Relative Motion

[YT2095]

 

So you agree, compared to the starting line the ball had a velocity of 0 m/s when it was thrown?

 

what do you mean "So you agree"?

I`m the one that`s told YOU this (which you now seem to understand), so I`m glad you Finally agree with me

 

and the fact that you Do precludes the rest of your questioning in that post, because you`ve already demonstrated that you understand this entirely.

 

Game Over

[Motor Daddy]

what do you mean "So you agree"?

I`m the one that`s told YOU this (which you now seem to understand), so I`m glad you Finally agree with me

 

and the fact that you Do precludes the rest of your questioning in that post, because you`ve already demonstrated that you understand this entirely.

 

Game Over

 

Uh, you told me the ball's velocity was 20 m/s in the opposite direction of the train. That means the distance between the ball and the starting line decreased. Which is it? The ball's velocity was 0 m/s, or the ball's velocity was 20 m/s in the opposite direction of the trains travel?

[Klaynos]

If the train's velocity is 20 m/s, as measured from the starting line and timer, what is the ground's velocity moving away from you? Either the ground is traveling at 20 m/s, or the train is traveling 20 m/s. Which is it? Maybe they are both traveling 10 m/s in different directions? If so, why didn't you state such an idea? We already established the distance between the train and the starting line is increasing at the total rate of 20 meters per second.

 

The train is moving at 20m/s in the reference frame of the tracks.

The station is moving at 20m/s in the reference frame of the train.

They are both moving at 10m/s in some other reference frame.

 

All 3 answers are correct, and no one of them is more correct or preferential to the others. This is what we've been trying to show you!

[Motor Daddy]

The train is moving at 20m/s in the reference frame of the tracks.

The station is moving at 20m/s in the reference frame of the train.

They are both moving at 10m/s in some other reference frame.

 

All 3 answers are correct, and no one of them is more correct or preferential to the others. This is what we've been trying to show you!

 

So please, clarify the example of the guy throwing the ball off the back of the train. What is the direction of travel of the ball, and what is the velocity of the ball? What is the velocity of the train, and what is the direction of travel? What is the station's velocity, and which direction?

 

Note, the train's velocity was measured from the starting line. That's the only measure we know for sure, as the tracks are marked, and the timer is activated at the start line and deactivated at the finish line.

 

Please clarify the actual velocities of each and their direction(s) of travel.

[Klaynos]

The ball being thrown off of the back.

 

In the trains rest frame:

 

The ball is travelling at -20m/s.

The platform/tracks are travelling at -20m/s.

 

The platforms rest frame:

 

The ball is travelling at 0m/s.

The train is travelling at 20m/s.

 

To move between frames you need to do Galilean transforms.

 

http://en.wikipedia.org/wiki/Galilean_relativity#Formulation

[Motor Daddy]

The ball being thrown off of the back.

 

In the trains rest frame:

 

The ball is travelling at -20m/s.

The platform/tracks are travelling at -20m/s.

 

The platforms rest frame:

 

The ball is travelling at 0m/s.

The train is travelling at 20m/s.

 

To move between frames you need to do Galilean transforms.

 

http://en.wikipedia.org/wiki/Galilean_relativity#Formulation

 

Just so I am perfectly clear, what is a distance of -20 meters?

[Klaynos]

Just so I am perfectly clear, what is a distance of -20 meters?

 

It's not technically a distance, it's a displacement, which is a vector length quantity, so -20m would be in the opposite direction to a 20m displacement. These are quite fundamental mechanics properties.

[Motor Daddy]

It's not technically a distance, it's a displacement, which is a vector length quantity, so -20m would be in the opposite direction to a 20m displacement. These are quite fundamental mechanics properties.

 

So then -20m/s is not technically a velocity, as velocity is d/t, so if -20 meters is not technically a distance, than -20 m/s can not be a velocity, correct?

 

Also, what is a negative displacement?

[Klaynos]

Velocity is a vector quantity, so 20m/s in a specific direction (this is a single dimensional problem so the direction is just assumed) is a velocity, speed is distance/time. Speed is not a vector but a scalar.

[Motor Daddy]

Velocity is a vector quantity, so 20m/s in a specific direction (this is a single dimensional problem so the direction is just assumed) is a velocity, speed is distance/time. Speed is not a vector but a scalar.

 

I never mentioned the word speed, only velocity.

 

What is a negative distance, and what is a negative displacement?

 

If you consider the train to be stationary from a single reference frame, how could the ball have a negative distance (velocity) from that same reference frame?

 

The direction was never assumed on my part, it was explained in the original example that the ball had a 20 m/s velocity in the opposite direction of the train's travel.

[Klaynos]

I never mentioned the word speed, only velocity.

 

What is a negative distance, and what is a negative displacement.

 

If you consider the train to be stationary from a single reference frame, how could the ball have a negative distance (velocity) from that same reference frame?

 

The direction was never assumed on my part, it was explained in the original example that the ball had a 20 m/s velocity in the opposite direction of the train's travel.

 

I brought up speed to give you an understanding of what velocity is. I've explained velocity, speed, displacement and distance above.

 

Edtharan discussed vectors earlier, do you understand what a vector is?

[Motor Daddy]

I brought up speed to give you an understanding of what velocity is. I've explained velocity, speed, displacement and distance above.

 

Edtharan discussed vectors earlier, do you understand what a vector is?

 

Thanks for the explanations. I have a pretty good handle on what a velocity is, though.

[Klaynos]

Thanks for the explanations. I have a pretty good handle on what a velocity is, though.

 

So you'd be comfortable with a velocity of say:

 

v=4x+7y+3z m/s

 

And from that working out a speed...

[Motor Daddy]

So you'd be comfortable with a velocity of say:

 

v=4x+7y+3z m/s

 

And from that working out a speed...

 

I envision the universe in real time, as a whole. I understand what relative motion is, and I also understand what acceleration and velocity are.

 

There is three things in this universe, mass, distance and time.

 

Standards of measure of mass, distance, and time have already been established for all to measure events of motion (mass, distance, and time).

[Klaynos]

I envision the universe in real time, as a whole. I understand what relative motion is, and I also understand what acceleration and velocity are.

 

There is three things in this universe, mass, distance and time.

 

Standards of measure of mass, distance, and time have already been established for all to measure events of motion (mass, distance, and time).

 

Right, ok, but do you understand what I wrote down? And there's more than 3 things.... but that's off topic.

[D H]

Thanks for the explanations. I have a pretty good handle on what a velocity is, though.

 

I am not so sure you do understand what a velocity is, based on this:

I never mentioned the word speed, only velocity.

 

What is a negative distance, and what is a negative displacement?

 

Think of a vector as something with a magnitude and a direction. It is easiest to start with position or displacement vectors. A pair of points might be separated by a distance of 20 kilometers, but we live in a three dimensional world. Distance alone does not suffice in describing the displacement between points. Suppose you are asked to deliver some vital equipment to a destination 20 kilometers away. Driving 20 kilometers in a random direction most likely will not take you to the destination. You have to drive 20 kilometers and drive in the proper direction.

 

The displacement between two points can be represented as a vector. The distance between the points is the magnitude of the displacement vector. Velocity is also a vectorial quantity. You drive 20 kilometers/hour to the northeast. That 20 kilometers/hour is your speed, not your velocity. The magnitude of a velocity vector is speed.

[Motor Daddy]

Right, ok, but do you understand what I wrote down? And there's more than 3 things.... but that's off topic.

 

First of all, I never claimed to be a mathematician, so if that is what you are driving at, you are correct.

 

However, I understand the concept of being able to measure the change of distances between any two (or more) points at the same time.

 

But if I was looking at the universe as a whole, there would be a specific direction of travel for each and every object I could look at.

 

For instance, You can wear a watch, and calculate every single rotational velocity of any specific point in motion in that watch. You could tell me that each and every point of measure can be relative to each other, but which way are you walking while you are wearing the watch, and are you sure you are traveling in the specific direction that you think you are, and what does that do to all your calculated velocities of each point you already calculated? Kind of makes them obsolete, correct?

 

Our measuring system is a single reference point for each measure taken. It's only limited to the levels of motion you present for observation and measurement.

 

How's that?

[Klaynos]

First of all, I never claimed to be a mathematician, so if that is what you are driving at, you are correct.

 

You said you understood velocity, I wrote down a simple velocity in 3D, if you understood velocities you should have understood that.

 

However, I understand the concept of being able to measure the change of distances between any two (or more) points at the same time.

 

If it's distance you're talking about then it's speed you want and not velocity, velocity is a vector and you need to use displacement.

 

But if I was looking at the universe as a whole, there would be a specific direction of travel for each and every object I could look at.

 

From your rest frame this is correct, but from someone elses rest frame their measurements would be different from yours, this is what we've been trying to explain to you.

 

For instance, You can wear a watch, and calculate every single rotational velocity of any specific point in motion in that watch. You could tell me that each and every point of measure can be relative to each other, but which way are you walking while you are wearing the watch, and are you sure you are traveling in the specific direction that you think you are, and what does that do to all your calculated velocities of each point you already calculated? Kind of makes them obsolete, correct?

 

Urmmm what? I don't really understand what you're asking here...

 

Our measuring system is a single reference point for each measure taken. It's only limited to the levels of motion you present for observation and measurement.

 

You seem to kinda understand reference fames when you write this, when you move between them you measure different things, and in SR and GR you measure completely weird things which we were discussing in the thread this was broken off from as I didn't think you fully understood reference frames.

 

How's that?

 

I think you need to understand about vectors, velocity, displacement and get to grips with relativity classically from the WP article I linked to a little while ago and other wp articles.

 

And then maybe return to some of your SR threads and read the links you where provided there with your new understanding of reference frames and vectors.

[Motor Daddy]

I am not so sure you do understand what a velocity is, based on this:

 

 

Think of a vector as something with a magnitude and a direction. It is easiest to start with position or displacement vectors. A pair of points might be separated by a distance of 20 kilometers, but we live in a three dimensional world. Distance alone does not suffice in describing the displacement between points. Suppose you are asked to deliver some vital equipment to a destination 20 kilometers away. Driving 20 kilometers in a random direction most likely will not take you to the destination. You have to drive 20 kilometers and drive in the proper direction.

 

The displacement between two points can be represented as a vector. The distance between the points is the magnitude of the displacement vector. Velocity is also a vectorial quantity. You drive 20 kilometers/hour to the northeast. That 20 kilometers/hour is your speed, not your velocity. The magnitude of a velocity vector is speed.

 

 

Like I said, I never mentioned speed.

 

A constant velocity is a constant velocity, it doesn't change direction or increase or decrease d/t. If you tell me a ball has a velocity of 20 m/s in a specific direction, does it matter how far away an object is? No. A velocity of 20 m/s means that ball will have traveled 20 meters in one second in the same direction of travel. The trains velocity has ZERO effect on the ball's velocity. If the ball has a velocity of 20 m/s towards the starting line, if the ball is 100 meters away form the starting line, the ball will reach the starting line in 5 seconds, regardless of how fast the train is traveling in the opposite direction. The velocity is the ball's velocity, not the rate of change of distance between the ball and the train traveling a different direction and velocity.

[Cap'n Refsmmat]

If I define a velocity of 20m/s, I have to define the direction that the object is traveling in. Suppose it's going 20m/s to the east. If I say -20m/s, I am specifying that the object is traveling in the opposite direction -- 20 m/s to the west. That's all it means.

 

When you work with vectors in 2D you usually define one direction as positive and one direction as negative. I imagine it gets more complicated in 3D, but it's still the same basic idea.

[Motor Daddy]

If I define a velocity of 20m/s, I have to define the direction that the object is traveling in. Suppose it's going 20m/s to the east. If I say -20m/s, I am specifying that the object is traveling in the opposite direction -- 20 m/s to the west. That's all it means.

 

When you work with vectors in 2D you usually define one direction as positive and one direction as negative. I imagine it gets more complicated in 3D, but it's still the same basic idea.

 

In reality the ball is traveling a specific distance in a specific time. Distance and time are not directional, and must be positive.

[Klaynos]

In reality the ball is traveling a specific distance in a specific time. Distance and time are not directional, and must be positive.

 

But with velocities you must use displacement which is directional not distance.

[Motor Daddy]

But with velocities you must use displacement which is directional not distance.

 

So if the ball was thrown off the back of the train in a direction towards the starting line, and the ball was released at the 100 meter mark away from the starting line, and the ball drops to the tracks on the 100 meter mark, what was its velocity?

[Cap'n Refsmmat]

0m/s.

[Motor Daddy]

0m/s.

 

I agree. So why wasn't that stated as the ball's velocity?