82
votes
Accepted
Why is Voyager 1 approaching Earth?
Voyager $1$ is headed away from the Sun at around $17$ km per second at an angle to the ecliptic of around $35$º.
The orbital velocity of the Earth is $29.8$ km per second, and multiplying by $\cos ...
- 358k
54
votes
Accepted
If absolute velocity does not exist, how can we say a rocket accelerates in empty space?
Within the context of Newtonian mechanics, there's a simple answer: velocities are not absolute, but differences in velocities are. So you can state that acceleration occurs unambiguously.
In ...
- 103k
49
votes
Accepted
Is there any physical evidence for motion?
According to classical physics: no. It is impossible to tell how fast something is moving from a snapshot.
According to special relativity: yes. If we choose a frame of reference where one of the ...
41
votes
How can any speed be defined as a constant?
Speed of light is actually a pretty special case compared to how we typically think of speed (as far as I understand it).
Movement is always relative to some frame of reference. In the case of a ...
- 15.3k
39
votes
Does Special Relativity Imply Multiple Realities?
I'm not going to really touch what one could mean by "multiple realities", but I think this can still be useful.
Let's say we are facing each other. I see a building to my left. You see a building to ...
- 57.2k
33
votes
Flying a drone in a moving car/airplane
Very good question! The point is that when the elevator begins to move (either upwards or downwards), it's accelerating, while the drone -- having no force acting on it directly -- is still moving at ...
- 11.4k
31
votes
Is there any physical evidence for motion?
If we could take a snapshot of both tennis balls, would there be any evidence that could suggest that one is moving and the other one is still?
We can't. Problem solved.
Well, almost problem solved....
- 50.2k
27
votes
Is it possible to stay up while riding a bike on a moving sidewalk without actually moving?
What keeps a bicycle up is a variety of things, but it all comes down to the front wheel, which can move left/right. The bike is always out of balance, and if it starts to fall to the left you ...
- 17.1k
25
votes
If absolute velocity does not exist, how can we say a rocket accelerates in empty space?
A rocket's thrusters function by ejecting reaction mass (exhaust). You can measure the movement of the rocket by its distance from its reaction mass. The rocket moves relative to its reactant.
You ...
- 932
21
votes
Can we use redshift measurements to determine absolute velocity?
A red or blue shift is created when the light source is moving relative to the detector. In your thought experiment, you emit light and you receive it, so there is no red or blue shift. For your idea ...
- 12.7k
21
votes
If absolute velocity does not exist, how can we say a rocket accelerates in empty space?
how can we say a rocket accelerates in empty space ?
According to third Newton law, body in a rocket will experience pseudo-force with direction opposite to that of rocket acceleration. That is - ...
20
votes
Is it possible to stay up while riding a bike on a moving sidewalk without actually moving?
By the principle of relativity, you will not fall over – assuming that you know how to use the bike and you won't be deliberately "confused".
The principle says that the laws of physics have the same ...
- 180k
19
votes
Accepted
What's the speed of light when viewed from the side?
The speed of light is the same in every direction; specifically, $c=299,792,458$ m/s. This fact was proven by Michelson and Morley in 1887, when they measured the speed of light very precisely using ...
- 35.6k
18
votes
Accepted
How can different points on a rigid body move with different speeds but also be relatively at rest?
Having a non-zero relative velocity is fine as long as the distance between the points isn't changing. This certainly holds for a rotating rigid body. As another example, take a ball on a string and ...
- 57.2k
18
votes
Do released objects take the direction and speed of their parent frame's velocity, or just the parent frame's speed component?
This conforms to my belief that, in classical mechanics, ignoring gravity and wind resistance, an object released from a parent frame starts moving with only its parent frame's speed -- not its parent ...
- 103k
17
votes
Is electric current relative?
Charge density $\rho$ and current density $\vec{J}$ form a Lorentz four-vector $(c\rho, \vec{J})$ that transforms under a Lorentz transformation just like $(ct, \vec{r})$ does.
For example, if two ...
- 51.7k
16
votes
How can any speed be defined as a constant?
My question is: If speed is not an entity in itself, but only
dependent on other constant factors, how can the speed of anything
(let alone light) be a constant? Am I completely missing something
...
- 11.7k
15
votes
Accepted
How can you accelerate without moving?
In relativity (both flavours) we consider trajectories in four dimensional spacetime, and acceleration is a four-vector not a three-vector as in Newtonian mechanics. We call this four-acceleration ...
- 358k
14
votes
Why do bodies traveling at constant velocity experience the same physics?
The whole point is that this is an observation that is made -- if you're on a windowless train moving on a smooth track, you don't really know you're moving at all.
Similarly, for all of newtonian ...
- 41.6k
14
votes
What is the essential difference between constant speed and acceleration?
The difference is that acceleration is absolute whereas velocity is relative. In other words, the local laws of physics are exactly the same for two objects moving with a constant velocity relative to ...
- 56.4k
13
votes
How can any speed be defined as a constant?
It is counter intuitive, this question or variants of it get asked a lot. Surely if you're travelling towards a beam of light it will appear to be travelling faster? The answer is it won't, every ...
- 6,963
13
votes
What is the "true" distance an object travels based on relative speeds?
To specify the distance an object has travelled, you need to also specify its position relative to some initial reference point. In the context of your question, there is no "true distance" ...
- 29.9k
12
votes
Question about spinning Earth
The speed of Earth's rotation is 1674 km/h on the equator. The surface, the airport, the atmosphere and the plane (before taking off) all move with 1674km/h relative to the centre of earth. They all ...
- 221
12
votes
What's the speed of light when viewed from the side?
In this case the speed would be same for anything sent from A to B, as seen from C. Since there is no relative velocity between the points A, B and C, the magnitude of the speed of any object going ...
- 1,624
12
votes
Is there any physical evidence for motion?
Cylinders Don't Exist
If I show you a picture of two round objects and tell you that one is a sphere and the other is a cylinder you are looking at head-on, how can you tell whether I am telling the ...
- 2,602
12
votes
Does Special Relativity Imply Multiple Realities?
Relax, take a deep breath :-) to me it seems that you do not realise that Alice and Bob have two different times. That is, what Alice calls time, Bob calls a mixture of space and time. So, If you ...
- 2,155
12
votes
Flying a drone in a moving car/airplane
A drone does not always fly with respects to the air in a moving car/airplane. It does so only if they are not accelerating. The elevator is accelerating in your video.
- 1,841
12
votes
Accepted
How should I understand the idea of relativity of space?
Yes, you can take a random point as your origin, but how do you know which random point you have picked? Imagine you are floating alone in empty space with nothing for a thousand km in any direction. ...
- 26.4k
Only top scored, non community-wiki answers of a minimum length are eligible