Relative Motion

[ydoaPs]

So when I look at a full moon at night from the Earth, it appears to me to be about 1 foot in diameter. If there was a astronaut on the edge of the left edge of the moon from my perspective, and he travels to the right edge of the moon in 12 hours (my time), did he travel .083 ft/hr?

No. The size of the moon, in your example, is an illusion. You can make an accurate measurement from the same reference frame that disagrees.

 

And likewise, giving a speed means that that speed has a reference point for all to play the game by those rules. You can't switch the reference point and still use that given velocity in the middle of the game.

Hence the numbers change when we change reference frames.

[Motor Daddy]

No. You're measuring his velocity wrong. The moon isn't 1 foot in diameter.

 

Let's say you went to an astronomical observatory and measured the angle between his position in the telescope at the start of the journey and at the end of the journey, then used trigonometry to produce a number for his speed. It would vary greatly from the number his speedometer stated -- because not only is he moving across the moon, but the moon is moving across the sky. In that case, you are both correct about his velocity.

 

 

 

That is indeed a key point.

 

What do you mean I am measuring his velocity wrong, that is my observation form the Earth. You mean, when talking about velocity that there can only be one reference point, and that is the perspective of the measuring tape and stop watch of the actual distance traveled in the actual duration??

 

Now you are switching the rules and telling me as an observer that I am having an illusion??

 

Why do you keep switching the rules? I am an observer, correct??? :rolleyes:

[ydoaPs]

You still have to make accurate observations.

[Motor Daddy]

No. The size of the moon, in your example, is an illusion. You can make an accurate measurement from the same reference frame that disagrees.

 

If it disagrees with the astronauts measurements, I am wrong, PERIOD! The astronaut already measure the distance and time that he traveled.

 

Hence the numbers change when we change reference frames.

 

The numbers don't change, the illusion does.

[Cap'n Refsmmat]

What do you mean I am measuring his velocity wrong, that is my observation form the Earth. You mean, when talking about velocity that there can only be one reference point, and that is the perspective of the measuring tape and stop watch of the actual distance traveled in the actual duration??

 

Now you are switching the rules and telling me as an observer that I am having an illusion??

 

Why do you keep switching the rules? I am an observer, correct??? :rolleyes:

You made an error in your observations. If you try it the way I described, you'll get the same effect with correct observations.

[ydoaPs]

Ok, Motor Daddy, what defines your absolute reference frame?

[Motor Daddy]

You made an error in your observations. If you try it the way I described, you'll get the same effect with correct observations.

 

When the astronaut finishes his travel across the Moon, he will report the actual numbers that he traveled. My observation doesn't change reality.

[Cap'n Refsmmat]

When the astronaut finishes his travel across the Moon, he will report the actual numbers that he traveled. My observation doesn't change reality.

 

He will report that he traveled a certain number of miles in a certain period of time. He is correct. However, your measurements from Earth will reveal that he traveled more miles -- because he was on the Moon, which is moving in an orbit around the Earth. So you will say his speed is greater because he is standing on a moving object. You both have perfectly valid numbers for speed -- his is not the "actual" number.

[Motor Daddy]

Ok, Motor Daddy, what defines your absolute reference frame?

 

The only measurements I can comment on as an observer are the measurements that were taken of the actual event. I can then look at the measurements and see what really happened during the event, AFTER the event has already occurred.

[ydoaPs]

Reality is different in different frames of reference.

 

The only measurements I can comment on as an observer are the measurements that were taken of the actual event. I can then look at the measurements and see what really happened during the event, AFTER the event has already occurred.

 

How many times do I have to ask? RELATIVE TO WHAT?

 

ATFQ

[Motor Daddy]

He will report that he traveled a certain number of miles in a certain period of time. He is correct. However, your measurements from Earth will reveal that he traveled more miles -- because he was on the Moon, which is moving in an orbit around the Earth. So you will say his speed is greater because he is standing on a moving object. You both have perfectly valid numbers for speed -- his is not the "actual" number.

 

The measurements that the astronaut takes are accurate, everything else is an illusion.

 

We are not measuring how far the moon traveled, we are measuring the distance the astronaut traveled on the moon. That's what we are talking about, correct?

 

Reality is different in different frames of reference.

 

 

 

How many times do I have to ask? RELATIVE TO WHAT?

 

ATFQ

 

How tall are you?

[Cap'n Refsmmat]

The measurements that the astronaut takes are accurate, everything else is an illusion.

 

We are not measuring how far the moon traveled, we are measuring the distance the astronaut traveled on the moon. That's what we are talking about, correct?

 

We are measuring the astronaut's velocity.

[ydoaPs]

How tall are you?

In a reference frame at rest to me, 2m. In the reference frame of a space alien zooming across the cosmos, about half a meter.

[Motor Daddy]

We are measuring the astronaut's velocity.

 

 

But the light you are viewing from the astronaut on the moon is old, outdated, obsolete light! It has a delay from the source to you. Your measurements of the event are inaccurate. The measure of the distance traveled is accurate if you count the revolutions of the wheel, and multiply that by the circumference of the wheel for a distance traveled.

 

Acceleration is caused by force, and torque is force times distance (from axis).

 

2 lbs on a handle of a 10 foot bar is 20 lb-ft of torque. 2 lbs on a handle of a 20 ft bar is 40 lb-ft of torque.

 

Force times distance!

 

In a reference frame at rest to me, 2m. In the reference frame of a space alien zooming across the cosmos, about half a meter.

 

So you get smaller if someone else looks at you from a distance?

Edited June 20, 2008 by Motor Daddy
multiple post merged

[ydoaPs]

So you get smaller is someone else looks at you from a distance?

 

No, I get smaller if someone is going really fast(in their frame of reference of course). They, in my frame of reference, are smaller than they are in their frame of reference as well.

[Rakdos]

So you get smaller is someone else looks at you from a distance?

 

The Alien is moving faster then him. Space-time contraction.

[Cap'n Refsmmat]

But the light you are viewing from the astronaut on the moon is old, outdated, obsolete light! It has a delay from the source to you. Your measurements of the event are inaccurate. The measure of the distance traveled is accurate if you count the revolutions of the wheel, and multiply that by the circumference of the wheel for a distance traveled.

Supposing I account for the two seconds it takes for light to reach me from the moon? (Not that it would have any effect at all anyway, as it'll affect all my numbers equally and my answer will end up being the same anyway.)

 

Why do you have to argue with physics that has been accepted and experimentally proven over nearly 100 years when you could just try to understand it?

 

So you get smaller is someone else looks at you from a distance?

 

Length contraction. I posted a link to an explanation of the phenomena before.

[Motor Daddy]

No, I get smaller if someone is going really fast(in their frame of reference of course). They, in my frame of reference, are smaller than they are in their frame of reference as well.

 

So the faster I go, the more you shrink?

 

How tall do you measure yourself if I travel 29,567 MPH past you?

[Cap'n Refsmmat]

So the faster I go, the more you shrink?

 

How tall do you measure yourself if I travel 29,567 MPH past you?

 

2m. But you measure him as a little bit shorter. You're both right.

[Motor Daddy]

The Alien is moving faster then him. Space-time contraction.

 

I'm not asking how fast an alien is traveling, nor do I care, because his speed has nothing to do with my actual height. If he has an illusion of me shrinking, that's his problem, not mine! I didn't shrink just because he went fast.

[ydoaPs]

[math]\Delta{x}'=\Delta{x}\sqrt{1-\frac{v}{c}}[/math]

[Motor Daddy]

2m. But you measure him as a little bit shorter. You're both right.

 

Luckily, I'm smart enough to realize that I am having an illusion, so I disregard my vision, as it is not accurate at 29,000 MPH.

[Cap'n Refsmmat]

Luckily, I'm smart enough to realize that I am having an illusion, so I disregard my vision, as it is not accurate at 29,000 MPH.

It's not an illusion. You're right and he's right. Reality can be different from two different perspectives, and neither perspective is any more correct than the other.

 

Please go read up on special relativity. If you insist on being argumentative and remaining ignorant about relativity (while trying to prove it wrong), I'm going to start closing these threads.

[Motor Daddy]

It's not an illusion. You're right and he's right. Reality can be different from two different perspectives, and neither perspective is any more correct than the other.

 

Please go read up on special relativity. If you insist on being argumentative and remaining ignorant about relativity (while trying to prove it wrong), I'm going to start closing these threads.

 

I'm done, then. Thanks for your time. No need to close threads.

 

I think the point of the game is that you have no idea what your initial velocity is, nor do you have any way of telling.

 

So it's silly to say a train is traveling at 20m/s. That makes the thought experiment inaccurate.

Edited June 22, 2008 by Sayonara³

[Cap'n Refsmmat]

I wasn't talking about the train. I was talking about the rocket ship. You have plenty of reference points to compare the train's velocity against.