How to tell if an object is moving

  • Thread starter Yoghurt114
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In summary, to determine if an object is moving, you must have a frame of reference. One way to determine if an object is moving relative to you is by looking at its parallax or the doppler shift of light emitted from the object. For objects at close range, a radar or laser range finder can also be used. However, if an object does not show any doppler shift in any direction, it does not necessarily mean it is not moving. This is because there is no absolute frame of reference in space. The closest thing to a frame of reference would be the cosmic microwave background.
  • #1
Yoghurt114
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Simple question, to which I have no answer.

Say, you encounter an object in space. How can you tell if it's moving, if you don't know your own location or speed, and you don't have anything you can use to navigate by or determine your location.

Asked out of pure interest.
Thanks.
 
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  • #2
You cannot. First it is critical to understand that there is no absolute frame of reference, that is, you cannot simply say an object is "moving", but you must specify what is it moving relative to. If you'd like to know if an object is moving relative to you, well then that's quite simple. One way is to look at its parallax, i.e. is its position changing relative to some very distant objects (stars or quasars or something like that). Another way would perhaps be to look for any kind of doppler shift coming from any light emitted from the object.
 
  • #3
The doppler shift is the answer I was looking for. Thanks.

I was under the assumption the doppler shift only applies to ambulances and police cars ;>

Never realized it also applies to electromagnetic waves, but it of course makes perfect sense.
 
  • #4
Doppler shift on incoming light if the object is something like a star or planet or nebula and very far away. If it is something like a spaceship at close range, then normal radar or a laser range finder would work. And at a close enough distance you could see it moving relative to you.
 
  • #5
By the way. If the object does not seem to have any doppler shift of EM waves in any direction. Wouldn't it be safe to assume it is not moving? That is. It is not moving relative to ... space?
 
  • #6
Yoghurt114 said:
By the way. If the object does not seem to have any doppler shift of EM waves in any direction. Wouldn't it be safe to assume it is not moving? That is. It is not moving relative to ... space?

The object would only have no doppler shift from YOUR frame. I could be in another frame traveling away from it and it would have doppler shift for me. There is NOT a frame for space itself. The closest thing you could say would be to take a frame of the cosmic microwave background.
 

FAQ: How to tell if an object is moving

1. How do you determine if an object is moving?

The most common way to determine if an object is moving is by observing its change in position over time. If the object is changing its position relative to a fixed point or reference frame, then it is considered to be in motion.

2. Can an object be moving if it appears to be still?

Yes, an object can still be in motion even if it appears to be still. This is because motion is relative and depends on the observer's frame of reference. For example, a person sitting in a train appears to be still to someone outside the train, but is actually in motion relative to the train's frame of reference.

3. How do you calculate an object's speed?

An object's speed can be calculated by dividing the distance it has traveled by the amount of time it took to travel that distance. This is known as the average speed. Instantaneous speed, or the speed at a specific moment in time, can be calculated by measuring the object's displacement over a very short period of time.

4. What is the difference between speed and velocity?

Speed is the rate at which an object is moving, while velocity is the rate at which an object is moving in a specific direction. In other words, velocity takes into account both the speed and direction of an object's motion.

5. How does acceleration affect an object's motion?

Acceleration is the rate at which an object's velocity changes over time. If an object experiences acceleration, it means that its speed, direction, or both are changing. This can cause the object to speed up, slow down, or change direction.

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