial frame of reference is such that free bodies move with constant velocity. If you detect free body accelerating, the frame is not inertial.

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You hang two balls on ceiling, and measure the angle between the string and ceiling. If it is all right angles, then you are in inertial frame at the direction of x and y to a good approximation.

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Short answer: One cannot determine whether a reference frame is inertial. It is simply not possible. This situation is avoided in General Relativity (or any of its extensions), since in GR the physics does not depend on the reference frame (any reference frame), and the concept of inertial frame is not necessary. Of course one can determine an inertial ...

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Special relativity and general relativity have different views about inertial frames, but in some ways the general relativity take on them is (perhaps surprisingly) easier to explain. So I'll start with GR then extend the description to SR. In general relativity there are usually no global inertial frames i.e. it is impossible to construct a frame that ...

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Events can never univerally only be said to happen in a particular order if they are space-like events, i.e. $(\Delta r)^2 > c^2(\Delta t)^2$. If this is true, the there is no universal agreement about the order in which the events happen. If, however, $(\Delta r)^2 ≤ c^2(\Delta t)^2$ holds true, then every observer will agree on the order.

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In Newtonian physics, neither are inertial frames. In relativity, only the first one is an inertial frame. Edit: Let me clarify. An inertial reference frame is a frame that "follows" an uniform motion, i.e. a motion where there could be said not be affected by any net force. In Newtonian physics gravity is a force and objects in free fall are thus ...

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Let me describe a geometrical way to approach your question. We start by drawing a spacetime diagram with distance along the horizontal axis and time on the vertical axis. We'll also take the speed of light to be $1$, so on our diagram a light ray travels along the line $x = t$ i.e. at 45º. If we have some other observer moving at a speed $v$ relative to ...

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The answer is, "as long as the light from the front of the train has not hit the back of the train first, then yes, there exists some reference frame which thinks of both as simultaneous." In special relativity, there is a number which everyone agrees on: take any two events that are separated by a vector (in your coordinates) $\vec r$ and time interval ...

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The situation is quite possible.As you have not mentioned any Exact value of acceleration or time between the two lights opening it is difficult to tell exact values which will satisfy your event.But I have made a similar thing which may help you understand. Imagine AE is the train and B,C,D are points on it such that AB=BC=CD=DE.Now AE is moving with ...

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