Relative Motion

[Cap'n Refsmmat]

No. Klaynos forgot a qualification: accelerating reference frames are different. All constant-velocity frames are equivalent. You're accelerating because of the way the Earth orbits around the Sun, so your frame is not equivalent with the Sun's.

[Motor Daddy]

No. Klaynos forgot a qualification: accelerating reference frames are different. All constant-velocity frames are equivalent. You're accelerating because of the way the Earth orbits around the Sun, so your frame is not equivalent with the Sun's.

 

Does that include a constant rotational velocity?

[Rakdos]

Does that include a constant rotational velocity?

Is a constant rotational velocity accelerating?

[Motor Daddy]

Is a constant rotational velocity accelerating?

 

No. 10 RPM is 10 RPM all day long.

[Klaynos]

Yes sorry, I was dealing with inertia reference frames (non-accelerating).

[Motor Daddy]

Yes sorry, I was dealing with inertia reference frames (non-accelerating).

 

The ball is accelerated (or more correctly, decelerated) if it is "thrown" off the back of the train.

[Klaynos]

The ball is accelerated (or more correctly, decelerated) if it is "thrown" off the back of the train.

 

And whilst it is accelerating the rest frame of the ball is no an inertial rest frame, but we're not talking about that frame at all, we've only discussed the rest frame of the carriage and the platform.

[Motor Daddy]

And whilst it is accelerating the rest frame of the ball is no an inertial rest frame, but we're not talking about that frame at all, we've only discussed the rest frame of the carriage and the platform.

 

So the train is traveling 20 m/s due East. The ball is thrown off the back of the train. How far West does it travel?

[Klaynos]

So the train is traveling 20 m/s due East. The ball is thrown off the back of the train. How far West does it travel?

 

East and west being used are implying the rest frame of the earth.... The question also doesn't give you the information you'd need to answer that.

[Motor Daddy]

East and west being used are implying the rest frame of the earth.... The question also doesn't give you the information you'd need to answer that.

 

Maybe the concept of two cars on the highway are a better image.

 

I am traveling down the road at 60 MPH at a constant velocity. There is a car in front of me also traveling 60 MPH in the same direction of travel. There is 10 feet between the cars. If the car in front of me suddenly hits the brakes and accelerates, how far does he travel in the opposite direction?

[Janus]

All these examples of relative motion are all showing how the same event looks different, depending on the reference. But there is a preferred reference which allows one to better know the relative motion within all the references in terms of energy conservation.

 

For example, someone watching the train will see the train in motion. From the reference inside the train it looks like the country side is in motion. The second reference creates the illusion their reference is stationary even though all experiments will indicate it has kinetic energy. The amount of energy used by the trains engine will never be enough to make the entire country side move. It violate energy conservation if we consider inside the train a valid reference for anything beyond what is going on inside the train. But even that is suspect in terms of reality energy.

 

You have a train with a relative velocity of 70 kph to the ground. Is the train moving or the ground? Despite your argument, you can't say. The train engine doesn't have to move the the country side, all it has to do it overcome the friction between itself and the rails, and that is the same whether or not you consider it the train or the country side as moving.

 

You can even consider the situation where the train and country side start off at rest WRT each other. The train starts up its engine and the two begin to develop a relative motion. Again, you do not have to resort to the train engine accelerating the whole country side to consider the Train as stationary. The countryside and train instead were both traveling at 70 kph before the engine started and the engine then slowed the train down, while the country side continued to move. Again, the energy exerted by the train is the same no matter what final speed you decide the train has.

[Klaynos]

Maybe the concept of two cars on the highway are a better image.

 

I am traveling down the road at 60 MPH at a constant velocity. There is a car in front of me also traveling 60 MPH in the same direction of travel. There is 10 feet between the cars. If the car in front of me suddenly hits the brakes and accelerates, how far does he travel in the opposite direction?

 

This is equivalent to asking "If I have an orange, what time is it in Japan?" There's nowhere near enough information, for one thing what do you mean by opposite direction, and relative to what?

[Motor Daddy]

You have a train with a relative velocity of 70 kph to the ground. Is the train moving or the ground? Despite your argument, you can't say. The train engine doesn't have to move the the country side, all it has to do it overcome the friction between itself and the rails, and that is the same whether or not you consider it the train or the country side as moving.

 

You can even consider the situation where the train and country side start off at rest WRT each other. The train starts up its engine and the two begin to develop a relative motion. Again, you do not have to resort to the train engine accelerating the whole country side to consider the Train as stationary. The countryside and train instead were both traveling at 70 kph before the engine started and the engine then slowed the train down, while the country side continued to move. Again, the energy exerted by the train is the same no matter what final speed you decide the train has.

 

So the train accelerates from 70 kph to 0 kph? What happens if the train keeps accelerating? Does it go -20 kph?

[foodchain]

The train's velocity is 20m/s, and the ball is traveling with the train at 20m/s.

 

Motion is relative to reference frames with the speed of light being c in all cases save for BECS? So any two observers of say a lighting strike observe such relative to the from of reference the occupy at the time in respects to the speed of light remaining constant.

 

Is this an okay way of putting it?

[iNow]

This is impressive. 141 posts in a day and a half, all talking about the same question over and over and over and over again.

[Edtharan]

There is one reference point and that is the starting line.

Lest go back to linguistics. You know what a Noun is? Well, in these examples each and every noun is a potential reference point.

 

So "Ball" is a reference point, "Train", "Track", "Carriage", "Start Line" and so forth are all reference points.

 

If you only want 1 reference point here is an example: There is a ball, how fast is it moving?

 

Now lets look at this example: There is a ball and I throw it at 10m/s how fast is the ball travelling.

 

Well in this there are two Nouns: The ball and Me. So the movement is implied as being relative to me.

 

Ok, how about this one: There is a ball and I throw it at 10m/s relative to the Earth. How fast am I travelling?

 

If your answer is 0, then you could be wrong. What if I was in a space suit in orbit above the Earth?

 

We can work out what my speed is by using maths. We could find the mass of the ball and find my mass, then apply Newtonian mechanics to work out the forces on me that would allow the ball to be thrown at 10m/s relative to the Earth.

 

So because there were 3 Nouns in that last example, there were 3 potential points of reference (frames of reference).

 

So, in the example that you have been following we have a Train, A Ball, The tracks, The person holding the Ball and the Starting Line.

 

There is not just one point of reference, but at least six!

 

So the train accelerates from 70 kph to 0 kph? What happens if the train keeps accelerating? Does it go -20 kph?

Since motion is relative, it can't be negative. You can express it as a negative, but in reality it is the same as the absolute value. Whether something can be called a negative velocity is only down to the coordinate system (which is arbitrary and does not effect the results).

 

So no. It doesn't go to -20km/h, it instead would go to 20km/h in the opposite direction.

 

I am traveling down the road at 60 MPH at a constant velocity. There is a car in front of me also traveling 60 MPH in the same direction of travel. There is 10 feet between the cars. If the car in front of me suddenly hits the brakes and accelerates, how far does he travel in the opposite direction?

Acceleration is not velocity. It is a change in velocity. Therefore if the car brakes and slows down (accelerates in the opposite direction), it is not immediately travelling in the opposite direction as to be travelling in the opposite direction the car would need a velocity in that opposite direction.

 

relative to the road, the car would have it's velocity reduced. Relative to the car following it the first car would have an increased velocity towards it (however, this second car can conclude that because it is not experiencing an acceleration that it is the other car that is changing it's velocity).

[Motor Daddy]

Lest go back to linguistics. You know what a Noun is? Well, in these examples each and every noun is a potential reference point.

 

So "Ball" is a reference point, "Train", "Track", "Carriage", "Start Line" and so forth are all reference points.

 

If you only want 1 reference point here is an example: There is a ball, how fast is it moving?

 

You tell me. In the example, the ball thrown off the back of the train had a velocity of 20m/s in the opposite direction of the train's travel, that was stated in the example.

 

Now lets look at this example: There is a ball and I throw it at 10m/s how fast is the ball travelling.

 

10 m/s, but I don't know the direction of travel.

 

Well in this there are two Nouns: The ball and Me. So the movement is implied as being relative to me.

 

Implied? The ball is traveling 10 m/s in an unknown direction. We have no idea of your velocity or direction of travel, though.

 

Ok, how about this one: There is a ball and I throw it at 10m/s relative to the Earth. How fast am I travelling?

 

Still unknown.

 

If your answer is 0, then you could be wrong. What if I was in a space suit in orbit above the Earth?

 

My answer was not zero.

 

We can work out what my speed is by using maths. We could find the mass of the ball and find my mass, then apply Newtonian mechanics to work out the forces on me that would allow the ball to be thrown at 10m/s relative to the Earth.

 

So because there were 3 Nouns in that last example, there were 3 potential points of reference (frames of reference).

 

So, in the example that you have been following we have a Train, A Ball, The tracks, The person holding the Ball and the Starting Line.

 

There is not just one point of reference, but at least six!

 

Six? We made it perfectly clear we are taking measurements from the track that is marked, and the timer that is activated at the "start line" and deactivated at the "finish line." Where are the other 5 timers, and how are they activated?

 

 

Since motion is relative, it can't be negative. You can express it as a negative, but in reality it is the same as the absolute value. Whether something can be called a negative velocity is only down to the coordinate system (which is arbitrary and does not effect the results).

 

So no. It doesn't go to -20km/h, it instead would go to 20km/h in the opposite direction.

 

So, in Janus's example, the maximum velocity the train could accelerate to is 0 kph, as if it went any faster it would have to be a negative number, as it certainly does not change direction of travel. That was my point.

 

 

Acceleration is not velocity. It is a change in velocity. Therefore if the car brakes and slows down (accelerates in the opposite direction), it is not immediately travelling in the opposite direction as to be travelling in the opposite direction the car would need a velocity in that opposite direction.

 

relative to the road, the car would have it's velocity reduced. Relative to the car following it the first car would have an increased velocity towards it (however, this second car can conclude that because it is not experiencing an acceleration that it is the other car that is changing it's velocity).

 

The car does not accelerate in a direction. The velocity is in a direction, and the acceleration is CALCULATED from distance and time. There is no direction to acceleration. It is a calculated value of distance and time.

 

When you talk about acceleration, you are talking about the rate of change of velocity. Velocity has a direction, so you do not accelerate in an opposite direction as Janus's example implies, or when you accelerate when you hit the brakes, or in the example of the ball on the train.

 

The ball's velocity is 19m/s in the same direction of travel of the train, not 1m/s in the opposite direction. We already established the ball had an initial velocity of 20 m/s, so all you are doing is reducing the velocity of the ball by 1 m/s, and that is not a change in direction. The distance between the start line and the ball continues to increase the entire duration. If there was a change in direction the distance between the ball and the start line would DECREASE, and that does not happen!

Edited June 22, 2008 by Motor Daddy

[YT2095]

reading all this, I get the overall feeling that you struggle with Frame shifting.

the ability to view something in it`s purest form, void of all trivia.

 

Try not to Over-complicate

[Motor Daddy]

reading all this, I get the overall feeling that you struggle with Frame shifting.

the ability to view something in it`s purest form, void of all trivia.

 

Try not to Over-complicate

 

But you gave me the velocity of the ball off the back of the train, 20 m/s in the opposite direction of travel of the train.

[YT2095]

ok, think simple maths here, we have +20 and -20 that when added make a Total of 0.

 

the same thing as if you stood still and dropped the ball.

[Motor Daddy]

ok, think simple maths here, we have +20 and -20 that when added make a Total of 0.

 

the same thing as if you stood still and dropped the ball.

 

But you said the train was +20, and the ball was +20 in the opposite direction, did you not?

[YT2095]

I did indeed

 

and the Opposite of +20 (the trains velocity) is -20.

making a grand total of 0 when added together.

 

here`s Another way to consider it, the ball (and train) Were stationary once, and the train started moving (with the ball inside it) to a speed of +20.

and by Doing so it gave Energy to the ball (kinetic energy) in a horizontal direction.

 

But, by throwing it horizontally in the opposite direction of it`s travel, you effectively Cancel that energy, making the Net total of kinetic energy Zero.

But... since you`r standing up and have let go of the ball, it loses Vertical energy now (that you gave it when you picked it up).

[Motor Daddy]

I did indeed

 

and the Opposite of +20 (the trains velocity) is -20.

making a grand total of 0 when added together.

 

So the ball had a velocity of -20, not +20, is what you are saying? What is a -20 m/s velocity?

 

here`s Another way to consider it, the ball (and train) Were stationary once, and the train started moving (with the ball inside it) to a speed of +20.

and by Doing so it gave Energy to the ball (kinetic energy) in a horizontal direction.

 

But, by throwing it horizontally in the opposite direction of it`s travel, you effectively Cancel that energy, making the Net total of kinetic energy Zero.

But... since you`r standing up and have let go of the ball, it loses Vertical energy now (that you gave it when you picked it up).

 

You mean you accelerated the ball, and the velocity of the ball decreased to 0 m/s? Why didn't you say that in the first place? I would then have answered appropriately.

 

To be more clear of your intention, you meant to say the ball is accelerated from an initial velocity of 20m/s to a velocity of 0 m/s, correct?

[swansont]

The car does not accelerate in a direction. The velocity is in a direction, and the acceleration is CALCULATED from distance and time. There is no direction to acceleration. It is a calculated value of distance and time.

 

Acceleration is a vector, so it has a direction, by definition. If you accelerate in the direction of motion, you speed up. If you accelerate opposite the direction of motion, you slow down. If you accelerate perpendicular to the direction of motion, you change the direction of motion.

[Motor Daddy]

Acceleration is a vector, so it has a direction, by definition. If you accelerate in the direction of motion, you speed up. If you accelerate opposite the direction of motion, you slow down. If you accelerate perpendicular to the direction of motion, you change the direction of motion.

 

Ready, GO! I take off as the timer starts, I run 40 meters and the timer stops. The event is over. The tape measure and timer say I just ran 40 meters in 4 seconds. I never mentioned acceleration.