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### Author Topic: What is a net difference?  (Read 2766 times)

#### Thebox

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##### What is a net difference?
« on: 22/02/2016 10:10:48 »
Hello, I was asked a question by my 15 year old cousin and did not know what to answer.

He ask's

a Journey from A to B takes 1 second to travel at v=c

a journey from B to A takes 1 second to travel at v=c

distance (d) = X=?

net difference d/ t = ?

#### Colin2B

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##### Re: What is a net difference?
« Reply #1 on: 22/02/2016 10:41:27 »
net difference d/ t = ?
you must have misunderstood the question, d/t is not a difference it is a speed if d is the distance (easily calculated using the speed of light).
You need to go back and ask for clarification.

PS - Net (or Nett in US) means a part of something, like Net income is income after deductions such as tax.
« Last Edit: 22/02/2016 10:50:40 by Colin2B »

#### Thebox

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##### Re: What is a net difference?
« Reply #2 on: 22/02/2016 11:13:51 »
net difference d/ t = ?
you must have misunderstood the question, d/t is not a difference it is a speed if d is the distance (easily calculated using the speed of light).
You need to go back and ask for clarification.

PS - Net (or Nett in US) means a part of something, like Net income is income after deductions such as tax.

I have asked and he says, he was asking about the net difference  between the two travelling bodies d/t

#### Colin2B

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##### Re: What is a net difference?
« Reply #3 on: 22/02/2016 12:02:31 »
I have asked and he says, he was asking about the net difference  between the two travelling bodies d/t
there is no net difference the distance is the same, the speed is the same. If you want to consider these as vectors, you would define one as +ve and the other -ve, net difference 0.
Always happy to help with homework questions by giving hints, but in this case I still don't understand the question.

#### Thebox

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##### Re: What is a net difference?
« Reply #4 on: 22/02/2016 12:06:41 »
I have asked and he says, he was asking about the net difference  between the two travelling bodies d/t
there is no net difference the distance is the same, the speed is the same. If you want to consider these as vectors, you would define one as +ve and the other -ve, net difference 0.
Always happy to help with homework questions by giving hints, but in this case I still don't understand the question.

Well I also thought the net difference was 0.   I think you answered the question, thanks Colin.

A question of my own, why would you define -ve and +ve , is this to show the invert velocity?

what does the e mean?

and how would you express net difference in a symbol form?

« Last Edit: 22/02/2016 12:09:20 by Thebox »

#### Colin2B

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##### Re: What is a net difference?
« Reply #5 on: 22/02/2016 12:13:11 »
A question of my own, why would you define -ve and +ve , is this to show the invert velocity?
No just direction.

what does the e mean?
+ve is just shorthand for positive.

and how would you express net difference in a symbol form?
Not aware of a symbol

#### Thebox

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##### Re: What is a net difference?
« Reply #6 on: 22/02/2016 12:15:34 »
A question of my own, why would you define -ve and +ve , is this to show the invert velocity?
No just direction.

what does the e mean?
+ve is just shorthand for positive.

and how would you express net difference in a symbol form?
Not aware of a symbol

arrghh I see. So would I express for the above question

+ve=c

-ve=c

d=x

net difference t=0

Explaining a back and forth journey?

#### Colin2B

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##### Re: What is a net difference?
« Reply #7 on: 22/02/2016 12:53:04 »
d=x

net difference t=0

Explaining a back and forth journey?
No, how can t=0 if you went there and back?

If you went there and back then d=2x and time =2t

#### Thebox

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##### Re: What is a net difference?
« Reply #8 on: 22/02/2016 13:33:45 »
d=x

net difference t=0

Explaining a back and forth journey?
No, how can t=0 if you went there and back?

If you went there and back then d=2x and time =2t

I think you may of misunderstood or I have not explained greatly.

We were individually accessing two trips, the first trip from A to B then the second return trip of B to A , and calculating the net difference in time of each trip at constant velocity of c in either direction.

#### Colin2B

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##### Re: What is a net difference?
« Reply #9 on: 22/02/2016 13:53:01 »
OK, this confused me:
"net difference d/ t = ?"

#### Thebox

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##### Re: What is a net difference?
« Reply #10 on: 22/02/2016 14:28:48 »
OK, this confused me:
"net difference d/ t = ?"

Yes I probably should of put

net difference d/t^2  =0  or ∑(+ve⊥-ve)=Δv=(d/t^2)=0

To show both  journeys was equal in every aspect.   Would that be more understandable?

« Last Edit: 22/02/2016 14:37:18 by Thebox »

#### alancalverd

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##### Re: What is a net difference?
« Reply #11 on: 22/02/2016 15:49:25 »
The important thing about the question is the realisation (following the experimental discovery) that c is constant regardless of direction, so A-> B = B->A in every respect.

Obviously d = 300,000,000 km or thereabouts.

#### Thebox

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##### Re: What is a net difference?
« Reply #12 on: 22/02/2016 16:18:01 »
The important thing about the question is the realisation (following the experimental discovery) that c is constant regardless of direction, so A-> B = B->A in every respect.

Obviously d = 300,000,000 km or thereabouts.

Yes indeed the importance is to realise that c is constant regardless of direction and a set length of x in either direction the photons would take the exact same amount of time to arrive at each others destination.

Would you say that at the exact half way point of X would be the exact position of passing photon's from each direction?

#### evan_au

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##### Re: What is a net difference?
« Reply #13 on: 23/02/2016 14:21:24 »
Quote from: TheBox
how would you express net difference in a symbol form?
In mathematics, physics and chemistry, the Greek letter Delta ("d" sound) is often used to represent a difference:
• "Δ":Capital Delta
• "δ": lower case delta
Sometimes the equivalent English letter "d" is used to represent a infinitesimal difference (as in velocity=ds/dt).
...and a "curvy d" () in more complicated scenarios.

#### Thebox

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##### Re: What is a net difference?
« Reply #14 on: 23/02/2016 14:38:13 »
Quote from: TheBox
how would you express net difference in a symbol form?
In mathematics, physics and chemistry, the Greek letter Delta ("d" sound) is often used to represent a difference:
• "Δ":Capital Delta
• "δ": lower case delta
Sometimes the equivalent English letter "d" is used to represent a infinitesimal difference (as in velocity=dt/dt).
...and a "curvy d" () in more complicated scenarios.

Then maybe simply dt/dt=0 net difference?

#### Colin2B

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##### Re: What is a net difference?
« Reply #15 on: 23/02/2016 17:38:35 »
Then maybe simply dt/dt=0 net difference?
no, dt/dt=1

for a difference you have to subtract eg t1-t2=δt

#### Thebox

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##### Re: What is a net difference?
« Reply #16 on: 23/02/2016 17:51:27 »
Then maybe simply dt/dt=0 net difference?
no, dt/dt=1

for a difference you have to subtract eg t1-t2=δt

In my scenario

t1-t2=o

#### jeffreyH

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##### Re: What is a net difference?
« Reply #17 on: 23/02/2016 18:03:57 »
Then maybe simply dt/dt=0 net difference?
no, dt/dt=1

for a difference you have to subtract eg t1-t2=δt

In my scenario

t1-t2=o

You can't simply subtract t2 from t1. Both of the time intervals are in the same direction since time does not go backwards. You gain nothing from the operation. It would be different if the time component was part of a velocity calculation. As that has a well defined direction since it is a vector.

#### Colin2B

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##### Re: What is a net difference?
« Reply #18 on: 23/02/2016 18:11:31 »
You can't simply subtract t2 from t1. Both of the time intervals are in the same direction since time does not go backwards. You gain nothing from the operation. It would be different if the time component was part of a velocity calculation. As that has a well defined direction since it is a vector.
I agree Jeff, I was thinking of an example such as 20s-5s=15s.
To subtract the 2 journey times is pointless.
My fault, I'd forgotten where this strange topic had started from!

#### Thebox

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##### Re: What is a net difference?
« Reply #19 on: 23/02/2016 18:11:55 »
Then maybe simply dt/dt=0 net difference?
no, dt/dt=1

for a difference you have to subtract eg t1-t2=δt

In my scenario

t1-t2=o

You can't simply subtract t2 from t1. Both of the time intervals are in the same direction since time does not go backwards. You gain nothing from the operation. It would be different if the time component was part of a velocity calculation. As that has a well defined direction since it is a vector.

its a net difference calculation of two journeys I need. two directions, the same speed and distance.

#### Colin2B

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##### Re: What is a net difference?
« Reply #20 on: 23/02/2016 18:43:15 »
ok, just to be clear.
If you travel from home to nearby town and it takes 1hr then return journey is also 1hr. Total journey time (round trip) is 2hr, but the difference between the 2 journey times is 0, in other words they are the same.
Sorry to labour this but we've got to be clear to avoid misunderstandings

#### jeffreyH

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##### Re: What is a net difference?
« Reply #21 on: 23/02/2016 19:28:38 »
If we take two vectors V1 and V2 this can be demonstrated. V1 starts at the origin [0, 0] and has an endpoint at [2, 2]. Vector V2 starts at [2, 2] and has its endpoint at the origin [0, 0]. To simplify the calculation we can translate V2 to The origin so that it starts at [0, 0] and its endpoint is at [-2, -2]. We can then simply add the vectors together using the endpoint values. [2, 2] and [-2, -2]. As can be seen the addition of 2 and -2 in both cases results ultimately in a null vector with both starting and ending points at [0, 0]. This indicates a velocity of zero for the round trip which could imply no time if you didn't have all the information about the past states of the system. Here information about the system is lost and so shows that information is generally not conserved with the flow of time.

#### Thebox

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##### Re: What is a net difference?
« Reply #22 on: 23/02/2016 20:10:38 »
ok, just to be clear.
If you travel from home to nearby town and it takes 1hr then return journey is also 1hr. Total journey time (round trip) is 2hr, but the difference between the 2 journey times is 0, in other words they are the same.
Sorry to labour this but we've got to be clear to avoid misunderstandings

Yes exactly that.

#### The Naked Scientists Forum

##### Re: What is a net difference?
« Reply #22 on: 23/02/2016 20:10:38 »

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