Is it possible to tell whether ship is moving or not?

[A.T.]

If the fly crashes, then the bus is moving with uniform speed.

Or the fly was moving, and the bus was still.

 

[Nick O]

So you and the remote are inside the bus, and you are flying a reference point from outside into the bus. That will indeed tell you whether or not the bus is moving with respect to the reference point, but it won't tell you which of the two is moving with respect to the earth.

 

[A.T.]

I can know that it is still, because I have the remote.

How does your remote determine if the fly is "still"?

 

[Meson080]

How do you know that the bus is moving, and not the point with respect to which the fly is motionless? The bus might be perfectly still with the fly's reference point moving at a constant velocity.

I am the one who has made the fly stand still using my remote. So, I know that bus is moving and not the fly.

 

[Meson080]

What we are trying to say is that you can't know that the fly actually comes to rest. And if you could know, then it would be exactly as if you had a window to see whether external reference points were moving, because the fly would be synchronized with a known external reference.

What happens if it is synchronized?

 

[Nick O]

When you press the "stop" button, I'm assuming that it becomes motionless with respect to its original reference point. Now suppose it was originally on a tree, so the tree was its point of reference. Now suppose that before you activated the fly, the tree was cut down and loaded onto a truck, which in turn accelerated to 50mph south. Now when start the fly, it has an initial "zero" velocity of 50mph south with respect to the earth. When you tell the fly to "stand still" in the bus, it would resume its original velocity of 50mph south.

 

[Nick O]

What happens if it synchronized?

Nothing remarkable, but then you are no longer in a closed system with no outside communication.

 

[Meson080]

Are you moving at the same velocity as the bus? If so, this tells you nothing. If not, then why do you need the fly?

As I am in contact with the bus. So, yes I am moving at the same velocity as the bus. Thus, I require the fly.

 

[Meson080]

Right, your eyes will tell you whether it is the bus or the fly that is moving relative to you. But how do you know whether you are still or moving? How would you convince someone else who happens to be nearby, also watching the bus and the fly, but moving at the same speed relative to you as the bus, that it is him and the bus that is moving and not you and the fly?

I am inside the bus in contact with it, and I have the fly in control, so as explained before I can know whether I am still or not.

Sorry, its not clear to reply for the orange part. Can you elaborate?

 

[Meson080]

Okay, what we need are scenarios. Tell me where these are wrong, and why.

Scenario 1: Bus moving at 100mph north.

The fly enters bus and comes to rest w.r.t. the bus. Therefore, the bus and fly are now moving 100mph north. The fly appears motionless, so you conclude that the bus is not moving.

Scenario 1: Bus motionless.

The fly enters bus and comes to rest w.r.t. the bus. Therefore, the bus and fly are now motionless. The fly appears motionless, so you conclude that the bus is not moving.

Scenario 1: No, I am not making the fly to move at 100 mph north. In my remote, I can read the speed of the fly to be 0 towards north and it must stand still according to my remote controller. But, the fly crashes. So, I can say the bus is moving.

Scenario 2: Yes, you are correct.

In the above cases the fly is not in contact with the bus and the experiment is conducted in vaccum.

 

[Meson080]

The bus is drifting at uniform speed. I am performing the experiment in it by making the insect stay at a particular point inside the bus (but not physically in contact). What's the problem?

How do you make the insect stay at a particular point inside the bus? It's not as easy as it seems.

My response to scenario 1 in the previous post might help you.

 

[A.T.]

I am the one who has made the fly stand still using my remote.

Well, I guess all the world problems are solved then...

Absolute rest? Just push STOP on the remote.

Faster than light? Crank the remote up to 11.

It reminds me of this at 2:30min:

https://www.youtube.com/watch?v=SJxcQt6nCC0

 

[DrStupid]

I can read the speed of the fly to be 0 towards north and it must stand still according to my remote controller.

That just postpones the problem. How does your remote controller know the speed of the fly?

 

[Meson080]

If the fly crashes, then the bus is moving with uniform speed.

Or the fly was moving, and the bus was still.

My response to scenario 1 might help you.

 

[Meson080]

I can read the speed of the fly to be 0 towards north and it must stand still according to my remote controller.

That just postpones the problem. How does your remote controller know the speed of the fly?

How does the speedometer in your bike say your bike's speed?

 

[Meson080]

So you and the remote are inside the bus, and you are flying a reference point from outside into the bus. That will indeed tell you whether or not the bus is moving with respect to the reference point, but it won't tell you which of the two is moving with respect to the earth.

Yes, I and my remote are inside the bus. Robotic fly will say me whether bus is moving or not.

 

[A.T.]

How does the speedometer in your bike say your bike's speed?

You think that your bike speedometer will work, when the bike floats in vacuum?

 

[Meson080]

You think that your bike speedometer will work, when the bike floats in vacuum?

I don't think Drstudpid's bike speedometer is going to work. But, my speedometer which says the speed of fly is surely going to work. Why what's the problem?

 

[A.T.]

But, my speedometer which says the speed of fly is surely going to work.

How does your speedometer work?

 

[TheDemx27]

Unless you are accelerating, you cannot tell whether the bus is at rest or not. At a uniform velocity, all of the forces are in equilibrium as they are when at rest. If they fly does fly into the car (perfectly) it will remain stationary with respect to the car as it is suspended within the air inside the car.

I often thought about this situation as well, anecdotally.

 

[DrStupid]

How does the speedometer in your bike say your bike's speed?

It counts the revolution of the wheel relative to the bike and measures the corresponding time. Together with the radius of the wheel this gives the tangential speed of the tire relative to the bike. As long as the bike is rolling on the ground without slip, the result is equal to the speed of the ground relative to the bike and to the speed of the bike relative to the ground respectively. In short: the speedometer measures the speed of the bike relative to the ground.

How does your remote control measures the speed of the fly - especially relative to which reference?

 

[Meson080]

How does your speedometer work?

That's a technical thing. I am not discussing about it.

 

[A.T.]

That's a technical thing. I am not discussing about it.

If you cannot describe your proposed experiment, then there is nothing to discuss at all.

 

[Meson080]

Unless you are accelerating, you cannot tell whether the bus is at rest or not. At a uniform velocity, all of the forces are in equilibrium as they are when at rest. If they fly does fly into the car (perfectly) it will remain stationary with respect to the car as it is suspended within the air inside the car.

I often thought about this situation as well, anecdotally.

Remember I am considering vaccum.

 

[Meson080]

If you cannot describe your proposed experiment, then there is nothing to discuss at all.

Do you think it is impossible to create such a device? Why do you think? I think it is possible.

 

[A.T.]

I think it is possible.

How?

 

[Meson080]

It counts the revolution of the wheel relative to the bike and measures the corresponding time. Together with the radius of the wheel this gives the tangential speed of the tire relative to the bike. As long as the bike is rolling on the ground without slip, the result is equal to the speed of the ground relative to the bike and to the speed of the bike relative to the ground respectively. In short: the speedometer measures the speed of the bike relative to the ground.

How does your remote control measures the speed of the fly - especially relative to which reference?

Thank you for the explanation. I just wanted to say that, as your speedometer works, even my speedometer works. Its a technical aspect. We are dealing with different thread. If you can say anything that such a device is not possible to construct, I can try to explain it and it will be valuable to stay on topic in this thread.

 

[A.T.]

I just wanted to say that, as your speedometer works, even my speedometer works.

If your speedometer works, why don't you attach it to the bus directly, to know if the bus is moving? Why do you need a robotic fly with a remote control?

Its a technical aspect.

It is the key part of your experiment, that you have to define precisely. If you cannot define your proposed experiment, then there is nothing to discuss at all.

 

[Meson080]

How?

I am sure it is possible. Similar to the speedometer in the bike as DrStupid explained, the meter in the fly's body can note down the reading, and then it can send info to the remote either through radiation or anything. It is just my idea that it is possible. Don't consider it seriously. If you want to consider it seriously, please explain why you think it is impossible to develop such a device. So that it makes us to stay on topic by speaking about technical aspects. Thank you.

 

[TheDemx27]

Remember I am considering vaccum.

Still, it would simply be suspended in space with respect to the vehicle.

Being inside a vehicle doesn't change anything. The fly and the car are moving with a constant velocity v. The fly, outside the vehicle, moves into the car. The fly is still moving at the constant velocity v, and it is experiencing no net forces whatsoever. It doesn't move with respect to the vehicle.