162

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, just like a tennis ball bouncing off the racket during a serve. In the comoving frame (i.e. if we are accelerating along with the rocket), this amounts to the ...


72

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 frames of reference: To measure quantities like the position, velocity, and acceleration of an object, you need a coordinate system $(x,y,z,t)$. Now the ...


62

"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 different equations." There IS indeed just one property called m which appears in both the equations. The point is that there is no intuitive reason why this ...


56

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 they never hit the/fall on the Earth. But they basically ARE falling. Our bodies can't tell the difference, because all your body parts are accelerating and ...


49

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 fundamental but is the result of an underlying property. The force itself most certainly exists as anyone who has been sat on by an elephant can attest. There is a ...


39

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 calculations, I believe that the details are inaccurate. Assume the bus has a mass of 5000 kg (pretty light for a bus), and is 3 m wide by 3 m tall - so the ...


37

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 said gravitons to find out whether acceleration is inertial or gravitational. It is often said that forces are due to the exchange of virtual particles, for ...


36

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 explain. The reason you feel compressed or stretched when you are accelerated in a lift is because of the presence of the normal force from the ground on you. ...


33

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 is more or less independent of inertial mass. So given a large, fixed, electrically charged object, you can make a small electrically charged test object feel ...


30

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 doesn't have to exist for all time. However, just to address another issue... It's limited by the speed of light. So at some point or another the elevator will ...


26

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 "felt" at all. Because (in the reference frames where they appear) they act on every single particle in your body at the same time. When you jump from ...


25

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 an instance. The bus will be moving relative to the outside air, and so will be accelerating towards the ground at a rate less than $g$. If we then released ...


25

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's accelerating rocket thought experiment, and not even bouncing balls will tell the accelerating frame apart from a planet's surface for you in this regard. I ...


25

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 gravitational field or in an accelerating spaceship. But an observer that has some characteristic size will experience tidal forces. Tidal forces are a result of a non zero ...


24

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 particle's gravitational mass; the more gravitational mass, the more the gravitational force. In Newtonian mechanics, the acceleration of a particle, for a given ...


22

In 1921 Einstein gave a series of lectures in Princeton, that you can read today under the title "The Meaning of Relativity". It is an early and very special description of General Relativity, where he emphasizes much the concepts and reasoning that lead him to the theory. Nobody is able to know what was really inside Einstein's mind, but in that lectures ...


21

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 section beats you over the head with mathematics, I want to show you why Newton's laws don't say that, and then I want to give you some immediate physical insight ...


21

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 to travel $s$ (the first bounce), it will have accelerated to a certain speed, $v$ let's say. Assuming a perfectly elastic collision, the ball (which was ...


17

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 are bi-metric theories, where the metric is a more ordinary field on a fixed background (this should be more in line with how string theorists tend to think of ...


17

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 is discussed in section 3.4 on pages 16–17. Rather than consider specific mechanisms the article discusses the general possibility that matter itself screens ...


17

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 downwards, and you will feel lighter. In both scenarios you describe it is the case that there is nothing pushing on you to cause your acceleration. On the ISS ...


16

The fact that gravitational field can be simulated/canceled by inertial forces relies upon the following elementary but fundamental fact. The gravitational coupling constant of a given body, i.e. its gravitational mass,$M$, coincides with the other universal constant associated with that body, appearing in the general law of motion, i.e. the inertial mass $...


16

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 parallel lines remain the same distance apart. But if you and I measure the distance, $d$, between us we find that $d$ starts off at 1km but decreases as we ...


16

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 physics. In physics, quantization is the process of turning a classical theory into a quantum one, which is a far more subtle process. And once you do have a quantum ...


14

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 coordinates don't matter. Everything physical must be expressible in a coordinate independent and/or tensorial manner. As I said in the comments, personally I ...


14

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 only a couple of hundred words.


13

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 free-falling if the only force on it is gravity, and the frame of an object is noninertial if the object is accelerating. So an object thrown upward is always ...


13

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 Gravity theory will unite GR & quantum field theory, but we don't have a successful QG theory yet. So we don't know if gravitons even exist, but considering how ...


12

Yes, it can be. In classical mechanics the principle is nothing but the statement that inertial and gravitational masses are identical. As a consequence all inertial forces can be mathematically interpreted as gravitational forces using the standard mathematical machinery of Newtonian mechanics. Your solution is correct.


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