Gravitation Potential energy. (2 Viewers)

[kwabon]

What happens to the potential energy during the first stage of the rocket?
will it increase or decrease? and reasons for your answer as well.

what i am thinking is due to Potential energy = mgh, shouldnt it increase?
but if you were to take potential energy = Gm(1)m(2)/r, it will decrease.

ahhh so confused atm

thanks for all your help.

 

[boxhunter91]

Remember the equation is negative.
From a point such as 6378000m to a point 26378000m the distance will increase therefore GPE will decrease.
Hope that helps.

 

[kwabon]

Remember the equation is negative.
From a point such as 6378000m to a point 26378000m the distance will increase therefore GPE will decrease.
Hope that helps.

thanks for your help, but i said the first stage, that will most likely be in the earth's atmosphere itself, just about 50km above the earth's surface.

 

[k02033]

What happens to the potential energy during the first stage of the rocket?
will it increase or decrease? and reasons for your answer as well.

what i am thinking is due to Potential energy = mgh, shouldnt it increase?
but if you were to take potential energy = Gm(1)m(2)/r, it will decrease.

ahhh so confused atm

thanks for all your help.

That's not the right way to use that equation.
The potential equation that you are familiar with Ep=-GmM/r in my opinion is a terrible way of describing the purpose of that equation.
To fix it, its simple,

Let gravitational potential energy of an object be


Chances are you haven't looked at it this way right? The gravitational potential equation is indeed a function of position. ie its a function whose input is the position of the object, and once you substituted a position into that function it will spit out the gravitational potential energy of the object at that certain location that you inputted (relative to the axis you have chosen to use).

So to find the change in change in gravitational potential energy, its really the gravitational potential energy at the final position minus the gravitational potential energy at the initial position.

let and denote initial position and final position

so the change in gravitational potential energy is..





but in a rocket launch Xf>Xi

so

ie gravitational Potential energy of the rocket is increasing as it gain altitude

 

[k02033]

and take the derivative of that function with respect to x and you will notice something cool (this is how we define potential energy)

 

[shady145]

^^ didnt know that GMm/x^2
thats also helps with a question my teacher couldnt get thnx

 

[k02033]

np! and also if you are interested, try integrating the other fundamental forces to get their respective potential functions. This will help you in understanding stopping voltage

 

[shady145]

np! and also if you are interested, try integrating the other fundamental forces to get their respective potential functions. This will help you in understanding stopping voltage

ooo ill do it now ^^ . i dnt know what stopping voltage is?

 

[shady145]

np! and also if you are interested, try integrating the other fundamental forces to get their respective potential functions. This will help you in understanding stopping voltage

ooo ill do it now... i dnt know what stopping voltage is XD

 

[k02033]

ooo ill do it now... i dnt know what stopping voltage is XD

Stopping voltage refers to the voltage between the cathode and the anode that is set up by the electric field that is used to decelerates photoelectrons release from the cathode due to photoelectric effect. So basically to calculate what the stopping voltage is, is asking how strong of an E field do i need to shove in between the electrodes to reduce the kinetic energy of the electrons down to zero. And this requires a change in potential energy of the system (conservation of energy)

 

[kwabon]

That's not the right way to use that equation.
The potential equation that you are familiar with Ep=-GmM/r in my opinion is a terrible way of describing the purpose of that equation.
To fix it, its simple,

Let gravitational potential energy of an object be


Chances are you haven't looked at it this way right? The gravitational potential equation is indeed a function of position. ie its a function whose input is the position of the object, and once you substituted a position into that function it will spit out the gravitational potential energy of the object at that certain location that you inputted (relative to the axis you have chosen to use).

So to find the change in change in gravitational potential energy, its really the gravitational potential energy at the final position minus the gravitational potential energy at the initial position.

let and denote initial position and final position

so the change in gravitational potential energy is..





but in a rocket launch Xf>Xi

so

ie gravitational Potential energy of the rocket is increasing as it gain altitude

totally agree with you, said that potential energy will increase, and apparently i got it wrong.
potential energy decreases, is the answer!!

post up the actual question tomorrow, feeling sleepy now.

 

[k02033]

oh wait i forgot.. dont try to do it for magnetic force, since the particle's trajectory is not one dimensional under that force, you would have to integrate its torque with respect to its angular position rather than force with respect to position... i think.. feels rusty = =

 

[k02033]

totally agree with you, said that potential energy will increase, and apparently i got it wrong.
potential energy decreases, is the answer!!

post up the actual question tomorrow, feeling sleepy now.

Wait i initially misread the question (i thought it was asking how the gravitational potential was changing, i blame title of this thread). It is true that the gravitational potential is increasing, BUT the system's overall potential is decreasing.(there are potential changes due to other forces) Assuming the total kinetic energy of the system is increasing. So the q was testing on your knowledge of conservation of energy, rather than how to calculate gravitational potential changes.

 

[k02033]

i just realised that the HSC syllabus has lead some of you to believe that potential energy is something that is only associated with gravity (is this true?) In fact that is not true. Potential energy is something associated with a force in general. There are many different kinds of forces, so there are many more ways that a change in potential energy can occur, rather than just when gravity is acting.

 

[helper]

totally agree with you, said that potential energy will increase, and apparently i got it wrong.
potential energy decreases, is the answer!!

post up the actual question tomorrow, feeling sleepy now.

Was this the Catholic Paper. I haven't seen it but have heard that there was an incorrect answer, similar to what you stated. Most schools picked it up and fixed it.

 

[kwabon]

Was this the Catholic Paper. I haven't seen it but have heard that there was an incorrect answer, similar to what you stated. Most schools picked it up and fixed it.

yeh, it was the catholic school trials, so we can agree on the fact that gravitational potential energy is decreasing?

 

[annabackwards]

yeh, it was the catholic school trials, so we can agree on the fact that gravitational potential energy is decreasing?

No, it's increasing.The gravitational potential energy of an object at some point within a gravitational field is equivalent to the word done in moving the object from infinity to that point. It is given by the formula

.

When an object moves from infinity to any point it moves with the gravitational field and gains Ke, losing Ep. Hence the Ep of any object in space is negative in value compared to Ep = 0 at infinity.
This can be mathematically shown by:

Ep at infinity > Ep at any point within a gravitational field​

But Ep at infinity = 0​

0 > Ep at any point within a gravitational field​

Ep at any point within a gravitational field < 0​

Now even from just looking at the formula, as the rocket gains alititude it's mass will decrease (as the fuel is being used) and the radius increases. Therefore, as time passes and the rocket gains alititude the GPE becomes a less negative number ie it is increasing, approaching the limit of 0.

You can say the GPE is decreasing ONLY if you specify that it's the magnitude that is decreasing, but even then you should say that it's negative and that the maximum possible value is 0

 

[Uncle]

That's not the right way to use that equation.
The potential equation that you are familiar with Ep=-GmM/r in my opinion is a terrible way of describing the purpose of that equation.
To fix it, its simple,

Let gravitational potential energy of an object be


Chances are you haven't looked at it this way right? The gravitational potential equation is indeed a function of position. ie its a function whose input is the position of the object, and once you substituted a position into that function it will spit out the gravitational potential energy of the object at that certain location that you inputted (relative to the axis you have chosen to use).

So to find the change in change in gravitational potential energy, its really the gravitational potential energy at the final position minus the gravitational potential energy at the initial position.

let and denote initial position and final position

so the change in gravitational potential energy is..





but in a rocket launch Xf>Xi

so

ie gravitational Potential energy of the rocket is increasing as it gain altitude

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.

^^ didnt know that GMm/x^2
thats also helps with a question my teacher couldnt get thnx

that's for one dimension, hence x was chosen as a coordinate.
we could have said GMm/(x² + y² + z²)

 

[kwabon]

No, it's increasing.The gravitational potential energy of an object at some point within a gravitational field is equivalent to the word done in moving the object from infinity to that point. It is given by the formula

.

When an object moves from infinity to any point it moves with the gravitational field and gains Ke, losing Ep. Hence the Ep of any object in space is negative in value compared to Ep = 0 at infinity.
This can be mathematically shown by:

Ep at infinity > Ep at any point within a gravitational field​

But Ep at infinity = 0​

0 > Ep at any point within a gravitational field​

Ep at any point within a gravitational field < 0​

Now even from just looking at the formula, as the rocket gains alititude it's mass will decrease (as the fuel is being used) and the radius increases. Therefore, as time passes and the rocket gains alititude the GPE becomes a less negative number ie it is increasing, approaching the limit of 0.

You can say the GPE is decreasing ONLY if you specify that it's the magnitude that is decreasing, but even then you should say that it's negative and that the maximum possible value is 0

but the answers of the CSSA physics for this particular question is increasing.
God dammit, i thought i knew Gpe, now due to the answers in the CSSA it is confusing the shit out of me.

 

[annabackwards]

but the answers of the CSSA physics for this particular question is increasing.
God dammit, i thought i knew Gpe, now due to the answers in the CSSA it is confusing the shit out of me.

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