# Tag Info

### Why would black hole rip me apart?

You don't need GR to see this effect. It's due to tidal forces. Suppose you are 2 meters tall. Then the force of the Earth on your feet is $GMm/r^2$, and the force on your head is $GMm/(r+2)^2$. The ...
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### Why would black hole rip me apart?

The problem is that when you are falling, all of you can't be in the same inertial frame. That is, whilst your centre of mass might be inertial, parts of your body will be feeling accelerating forces ...
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### How could any frame of reference be inertial?

In newtonian mechanics, inertial frames are an equivalence class. They can be defined as frames where Newton's laws are valid. If you can find one inertial frame, then you automatically get an ...
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### Dependence (or lack thereof) of forces on frames of reference

When you say "blocks A and B move with a relative acceleration of -3 m/s2" you are considering the motion of one block in the reference frame attached to the other block (block C). But the ...
• 6,270
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### Riemann curvature tensor in an inertial frame

The fact that a function's first derivative vanishes at a point does not mean that its second derivative vanishes at that point. Note that for $f(x)=x^2$, $f'(0)=0$ but $f''(0)=2$.
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### Are Newton's laws just definitions?

From a mathematical point of view, they are definitions: they relate mathematical abstractions. But from a physical point of view, they are not definitions: they capture real behavior of real physical ...
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1 vote

### Simple resolution to the twin paradox?

The calculation you performed clearly shows that distance depends on the observer. This does not resolve the twin paradox since if Alice does the math instead of Bob's perspective then you would get ...
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1 vote
Accepted

### Are Newton's laws just definitions?

Your statements starting with "in an inertial reference frame" are indeed trivially true, because of the definition of the inertial frame: inertial frame is a frame where the first and the ...
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1 vote

### Are Newton's laws just definitions?

Newton's first law is usually taken as the definition of an inertial frame. Hence it is not a special case of the second law, and resolves your concern of the second bullet point regarding the second ...
1 vote

### Is angular momentum conservation Galilean invariant?

But this doesn't make sense to me because the rotational invariance of a system doesn't seem to change when I change to a new inertial reference frame. It can change. Torque depends on origin. As an ...
• 415
1 vote
Accepted

### FTL travel without time travel (again)

It seems to me we can change the thought experiment so that it is real FTL travel without time travel by slowing the FTL speed to less fast but still FTL speed I think this answer does a good job of ...
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