# Tag Info

18

A physicist, me for example, identifies events by choosing a set of coordinates. For example I have a clock that I use to record time and a ruler that I can choose to measure distance. This allows me to set up some coordinates $(t, x, y, z)$ so I can assign every event to some point in my coordinate system. If I received a laser pulse from Mars at 16:05 ...

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After a comment it becomes clear that there is deep misunderstanding here and the question title has nothing to do with the actual problem. Let's get something basic clear: there is no "the centripetal force". That is no force out there that magically decides to come into being when an object moves in a circle. Rather "centripetal" is a label for those ...

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This is a very common "gotcha" for novices, and something that is not covered well at all in textbooks, in my opinion. The problem is one of semantics, not of physics. The term centripetal force does not name a physical force the way gravitational force and electrostatic force do. The words centripetal force stand for the net result of real forces in the ...

3

Let's say we're sending a scouting mission to an Earth-like planet 100ly away to see if it's suitable for colonization. We could send the scouts out at near light speed, and due to time dilation they could easily survive the trip without dying of old age. If we send them out at .9c then the entire round trip will only take 20 years in their frame of ...

2

A space train leaves Mars at 14:00pm and arrives on Earth at 19:45. The train moves at 0.001C and has 40Km of length. How long will it take for the whole train to arrive on Earth? - Disregard re-entry and friction. Nobody on Earth will say the train is leaving mars now. Same thing with the light, just it moves faster and is smaller than the train above. ...

2

A pedagogical note: Because students regularly try to look for the centripetal force and get confused (as noted by other answers here), I try to emphasize that it is the acceleration which is centripetal rather than the force (even though the sum is center directed). Because the acceleration is always the result of a sum, I have found that fewer students in ...

2

Let you apply force $\bf F$ at point $P$ the coordinate of which is $\bf r$ measured from a specific point $O$ - the point about which you want to rotate. Let $\bf r$ and $\bf F$ be in the same plane. Now, if you were to rotate $P$ about $O$, it would rotate around some axis perpendicular to the plane in which the force and the point lies; if ...

1

Imaging the balls on a string. You are launching N balls per second, at a velocity $u$. This means the distance between the balls is $u/N$. And $N$ balls per second will pass a certain point in space. Now if the car is moving at a velocity $v$ (same direction as $u$), fewer balls per second can hit it - because subsequent balls on the string have further to ...

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Relativity just requires "constant speed of light in vacuum". It makes no claims about the speed of light in a medium. When you are moving relative to water, you will observe a different speed of light depending on your relative velocity. But you will still have all the other effects of relativity at work - such as time dilation.

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The Coriolis acceleration is $2\vec{v}\times\vec{\omega}$, where the velocity vector $\vec{v}$ is measured in the rotating reference frame. In your example, if I'm reading you right, the particle is moving with the disc, so $\vec{v}=0$, so the Coriolis acceleration is zero. In general, if you have either a radial or tangential velocity component in the ...

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Do you know Bernard Schutz's book: A First Course in General Relativity? Check out the first chapter of that book. There is a derivation of invariance of proper time using first principles in section 1.6. Basically, the idea is to start from expressing $\Delta \bar s ^2$ (interval in the barred frame) as a linear combination of $\Delta x_i$'s (vector ...

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One thing you have to note is that speed is relative, Clock A would see clock B moving from A's point of reference, and B would see A moving in B's reference, so you shouldn't be using the word "stationary" in this context. Both the clocks would see the other clock tick slower, B would see A's future only if it returns back to A, this makes it obvious to A ...

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Centripetal Force isn't a real force. It is a construct for representing the sum of different forces that cause circular motion. In the case of an orbit, the gravitational force is the centripetal force, which is what you stated in the first equation. They are NOT opposites, as you asked in the question. They do not become arranged to have a net force of ...

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A reference frame is equivalent to a choice of coordinates. So, choosing an accelerated frame in Minkowski space is equivalent to choosing a specific coordinate system on Minkowski space. Most importantly, this means that there is not genuine curvature in an accelerated frame, i.e. it is fundamentally different than gravity. The equivalence principle ...

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