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Gravitation Potential energy. (1 Viewer)

k02033

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hahahah i like how this guy keeps using tertiary physics to explain hsc physics.
cant wait till you explain electromagnetism in motors and generators where it has all these calculus and vector terms lol.

)
i just substituted 2 input values into an hsc functions and subtracted the output values, demonstrating how to properly use the equation. don't think that is uni physics.
this is uni physics :

 
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helper

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Heard a bit more about the question today, and found out it also included mass loss from the fuel being expelled. So would really like to see the question before comitting.
 

Bank$

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This question is soooo simple !

Just use common sense guys.

GPE is a very WEAK force of attraction, the potential energy in the chemical bonds (i.e. fuel) is much greater than what is due to gravity. This is proven as it can cause the rocket to go up.

hence the loss in energy from the breaking of bonds is greater than the energy gained from the change in altitude.

also there is a change in mass thus even less potential energy due to gravity

K02003 is right but his explanation is a bit hard for hsc physics to understand.
 

kwabon

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ok enough is enough.
have a look at what i wrote for that particular question, and then have a look at the answers.

first one is the answer,
second one is an extract from my trials

sorry its upside down.
 

kwabon

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It is increasing as i said. As it's becoming a less negative number, it's increasing.

For example if it's 500m above the ground the GPE could be say -500 but when it's 1000m off the ground it might be -300. See how it's increasing (becoming less negative) as -300 > -500?

**Just fake values are used XD
thats what i wrote in the test, and i did not get full marks for it, apparently because decreases.
 

annabackwards

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thats what i wrote in the test, and i did not get full marks for it, apparently because decreases.
You have a dodgy teacher, argue with her/him because every other school realised the mistake and had enough sense to change it. You should get at least 2/3 and you gave one reason - i wouldn't get 3 marks for that answer at my school because it didn't utilise the equation, but you can try :)
 

helper

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The sample answer is wrong.

Yes GPE is proportional to mass, so as fuel is lost the GPE decreases if it remained at the same height.

GPE is negatively inversely proportional to the distance from the earth. So for the same mass as the rocket propels further from the Earth its GPE increases.

Combining these two, because the rate of fuel expulsion is greater than the rate of altitude gain, the overall GPE of the rocket will decrease.

The last paragraph wasn't provided enough detai in the question but is based on the fact, that the rocket is first taking off it ejects a large amount of gas but doesn't move much.

So your answer is worth 2 out of 3 if they were expecting to realise the mass change but it could have been better worded to ensure you were to think of the fuel and rocket together.

Also explaining what is happening also needs to say where it is going to. So if you are saying it is decreasing, you should be saying what it is being transformed into. If you are saying it is increasing, then you should be saying what it is transformed from.
 
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k02033

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Poor question...

There is always something wrong in the CSSA trials..
I think the question is still valid. The question is asking whether the overall potential energy of the system is increasing or decreasing. Its not asking whether the GRAVITATIONAL potential is increasing or decreasing.

The sample answer is wrong.

Yes GPE is proportional to mass, so as fuel is lost the GPE decreases if it remained at the same height.
Thats a wrong interpretation.
if the object is staying still within a gravitational field ie remain in the same height as you said, then gravity CAN NOT possibly do work on the object, so the object can not experience any GRAVITATIONAL potential change. So whatever potential change is occurring within the system, gravity is not responsible, so you are not allow to use U(x)=-GmM/r to interpret the situation
 
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helper

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I think the question is still valid. The question is asking whether the overall potential energy of the system is increasing or decreasing. Its not asking whether the GRAVITATIONAL potential is increasing or decreasing.
Did you bother to read the actual question?
http://community.boredofstudies.org...l-energy/19208d1252193036/scanned-picture.jpg

It states gravitational potential energy



Thats a wrong interpretation.
if the object is staying still within a gravitational field ie remain in the same height as you said, then gravity CAN NOT possibly do work on the object, so the object can not experience any GRAVITATIONAL potential change. So whatever potential change is occurring within the system, gravity is not responsible, so you are not allow to use U(x)=-GmM/r to interpret the situation
You would use -GmM/r in the original situation and at the final situation and compare the two. The amount of potential energy is different. That isn't necessarily saying that gravity has changed the potential energy. If you have a rock. Remove part of the rock. The rocks GPE has decreased. The other part of the rock will still have GPE, which may be the same or different depending on where it has been moved to.

In this case the rockets GPE has changed because the fuel has been moved elsewhere. The fuel will still have GPE but it is no longer part of the rocket.
 
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k02033

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Did you bother to read the actual question?
http://community.boredofstudies.org...l-energy/19208d1252193036/scanned-picture.jpg

It states gravitational potential energy
i assume the 1st post of the this thread stated the question correctly, (i cant predict the student copy the q wrong!)




That isn't necessarily saying that gravity has changed the potential energy.
this is true, in fact we can say gravity definitely have not cause any potential change

If you have a rock. Remove part of the rock. The rocks GPE has decreased.
not true

(assume all parts of the rock stays at the same location as you claimed in the below quote, this equivalent to a rocket staying the same place, losing mass somehow)
Yes GPE is proportional to mass, so as fuel is lost the GPE decreases if it remained at the same height.
The key is that the there is some force of constraint doing whatever it takes to oppose the force of gravity, keeping the object stationary. it looks like GPE should change because the actual magnitude of the force of gravity is changing, but this is irrelevant because there is the force of constraint somewhere.

As soon as this is true, gravity doesn’t get a chance to pull the object anywhere, so it won’t do any work on the rock.
its mass changed probably because of a potential change, but we can definitely deduce that potential change is not gravitational potential change,

In this case the rockets GPE has changed because the fuel has been moved elsewhere. The fuel will still have GPE but it is no longer part of the rocket.
and i thought we were assuming all parts remained at the same location, so doesnt that contradict you initial claims?
 
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kwabon

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You have a dodgy teacher, argue with her/him because every other school realised the mistake and had enough sense to change it. You should get at least 2/3 and you gave one reason - i wouldn't get 3 marks for that answer at my school because it didn't utilise the equation, but you can try :)
which equation you talking about?
Gpe = -gMm/r or Potential energy = mgh
 

helper

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i assume the 1st post of the this thread stated the question correctly, (i cant predict the student copy the q wrong!)
No but when you say someone is wrong after the person has already posted the question and sample answer, then it shows you aren't reading all the answers.

ok enough is enough.
have a look at what i wrote for that particular question, and then have a look at the answers.

first one is the answer,
second one is an extract from my trials

sorry its upside down.


(assume all parts of the rock stays at the same location as you claimed in the below quote, this equivalent to a rocket staying the same place, losing mass somehow)


The key is that the there is some force of constraint doing whatever it takes to oppose the force of gravity, keeping the object stationary. it looks like GPE should change because the actual magnitude of the force of gravity is changing, but this is irrelevant because there is the force of constraint somewhere.

As soon as this is true, gravity doesn’t get a chance to pull the object anywhere, so it won’t do any work on the rock.
its mass changed probably because of a potential change, but we can definitely deduce that potential change is not gravitational potential change,



and i thought we were assuming all parts remained at the same location, so doesnt that contradict you initial claims?
No the key is that when something is made up or more than one thing it has a total GPE, that is made up of the some of the parts. If you separate it into the parts, then the total remains the same but the individual parts have less than the total overall.

The Catholic answer, which I have said was wrong for another reason as well, was trying to have the students say:

1. The Rocket is a system of fuel and ship
2. As the Rocket is launching, it is expelling the fuel, making the mass of the rocket less by the amount of the fuel being expelled.
3. Thus the mass of the rocket is now less, so the rocket GPE is now less.

IE.







So there are two things effecting the GPE of the rocket, the mass of fuel loss and the increase in distance from planet. Depending on which is occurring at a faster rate, then the GPE of the rocket may go up or down.
 

Bank$

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So there are two things effecting the GPE of the rocket, the mass of fuel loss and the increase in distance from planet. Depending on which is occurring at a faster rate, then the GPE of the rocket may go up or down.
Exactly and including the loss of potential bond energy (from fuel burning) the net potential is going to be less
 

helper

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Exactly and including the loss of potential bond energy (from fuel burning) the net potential is going to be less
Did the question ask about net potential?
No, this statement is irrelevant and would be treated as such by the markers.
 

k02033

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So there are two things effecting the GPE of the rocket, the mass of fuel loss and the increase in distance from planet. Depending on which is occurring at a faster rate, then the GPE of the rocket may go up or down.
this is fine, but you and i were analysing something that wasnt gaining in altitude, in which i believe you gave a wrong interpretation
 

riseek

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i just checked my answer, i said it increases (which might be wrong), n got 3/3
 

annabackwards

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which equation you talking about?
Gpe = -gMm/r or Potential energy = mgh
I'm talking about the first one, because we are talking about a rocket that is moving a long distance away from eath and out of Earth's gravitational field.

The second equation only works for objects that remain inside the Earth's gravitational field :)
 

k02033

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wait i realized what i said is a contradiction, in that how can something split into 2 distinct objects and then defining those 2 to have the same location. And after the rock has split, how can we say the GPE of the rock has changed? which part do we define to be the "rock"? i think some of the initial proposal doesnt make sense.
 
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