Kinetic energy paradox

  • #1
pankazmaurya
26
0
watz gung wrong here
 

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Answers and Replies

  • #2
zhermes
1,255
3
What do you think?
 
  • #3
JaredJames
2,817
22
watz gung wrong here

Your spelling. Please not the rules on English and text speak.

This looks like homework. Could you provide your opinion on the matter?
 
  • #4
pankazmaurya
26
0
Your spelling. Please not the rules on English and text speak.

This looks like homework. Could you provide your opinion on the matter?

if this looks like homework to you then you should better know its answer
 
  • #5
pankazmaurya
26
0
What do you think?

there is something very basic in this problem that anyone is not able to catch
 
  • #6
JaredJames
2,817
22
if this looks like homework to you then you should better know its answer

Yes, it looks like homework and as such the PF rules require your attempt first.

Once we've got that I'll gladly offer my answer to the problem.
 
  • #7
pankazmaurya
26
0
Yes, it looks like homework and as such the PF rules require your attempt first.

Once we've got that I'll gladly offer my answer to the problem.

though i m not able to get the problem clearly but i have understood that there is a blunder in considering the train frames of reference to apply the work energy thereom
 
  • #8
JaredJames
2,817
22
though i m not able to get the problem clearly but i have understood that there is a blunder in considering the train frames of reference to apply the work energy thereom

And what do you think the blunder is?
 
  • #9
pankazmaurya
26
0
And what do you think the blunder is?

that is wat im not geting.now ur chance...
 
  • #10
JaredJames
2,817
22
that is wat im not geting.now ur chance...

I'd lose the attitude, I'm not going to tell you the answer. The rules of PF are that I cannot do your work for you and I have no intention of doing so. I can only provide you guidance.

So let's get you started.

What is the formula for kinetic energy? What are the two relevant formulas for work done?
 
  • #11
pankazmaurya
26
0
I'd lose the attitude, I'm not going to tell you the answer. The rules of PF are that I cannot do your work for you and I have no intention of doing so. I can only provide you guidance.

So let's get you started.

What is the formula for kinetic energy? What are the two relevant formulas for work done?

its simple u cant tell the answer becuase you dont know. and my dear m not interested in the answer .
 
  • #12
JaredJames
2,817
22
its simple u cant tell the answer becuase you dont know. and my dear m not interested in the answer .

I've just derived the answer on a pad in front of me for you.

I can tell you it's relating to the KE work done and velocity equations.

Drop the attitude or no one here will help you. If you don't care about the answer I recommend this thread be locked right now as any further help will be pointless if you don't care about it.

I again refer you to the rules you agreed to when you signed up relating to you must show some attempt before we can help.
 
  • #13
pankazmaurya
26
0
I've just derived the answer on a pad in front of me for you.

I can tell you it's relating to the KE work done and velocity equations.

Drop the attitude or no one here will help you. If you don't care about the answer I recommend this thread be locked right now as any further help will be pointless if you don't care about it.

I again refer you to the rules you agreed to when you signed up relating to you must show some attempt before we can help.

would you elaborate on your bakwas....what kind of efforts do you want
 
  • #14
JaredJames
2,817
22
would you elaborate on your bakwas....

Last chance or I'll report the thread and stop offering help. Drop the attitude.
what kind of efforts do you want

EDIT: Forget that. The answer is more obvious than all my workings here.

Look at the values for speed quoted. What is wrong with Pauls speed values?
 
  • #15
pankazmaurya
26
0
Last chance or I'll report the thread and stop offering help. Drop the attitude.


You indicated you're not sure where to begin, so I pointed you in the direction by asking you for some equations:

stop being bossy....the energy of the fuel will be converted to kinetic energy of the car according to half my square
 
  • #16
JaredJames
2,817
22
stop being bossy....

You asked for help, I'm giving it to you and for that all I ask is you converse in a respectful manner and follow the rules of the forum. So far you have not done that and you won't get much help if you keep it up.
the energy of the fuel will be converted to kinetic energy of the car according to half my square

I edited the previous post last minute.

Look at the speeds quoted in the question and where Pauls reference point is. What is wrong with his speeds.
 
  • #17
lalbatros
1,256
2
This homework has already been discussed once of this forum.
No need to do it twice.
 
  • #18
JaredJames
2,817
22
This homework has already been discussed once of this forum.
No need to do it twice.

Has it really? Could you link to it please? Save this having to continue for no reason.
 
  • #19
lalbatros
1,256
2
I could not give you the link, since this would amount to giving you an answer to an homework.
 
  • #20
JaredJames
2,817
22
I could not give you the link, since this would amount to giving you an answer to an homework.

In which case we need to discuss it twice, making your first post pointless.
 
  • #21
JaredJames
2,817
22
they are right according o me...

So Peter is right saying it will take 3x litres of fuel to go from 100 to 200 and Paul is right saying that it will take (5/3)x litres of fuel to go from 200 to 300?

Are you telling me it would take less fuel to go from 200 to 300 than it would 100 to 200?

Again, look at the values for the speeds - there's something wrong with one set of speeds.
 
  • #22
pankazmaurya
26
0
So Peter is right saying it will take 3x litres of fuel to go from 100 to 200 and Paul is right saying that it will take (5/3)x litres of fuel to go from 200 to 300?

Are you telling me it would take less fuel to go from 200 to 300 than it would 100 to 200?

Again, look at the values for the speeds - there's something wrong with one set of speeds.

the train in moving in opposite direction of the car and hence in train frame of refrence corrolay pauls frame of refrence the initial speed of car is 100 and after consuming x amount of fuel it will be 500
 
  • #23
JaredJames
2,817
22
the train in moving in opposite direction of the car and hence in train frame of refrence corrolay pauls frame of refrence the initial speed of car is 100 and after consuming x amount of fuel it will be 500

Where did you get 500 from?

The kinetic energy to go from 100 to 200 for the car will be the same, regardless of frame of reference. The train is independent and Paul is making a mistake by adding the trains velocity into the equation.

The car goes from 0 to 100 and then 100 to 200. The fuel to go from 0 to 100 is not the same as the fuel to go from 100 to 200. The fuel to go from 100 to 200 is not the same as the fuel to go from 200 to 300. So with Paul using these values he is incorrect.

You can't add the trains motion into the equation because it is independent of the car. The kinetic energy for acceleration of the car isn't affected by the train.
 
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  • #24
pankazmaurya
26
0
Where did you get 500 from?

The kinetic energy to go from 100 to 200 for the car will be the same, regardless of frame of reference. The train is independent and Paul is making a mistake by adding the trains velocity into the equation.

The car goes from 0 to 100 and then 100 to 200. The fuel to go from 0 to 100 is not the same as the fuel to go from 100 to 200. The fuel to go from 100 to 200 is not the same as the fuel to go from 200 to 300. So with Paul using these values he is incorrect.

the fuel required for 0 to 100 in peters frame is same as the fuel required 100 to 200 in pauls frame........and if u are trying to sat that paul is making mistake by adding trains velocity then how how can conclude that he is making a mistake...becuase he can never know whether he is moving or the car is moving
 
  • #25
JaredJames
2,817
22
the fuel required for 0 to 100 in peters frame is same as the fuel required 100 to 200 in pauls frame

Yes it is, but in his equations he uses it wrongly and includes the trains velocity. If you include the trains velocity, you are including the kinetic energy of the train.
........and if u are trying to sat that paul is making mistake by adding trains velocity then how how can conclude that he is making a mistake...becuase he can never know whether he is moving or the car is moving

Irrelevant to the question.
 
  • #26
pankazmaurya
26
0
Yes it is, but in his equations he uses it wrongly and includes the trains velocity. If you include the trains velocity, you are including the kinetic energy of the train.

but how will paul know that he is including trains velocity...
Irrelevant to the question.
m not discusing the exact question...i want to discuss the spirit of it
 
  • #27
JaredJames
2,817
22
m not discusing the exact question...i want to discuss the spirit of it

What spirit? You specifically asked what's gone wrong in the question. I am trying to help you answer it - that will show what has gone wrong.

If you don't want to answer the question then you've completely wasted my time for the last two pages of posts. You should make that perfectly clear in the OP.

I will no longer respond to this thread. Between your attitude and this I'm just fed up.
 
  • #28
JaredJames
2,817
22
Here is the final explanation:

The energy to go from 100 to 200 is not the same as 200 to 300. Therefore the fuel requirements are different.

If Paul ignores the trains speed and uses 200 to 300 for the second section his V2 values for KE are going to be much higher.

V2 = 2002 = 40000
V2 = 3002 = 90000

That is a significant difference in energy values. You have to put in more than double the fuel to get to 300 than you do to get to 200. So if you don't compensate for the trains speed you end up with the incorrect energy values - because the car is not going to 300. Change in KE from 100 to 200 = 0.5m(2002 - 1002) =/= change in KE from 200 to 300 = 0.5m(3002 - 2002). So the readings gained by both people are not the same and hence cannot both be right for the second stage.

Pauls reference frame can be what ever he likes, it could show the car going from 1000 to 1100 but the energy used by the car will always be the amount to go from 100 to 200. You have to compensate for this based on your reference frame.

What ever speed Paul records must be altered to allow for the trains velocity.
 
  • #29
rcgldr
Homework Helper
8,791
579
If Paul ignores the trains speed and uses 200 to 300 for the second section.
From Paul's point of view the car accelerates from -100 to 0 for the first section, then from 0 to 100 for the second section.

This same issue has been brought up a couple of times recently. If you include the energy change to the earth, and consider earth and car as part of a closed system, then ignoring losses to heat, the total energy of the system is conserved, and the total kinetic energy gain is equal to the total chemical or electrical potential energy consumed by the engine, as observed from any inertial (non-accelerating) frame of reference.

If the effect on the earth is going to be ignored (earth with infinite mass), then the frame of reference must correrspond to the point of application of force, which in this case, is the road, since that is where the force is being applied by the tires of the car, and since the assumption is the road is attached to an earth with infinite mass (no energy gained by the earth).
 
  • #30
JaredJames
2,817
22
From Paul's point of view the car accelerates from -100 to 0 for the first section, then from 0 to 100 for the second section.

Correct, that is compensating for the trains motion. But, in the question Paul goes wrong and uses 100 to 200 and then 200 to 300 for the energy values.
 
  • #31
rcgldr
Homework Helper
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579
In the question Paul goes wrong and uses 100 to 200 and then 200 to 300 for the energy values.
True, but I was ignoring that mistake in the the orignal post as well as the bad calculations of energy, and commenting on a corrected observation.
 
  • #32
JaredJames
2,817
22
True, but I was ignoring that mistake in the the orignal post as well as the bad calculations of energy, and commenting on a corrected observation.

Precisely, your answer is exactly why Paul is incorrect in his calculation.
 
  • #33
Gokul43201
Staff Emeritus
Science Advisor
Gold Member
7,176
21
Precisely, your answer is exactly why Paul is incorrect in his calculation.
That's not quite true. The reason Paul would be wrong even if he used the correct velocities is because he hasn't accounted for all parts of the system. The train is definitely not an isolated body - it is experiencing a force from the rails.

Nevertheless, pankaz needs to read the forum guidelines before continuing to post. The attitude displayed is not useful.
 
  • #34
JaredJames
2,817
22
That's not quite true. The reason Paul would be wrong even if he used the correct velocities is because he hasn't accounted for all parts of the system. The train is definitely not an isolated body - it is experiencing a force from the rails.

Very true.

I guess I'm simplifying it as much as possible for the question (the whole ignore all but kinetic energy). Perhaps too much.
 
  • #35
pankazmaurya
26
0
From Paul's point of view the car accelerates from -100 to 0 for the first section, then from 0 to 100 for the second section..

from pauls point of veiw the accelerates from -100 to -200 because he is moving in opposite direction of the car
 

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