# Tag Info

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The main thing we must know is what causes gravity, what makes things to fall towards Earth, I think these things haven't we understood yet. Although Einstein tried to Explain Gravity but couldn't explain it completely.

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Let me present a slightly different perspective to Alfred's answer, although I'm basically saying the same thing. I suspect you've got hung up on the idea that velocity causes the relativistic effects like time dilation, but the underlying cause is something different. All the weird effects in SR are caused by a fundamental symmetry of spacetime, which is ...

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I am 100% confused by this. I do not understand why the plane clock runs slower than the earths clock? Why the preference on the plane? Why not on Earth? Why does the plane lose time, and not the other way around? (I cant think why one body or the other gets the preference as from the perspective of one or the other they are both moving the same). From the ...

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We need to untangle this a bit but first: the cause of time dilation is the geometry of spacetime which is such that there is an invariant speed c. Now, remember that velocity or speed is not a property of an object; there is no absolute rest. Further, consider the case of three objects in uniform relative motion with respect to each other. If I choose ...

2

It always helps to draw the right picture. This picture assumes that Boxguy is standing next to the lamp, and that the flash leaves the lamp just as it passes PlatGirl. (If, for example, BoxGuy were standing next to the mirror, the picture would look a little different.) The black vertical line is Platgirl's worldline, and any black horizontal line is ...

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Please correct me if I am wrong. But I think that the speed of light, measured distance and time from a frame of reference are concepts defined relatively to each other. In the sense that we fix the speed of light and define distance and time relatively to it. In particular I am not really convinced that the measured speed of light is the same in every ...

2

Is there any significance in saying an observer as an imaginary entity? Yes. From Wikipedia: Physicists use the term "observer" as shorthand for a specific reference frame from which a set of objects or events is being measured. Speaking of an observer in special relativity is not specifically hypothesizing an individual person who is ...

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In the frame of reference of the body, is the centripetal force felt or is only the centrifugal force felt? It depends on what you mean exactly. Consider, for example, the amusement park ride Dumbo at Disneyland: . On this ride, passengers sit in mini Dumbo replicas and are swung around in a circle. What forces do they feel? Well, firstly, they ...

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In the frame of reference of the body, is the centripetal force felt or is only the centrifugal force felt? In the frame of reference of body both centripetal and centrifugal forces are felt. does a body only feel the effect of pseudo forces in an accelerated reference frame? No the body feels all Pseudo forces and real forces in an ...

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As with any questions about (classical) motion, think about what forces are involved. Once you throw the ball in the air, what forces act on the ball? What forces act on you? There's gravity, pointed vertically downward. There's some air resistance pointed in the opposite direction of motion (so down while the ball is going up, and up while the ball is ...

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Provided the train does not accelerate or decelerate during the ball's flight, it will fall back into your hands, since the ball is moving at the same velocity as you and the train.

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User Sahil Chadha has already answered the question, but here's the math and a pretty picture for anyone who is unconvinced that you're right. Since the train is accelerating, from the perspective of an observer on the train, the ball will experience a (fictitious) force in the direction opposite the train's travel having magnitude $ma$ where $m$ is the ...

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Yes your reasoning is correct,from the point of view of train the ball will travel in a tilted parabolic path as direction of apparent gravity will be different in the train and will not end up in your hand

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Short answer: If you're talking about the spatial components of force, then yes, through a Lorentz transformation on the force four-vector. By "real force" I assume you mean a non-inertial force, so that you are computing force from an inertial frame, or, more generally, in a freefall frame - i.e. a locally flat Minkowskian tangent space to the spacetime ...

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Well, take the electromagnetic force...it has been shown that the induced magnetic field around moving charges is a relativistic reference frame effect. See this post How Special Relativity causes magnetism Therefore a static electric force in one frame becomes a magnetic force in another. However, from a Newtownian view, we either allow pseudoforces and ...

4

If you say that earth's velocity around the sun is 67,000 mi/h, your reference point is the sun itself, which makes the aeroplane's velocity 68,000 mi/h, not 1000. Using special relativity only, and (A) observing from the sun, a clock on the plane would seem to run slower than a clock on earth. A person (B) on earth would measure also measure an ...

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The aeroplane is moving in the atmosphere of the earth, and so how can you say that the earth is moving faster, when viewed from outerspace. I think the aeroplane will still be faster.

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Basically, the universe has a speed limit. No object can ever exceed the speed of light. Now imagine you decide to prove Einstein wrong by building a train capable of nearly reaching the speed of light, and then shooting a bullet forward in that train, so that the bullet will break the speed of light. In order to preserve this speed limit, time for you and ...

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In one sentence: More mass means stronger attraction and less buoyancy (they fall faster), but the effect is negligible in most cases.

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Suppose you pick two people at random. From one, you pluck a single hair from their head. Is it possible to tell who had the hair plucked by weighing the people? Technically, plucking a hair makes a person very slightly lighter, so you get a tiny bit of information about who had the hair plucked by weighing the people. But the information is very slight ...

2

All the previous answers are correct. Let me add just some mathematics. Take a look at the equation for Fermi normal coordinates, for example in the original article by Misner and Manasse [1] or here. These coordinates provide an example of a local Lorentz frame, that is, a reference frame with a locally flat metric. As you can find in these articles, Fermi ...

2

First of all your statement "because pseudo forces can (locally) be interpreted as gravitational fields and it is therefore impossible for the local experimenter to decide whether he is moving, or being accelerated, or motionless." is incorrect. I will paraphrase MTW's 'Gravitation', section 13.6, page 327: We have a very small man inside a very small, ...

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I'd like to answer as far as buckets are concerned but leave the CMBR to a cosmologist or a real relativity-ist. Mopping the floor up after the chaos left by my children, I think of myself as an expert on the former! In GR it is immaterial whether one describes a "force" as an "inertial force" or a gravitational field. All one "knows" is whether one is ...

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GR treats all free-fall reference frames as equivalent. Also, a reference frame is a local thing, as you said. A rotating bucket filled with water is a non-local thing. Anyway, you can go down a very deep rabbit hole by looking up Mach's principle, but I'm not sure I would advise this (I think it may be antiquated). Finally, assuming the universe is ...

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The moment of inertia tensor is not constant in the external reference frame (http://en.wikipedia.org/wiki/Precession#Torque-free )

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Great question; I remember being so confused by this when I first took analytic mechanics. The components of the angular velocity "in the body frame" aren't zero because when one writes these components, one isn't referring to measurements of the motions of the particles in the body frame (because, of course, the particles are stationary in this frame). ...

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Pebble will not move according to a inertial frame outside the disk which is rest with respect to ground,Since looking from this inertial frame,there is no horizontal force acting on pebble because of frictionless.Hence accoring to newton's $2^{nd}$ law pebble will stay in its state according to a observer from ground. But for an observer rotating along ...

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A simple answer: motion is relative thus, there is always an implied reference. In other words, if you read or hear the phrase "X is stationary", you should immediately think "stationary with respect to what?". Most often, it is the case that an unqualified "stationary" means "at rest with respect to the measuring apparatus" or "with respect to the lab". ...

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This term is used all the time in introductory classical physics. In that context, stationary usually means not moving in the laboratory frame. Thus, a block sitting on a table not doing much would be referred to as being stationary. If one studies relative motion, then stationary could mean not moving in whatever frame you're discussing.

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In that case the pebble won't move. If there is no friction, there won't be any forces between the pebble and the disk.

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Let's assume that this whole setup is being viewed from an inertial frame and that if there is gravity, then it points perpendicular to the plane of the disk, then The disk will slide under the pebble, and the pebble will stay where it is. Why? Well in an inertial frame, Newton's second law holds. Since the force on the pebble tangent to the surface of ...

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why we always choose the center of gravity of the bicycle be the rotational center. We do not do that always, sometimes it is better to use the point in contact with the ground or some other point. We use center of mass when it leads to simpler equations than the other points. In problems dealing with torques or rotations we use the theorem T: the sum ...

2

What if without meeting they send a light pulse to each other, such that they can know each other's age The result will still be the same - each twin judges the other twin to be ageing more slowly than themselves. However, sending a light pulse to each other involves other factors that must be taken into account such as time of flight and ...

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How do we prove that any directions are orthogonal? [...] we can use the pythagorean theorem. This involves of course a definition of (how to measure or compare) "angle(s)" in the first place; such that one may comprehend statements about (distinct) angles being "equal" (or else: "not equal") for instance in Euclid's 4th axiom (on "right angles") or in ...

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If the twins never meet, but just continue travelling in a straight line at constant velocity then each twin will see the other as being younger. The *paradox*$^1$ only occurs if one or both of the twins is accelerated, which of course is necessary for the twins to meet again. $^1$ it's not a paradox of course, just an unintuitive result!

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