Relative Motion

[Cap'n Refsmmat]

Let me get this straight. You know you are in motion flying though space in a box, and yet you have no clue if you are in motion or not, and so, by default, you automatically assume you are stationary?

 

I'm having a hard time grasping that concept.

 

I didn't say I know I was in motion. (And if I did, ignore it.) All I said is that I know I'm in space in a box. It's impossible to tell if I'm in motion or now.

[Klaynos]

Let me get this straight. You know you are in motion flying though space in a box, and yet you have no clue if you are in motion or not, and so, by default, you automatically assume you are stationary?

 

I'm having a hard time grasping that concept.

 

It's easiest to assume you're stationary, and there's no physical reason why this isn't a reasonable assumption. And there's no way to "know" you're in motion, unless your accelerating.

[Motor Daddy]

It's not zero. It's as close to zero as it can possibly be without being zero.

 

So you are the only one in this universe that can actually approach infinity?

[ydoaPs]

If it's mathematically sound, could you tell me which object is static in this universe?

all non-accelerating objects

 

Also, if an instant is possible, and an object travels 10 m/s, could you tell me which d/t the object has no motion or time?

itself

 

So you are the only one in this universe that can actually approach infinity?

 

What?

[Motor Daddy]

We've got to use more correct definitions if we're going to talk about this in great depth and I think we'll get out of your maths level quite quickly... Velocity in 1D for example is defined as:

[math]v= \frac {dx}{dt}[/math]

 

True, as I stated before, I am nowhere close to a mathematician, never will be.

 

I do have a strong understanding of distance and time, and also force times distance (torque) though.

 

f=ma? What is the force if the acceleration is zero?

[Cap'n Refsmmat]

true, As I Stated Before, I Am Nowhere Close To A Mathematician, Never Will Be.

 

I Do Have A Strong Understanding Of Distance And Time, And Also Force Times Distance (torque) Though.

 

F=ma? What Is The Force If The Acceleration Is Zero?

 

0 N.

[Motor Daddy]

I didn't say I know I was in motion. (And if I did, ignore it.) All I said is that I know I'm in space in a box. It's impossible to tell if I'm in motion or now.

 

No it's not. You had to get into the box at some point in time. You just didn't appear in space in a box, like some kind of "God" created you and put you there.

 

I assume you are human, so you must have came from the Earth (which was rotating before you were born, correct?

 

In order to NOT be on the Earth you had to accelerate away from the Earth, correct? Did you not calculate the acceleration away from the Earth? Did you not feel the acceleration?

[ydoaPs]

I do have a strong understanding of distance and time

Apparently not.

 

f=ma? What is the force if the acceleration is zero?

[math]\sum{\vec{F}}=\frac{d\vec{p}}{dt}[/math] If your acceleration is zero, you could still be under forces that cancel. Hoo-ya vectors.

[Motor Daddy]

I'm trying to keep up with all of you, but it's too much for one person to handle all these responses.

[Cap'n Refsmmat]

No it's not. You had to get into the box at some point in time. You just didn't appear in space in a box, like some kind of "God" created you and put you there.

 

I assume you are human, so you must have came from the Earth (which was rotating before you were born, correct?

 

In order to NOT be on the Earth you had to accelerate away from the Earth, correct? Did you not calculate the acceleration away from the Earth? Did you not feel the acceleration?

 

You are making this complicated, so: I passed out under the forces of launch (low blood pressure, it runs in my family) and woke up a few hours later.

 

It doesn't matter anyway. I could just as easily say I started from a negative velocity and accelerated to zero velocity as I could say I started at zero velocity and accelerated to a positive one. It works either way.

[Motor Daddy]

0 N.

 

Care to elaborate why 3,000 RPM is not accelerating, and yet there can be torque (force times distance) at that constant rotational velocity?

[ydoaPs]

Care to elaborate why 3,000 RPM is not accelerating, and yet there can be torque (force times distance) at that constant rotational velocity?

 

It is accelerating.

[YT2095]

Torque doesn`t come into it unless it`s engaged in work.

[Motor Daddy]

It is accelerating.

 

It's not increasing or decreasing rotational velocity, correct?

[D H]

This thread sprang into existence because it became clear to Klaynos that MD was having a hard time grasping the concept of relative motion. This concept of relative motion is in a way the first theory of relativity in physics. It dates back to almost 400 years ago. It is called Galilean Relativity in honor of Galileo who first introduced the concept in 1632. A good understanding of Galilean Relativity is crucial for even high school level physics.

 

Galileo's insight was that a person belowdecks on a ship could not tell whether a ship was docked or moving steadily at sea. The exact same concept applies here to the ball thrown on a train. Suppose the person throwing the ball on the train is in a windowless train car. There is nothing the person in the car can do with the ball to distinguish whether the train is at rest with respect to the tracks or the train is moving steadily along the tracks. The person can assess the motion of the ball with respect to the train and nothing else.

 

If the person moves to a car with windows, he can deduce that the train is moving based on what he sees outside the windows. He can also deduce the velocity of the ball with respect to the tracks by vectorially adding the velocity of the ball with respect to the train to the velocity of the train with respect to the tracks.

 

One final thing to remember is that various physics experiments at the end of the 19th century showed that Galilean Relativity is not exactly correct. Einstein developed his theory of Special Relativity to explain this. That is an aside for future work and is off-topic in this thread. What Klaynos is doing in this thread is teaching Galilean Relativity.

[Cap'n Refsmmat]

Care to elaborate why 3,000 RPM is not accelerating, and yet there can be torque (force times distance) at that constant rotational velocity?

 

By "not accelerating" you mean the speed of rotation is constant, I presume. But the metal of whatever object you're spinning undergoes constant acceleration while it rotates -- acceleration is a change in velocity, and velocity includes distance. As the metal revolves around the central axis its direction is constantly changing.

[Motor Daddy]

DH, Why would the person not believe he was traveling 20 m/s on the train, when he is told he is traveling at 20 m/s relative to the tracks. Why does he then disregard the information of 20 m/s, and say that his ball is not in motion?

 

If he does acknowledge the ball is traveling away from the starting line at 20 m/s like he is, why does he insist on saying the ball is thrown 20 m/s in the direction towards the starting line? Surely he realizes the ball has to decelerate to 0 m/s from a 20 m/s initial velocity.

 

He has information, and yet he refuses to use it.

Edited June 22, 2008 by Motor Daddy

[swansont]

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.

 

Look up non-sequitur in the dictionary, when you get a moment.

 

"There is no direction to acceleration." is wrong. Deal with it and move on, rather than trying to change the subject. Trouble layeth in that direction.

[YT2095]

He has information, and yet he refuses to use it.

 

LOL, Absolutely Classic, I love it

[Motor Daddy]

Look up non-sequitur in the dictionary, when you get a moment.

 

No, that is the point. Motion occurs, and we measure the distance and time.

 

Do you know what happens BEFORE it happens?

 

The distance and time were measured. 40 meters in 4 seconds.

 

I asked you for the acceleration(s).

 

The event was measured, and you know the results AFTER the event is completed, and not a moment sooner.

[Cap'n Refsmmat]

DH, Why would the person not believe he was traveling 20 m/s on the train, when he is told he is traveling at 20 m/s relative to the tracks. Why does he then disregard the information of 20 m/s, and say that his ball is not in motion?

Because the ball is not in motion -- compared to the train. The relative velocity is 0m/s. And the train's velocity is only 20m/s compared to the Earth. You prefer using the Earth as a reference frame, but it's just as allowable to use the train.

[Motor Daddy]

Because the ball is not in motion -- compared to the train. The relative velocity is 0m/s. And the train's velocity is only 20m/s compared to the Earth. You prefer using the Earth as a reference frame, but it's just as allowable to use the train.

 

So when the ball is thrown off the back of the train, relative to the train at 20 m/s, what is the distance between the train and the ball 1 second later?

[Cap'n Refsmmat]

20m.

 

From the train's frame:

• The train is stationary.
  
• The ball is moving 20m/s away.
  

 

From the ball's frame:

• The ball is stationary.
  
• The train is moving at 20m/s away from the ball.
  

 

From the station's frame:

• The train is moving at 20m/s away from the ball.
  
• The ball is stationary.
  

 

They all are equally valid and give you the same answer to your question. That's the point I'd like to illustrate: all the frames are equally valid.

[Motor Daddy]

20m.

 

From the train's frame:

• The train is stationary.
  
• The ball is moving 20m/s away.
  

 

From the ball's frame:

• The ball is stationary.
  
• The train is moving at 20m/s away from the ball.
  

 

From the station's frame:

• The train is moving at 20m/s away from the ball.
  
• The ball is stationary.
  

 

They all are equally valid and give you the same answer to your question. That's the point I'd like to illustrate: all the frames are equally valid.

 

What is the distance between the ball and the train one second after release?

[Cap'n Refsmmat]

20m. I already answered that.