162 votes

If I drop a ball in an accelerating rocket, will it bounce? If so, how?

This is one of those things that should become clear once you see it, so I made an animation: As you can see, the ball simply bounces off the back of the rocket once the rocket catches up with it, ...
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  • 32.6k
76 votes

Why does a free-falling body experience no force despite accelerating?

Before telling you why an observer in free fall does not feel any force acting on him, there are a couple of results that should be introduced to you. Newton's second law is only valid in inertial ...
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  • 2,912
64 votes
Accepted

Why did we expect gravitational mass and inertial mass to be different?

"isn't there just one property called m and it just appears in different equations (e.g. Newton's second law and the law of gravitation)? In a similar way that (say) frequency appears in many ...
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  • 4,383
57 votes

Is floating in space similar to falling under gravity?

In essence, yes. Being on a space station in orbit basically IS falling due to gravity, it's just that the astronaut and the space station keep missing the Earth due to constantly moving sideways so ...
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  • 491
50 votes

If gravity is a pseudoforce in general relativity, then why is a graviton necessary?

While it's common to describe gravity as a fictitious force we should be cautious about the use of the adjective fictitious as this is a technical term meaning the gravitational force is not ...
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39 votes

Does a person inside a falling bus fall to the front of it?

The bus experiences considerable drag, and will therefore fall more slowly than a person inside the bus. The scenario is possible in principle - but after carefully viewing the clip and doing some ...
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  • 117k
38 votes
Accepted

Why can’t gravitons distinguish gravity and inertial acceleration?

Gravitons do not mediate the gravitational force and you cannot detect gravitons flashing to and fro between objects interacting gravitationally. Since you cannot detect the gravitons you cannot use ...
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35 votes
Accepted

Why does a free-falling body experience no force despite accelerating?

It is incorrect to link the feeling of being accelerated to being accelerated itself. You can be under constant velocity or be continuously accelerated, yet you need not feel anything at all. Let me ...
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  • 2,069
35 votes

Why did we expect gravitational mass and inertial mass to be different?

Objects have a property called "electric charge". This electric charge decides how strong a force they feel when close to other electrically charged objects. The electric charge of an object ...
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  • 2,720
30 votes
Accepted

Is the elevator analogy of the equivalence principle really true?

The point of the thought experiment isn't to say that the elevator can accelerate forever. The point is that acceleration is indistinguishable from being in a gravitational field. The acceleration ...
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  • 53.8k
30 votes
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Flywheel half-way through the event horizon of a black hole vs the equivalence principle

since classically no object can escape the black hole once it passes the event horizon, it seems as though the flywheel should break as it passes through the event horizon, because for every piece ...
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  • 68.8k
26 votes

Why don't you feel gravity the same way you feel a car's acceleration?

They are both exactly the same and feel exactly the same. In fact, they don't feel like anything. Gravitational force and centrifugal "force" or other inertial "forces" cannot be &...
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  • 47.2k
25 votes

If I drop a ball in an accelerating rocket, will it bounce? If so, how?

The ball will bounce exactly as it would on the surface of a planet with local gravitational acceleration equal to the rocket's acceleration. The physics really does play out exactly as in Einstein'...
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24 votes
Accepted

In circular motion, with a constant distance, why does the mass of the orbitting object have no effect on its revolution at all?

But can someone please explain why this is without using pure algebra? I will try without a single formula. In Newtonian gravity, the gravitational force on a particle is proportional to the ...
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24 votes
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Does a person inside a falling bus fall to the front of it?

If the bus was in a vacuum (both inside and outside), then the passenger would float. However, the effects of air resistance on the two objects (passenger and bus) are probably not negligible in such ...
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24 votes

If gravity is a pseudoforce in general relativity, then why is a graviton necessary?

Gravity is not equivalent to an accelerated frame. It's locally equivalent to an accelerated frame. That means that a point-like observer will never be able to tell whether he/she is in a ...
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  • 6,187
22 votes
Accepted

If centrifugal forces are fictitious, then isn't gravitational force also fictitious?

The best way to avoid this kind of confusion is to start from the beginning in a purely Newtonian description of the motion, i.e., working in an inertial frame. Only after understanding the situation ...
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  • 24.8k
21 votes

Some confusion regarding the equivalence principle

Clearly, it will experience a torque due to its non-uniform mass if the room is in a uniform gravitational field. This should not be clear to you, because it's not true. I apologize if the next ...
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  • 35.1k
21 votes

If I drop a ball in an accelerating rocket, will it bounce? If so, how?

Let's say both the rocket and the ball start at zero velocity and the rocket accelerates at a constant rate. The ball starts at some distance $s$ from the floor. In the time it takes for the rocket ...
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  • 745
17 votes
Accepted

Gravity is curved geometry: A fact of nature or model-dependent interpretation?

The interpretation of gravity as curvature of spacetime is model-dependent. You already mentioned the teleparallel equivalent of general relativity, modelling gravity by torsion. Another possibility ...
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  • 12.7k
17 votes

When objects fall along geodesic paths of curved space-time, why is there no force acting on them?

Suppose you and I start on the equator, a kilometre apart, and we both head exactly due North in a straight line, so we head off in exactly parallel directions: Now we know that in Euclidean geometry ...
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17 votes

Gravitational shielding and equivalence principle

The Wikipedia article refers to the paper General Theory of Relativity: Will it survive the next decade? by Orfeu Bertolami, Jorge Paramos and Slava G. Turyshev. In that paper gravitational shielding ...
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17 votes

Is quantizing acceleration equivalent to quantizing gravity?

No, that betrays a misunderstanding of what "quantizing" means. The usual definition of quantization is the conversion of continuous values to discrete values, but that's not what it means in ...
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  • 95.5k
17 votes

Difference between Free Fall and Constant Velocity

The feeling of weight is just the feeling of "something" pushing on you. For example, stand in an elevator accelerating upwards, and you will feel heavier. Stand in an elevator accelerating ...
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  • 53.8k
15 votes

If centrifugal forces are fictitious, then isn't gravitational force also fictitious?

You are mixing together centrifugal with centripetal. There is no such thing as a centrifugal force, correct. Rather the centrifugal effect is the tendency to appear to fly outwards in the circular ...
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  • 47.2k
14 votes

Why do we say "Spacetime Curvature is Gravity"?

No, we should not say that Christoffel symbols are gravity. The big reason, which really should be enough, is that they are coordinate dependent. One of the main tenets of General Relativity is that ...
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  • 26k
14 votes
Accepted

How does the Equivalence principle explain "what goes up must come down"?

Clearly the apple's reference frame is non-inertial when it is on its way up, but it becomes an inertial frame when it starts free-falling downward. In Newtonian mechanics, an object is said to be ...
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  • 95.5k
14 votes

Suppose you are in a closed box which is in motion

You can't. This is a literal textbook question about Galileo's ship. I cannot understand why this seems not to be taught in schools, the idea is nearly 400 years old, and the description of it is ...
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  • 1,897
13 votes

Why can’t gravitons distinguish gravity and inertial acceleration?

Firstly, pure General Relativity theory doesn't have gravitons, it just has spacetime curvature. Gravitons are a quantum particle, and GR isn't a quantum theory. Hopefully, some kind of Quantum ...
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  • 10.1k

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