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161 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, ...
N. Virgo's user avatar
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66 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 ...
silverrahul's user avatar
  • 4,466
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 ...
guest's user avatar
  • 491
53 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 ...
John Rennie's user avatar
42 votes
Accepted

If gravity is not a force, then how come gravitational assists work?

Well, gravity is a force and it isn't. What is a force anyway? It's what makes you accelerate, which is already a statement about a second-order derivative of one variable with respect to another, and ...
J.G.'s user avatar
  • 25k
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 ...
John Rennie's user avatar
36 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 ...
Arthur's user avatar
  • 2,978
31 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 ...
BioPhysicist's user avatar
  • 57.2k
30 votes
Accepted

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 ...
Dale's user avatar
  • 103k
28 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 &...
Steeven's user avatar
  • 51.4k
26 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'...
Selene Routley's user avatar
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 ...
MannyC's user avatar
  • 6,796
22 votes

Is quantum gravity research implying that gravity is actually a force and not spacetime curvature according to GR?

First, the concept of "force" is not as easy as it might seems. It is inherited from Newtonian mechanics in which it is defined as a vector sourcing the motion of the particles. In a sense, ...
Léo Vacher's user avatar
  • 1,143
22 votes

Is quantum gravity research implying that gravity is actually a force and not spacetime curvature according to GR?

Disclaimer: I am mainly a relativist, and I think science has benefitted much more by thinking of gravity as a geometrical entity than as a "force". It is impossible to answer this question ...
Níckolas Alves's user avatar
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 ...
CR Drost's user avatar
  • 38.3k
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 ...
IanF1's user avatar
  • 745
21 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 ...
GiorgioP-DoomsdayClockIsAt-90's user avatar
19 votes

Why don't two accelerated clocks behave like two clocks in a gravitational field?

Acceleration in special relativity works a bit differently from the Newtonian version. It takes a while to build up a new set of intuitions. A uniformly accelerating particle moves along a hyperbola ...
Nullius in Verba's user avatar
18 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 ...
knzhou's user avatar
  • 103k
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 ...
John Rennie's user avatar
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 ...
BioPhysicist's user avatar
  • 57.2k
17 votes
Accepted

Does the equivalence principle only apply for the gravitational field of an infinitely sized body?

You have noted something really important, but there is a caveat in the formulation of the equivalence principle that avoids your remark. The equivalence principle states that locally a gravitational ...
Níckolas Alves's user avatar
16 votes
Accepted

Test for inertial frame of reference

The falling elevator's frame is inertial! Congratulations, you've found Einstein's equivalence principle. In relativity, we have to consider the proper acceleration, which is a 4-vector. Gravitational ...
Logan J. Fisher's user avatar
16 votes

Why does acceleration in special relativity give rise to general relativity (and thus gravity)?

If we include accelerated motion in special relativity, the result is general relativity. This is simply not true. Special relativity can easily handle accelerated motion on its own. The result of ...
Dale's user avatar
  • 103k
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 ...
Steeven's user avatar
  • 51.4k
15 votes

Is the gravitational force a phenomenon or a theoretical invention?

how is it possible for a phenomenon to be replaced by a theory? Theories don’t replace phenomena, they explain them. Newton’s law of gravitation and general relativity both explain the same phenomena....
Dale's user avatar
  • 103k
15 votes
Accepted

What does it mean that a falling mass in space doesn't sense any force?

If you've ever been in an airplane taking off or a car taking a corner fast you'll know that you can feel the acceleration. This is commonly referred to as the g-force. In general relativity this ...
John Rennie's user avatar
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 ...
Javier's user avatar
  • 28.3k
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 ...
knzhou's user avatar
  • 103k

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