This discussion is now closed.
Check out other Related discussions
- OCR A Gravitational fields help
- GCSE Physics Question Help
- Gravitational potential is independent of mass
- Physics Past Paper Question on Efficiency
- the issac physics astronomy question
- physics
- GCSE
- Electric Fields Help
- CIE A-levels Physics
- A Level Physics gravitational forces
- Does having excess empathy often blow up in your face?
- How far from the Earth do you have to be to be at true 0 gravity?
- PAT space question
- Physics
- Help with dimensional analysis....
- Issac Physics Problem that I am stuck with
- AQA GCSE Combined Science: Trilogy - Physics Paper 1 (Higher) 8464/P/1H [Exam Chat]
- Maths - mechanics
- Career opportunities in psychology after masters
- Dr Kathy Romer answers your questions about Physics!
Gravitational Potential
A rocket has a mass 1000 kg and the planet has a surface potential equal to -100 MJ kg^-1 .
i. Assume the fuel is used quickly to boost the rocket to high speed, calculate the Gravitational potential energy (GPE) of the rocket that must be increased for it to escape completely.
I got the answer to this, 10^11 J.
ii. If the rocket is only given 40000 MJ of kinetic energy from the fuel, determine gravitational potential at the point.
I also got the answer for this, its -60 MJ kg^-1.
iii. Determine the maximum distance from the surface it can reach, given the planet's radius is 2 x 10^6 m.
Alright I can't go further here, can somebody help me? hehe thx!
i. Assume the fuel is used quickly to boost the rocket to high speed, calculate the Gravitational potential energy (GPE) of the rocket that must be increased for it to escape completely.
I got the answer to this, 10^11 J.
ii. If the rocket is only given 40000 MJ of kinetic energy from the fuel, determine gravitational potential at the point.
I also got the answer for this, its -60 MJ kg^-1.
iii. Determine the maximum distance from the surface it can reach, given the planet's radius is 2 x 10^6 m.
Alright I can't go further here, can somebody help me? hehe thx!
use GPE = mgh. You know what the GPE is at the surface, it's just -10^11J. And that's at a height of 2x10^6m. So then use that to determine mg, and then use that value of mg to find your new value of h.
Specifically: GPE(h1) = mgh1, GPE(h2) = mgh2....... h2 = GPE(h2)/mg = GPE(h2)/(GPE(h1)/h1) = h1 * GPE(h2)/GPE(h1) = 2x10^6m * -60/-100 = 2x10^6 * 6/10 = 3.3x10^4m by my reckoning. Although I just did all that in my head so I made have made a mistake
Specifically: GPE(h1) = mgh1, GPE(h2) = mgh2....... h2 = GPE(h2)/mg = GPE(h2)/(GPE(h1)/h1) = h1 * GPE(h2)/GPE(h1) = 2x10^6m * -60/-100 = 2x10^6 * 6/10 = 3.3x10^4m by my reckoning. Although I just did all that in my head so I made have made a mistake
Willa is wrong (sorry Willa)
you cannot use mgh as the value of g decreases as you move further from the planet.
when all the KE has been transferred into GPE the rocket will have potential energy equal to mGM / r where r is the distance from the planet.
The potential GM / r at this point = - 60 MJ / kg
What we need of course is the mass of the planet. Knowing the radius of the planet is 2 x 10^6 m and knowing the potential at its surface is -100MJ/kg we can calculate the mass of the planet. ( I get 3 x 10^24kg)
so now r can be calculated. I get 3.33 x 10^6m, which is 1.33 x 10^6m above its surface.
you cannot use mgh as the value of g decreases as you move further from the planet.
when all the KE has been transferred into GPE the rocket will have potential energy equal to mGM / r where r is the distance from the planet.
The potential GM / r at this point = - 60 MJ / kg
What we need of course is the mass of the planet. Knowing the radius of the planet is 2 x 10^6 m and knowing the potential at its surface is -100MJ/kg we can calculate the mass of the planet. ( I get 3 x 10^24kg)
so now r can be calculated. I get 3.33 x 10^6m, which is 1.33 x 10^6m above its surface.
Drummy
Willa is wrong (sorry Willa)
you cannot use mgh as the value of g decreases as you move further from the planet.
when all the KE has been transferred into GPE the rocket will have potential energy equal to mGM / r where r is the distance from the planet.
The potential GM / r at this point = - 60 MJ / kg
What we need of course is the mass of the planet. Knowing the radius of the planet is 2 x 10^6 m and knowing the potential at its surface is -100MJ/kg we can calculate the mass of the planet. ( I get 3 x 10^24kg)
so now r can be calculated. I get 3.33 x 10^6m, which is 1.33 x 10^6m above its surface.
you cannot use mgh as the value of g decreases as you move further from the planet.
when all the KE has been transferred into GPE the rocket will have potential energy equal to mGM / r where r is the distance from the planet.
The potential GM / r at this point = - 60 MJ / kg
What we need of course is the mass of the planet. Knowing the radius of the planet is 2 x 10^6 m and knowing the potential at its surface is -100MJ/kg we can calculate the mass of the planet. ( I get 3 x 10^24kg)
so now r can be calculated. I get 3.33 x 10^6m, which is 1.33 x 10^6m above its surface.
oops. yes you win! But then all you do is flip the height dependances from my answer, since the equation is ~1/r rather than r. which gives 20/6 * 10^6m, which gives 3.33x10^6m as you said - and then yes you should subtract hte radius of the planet as well!
well done. much better than my answer.
Related discussions
- OCR A Gravitational fields help
- GCSE Physics Question Help
- Gravitational potential is independent of mass
- Physics Past Paper Question on Efficiency
- the issac physics astronomy question
- physics
- GCSE
- Electric Fields Help
- CIE A-levels Physics
- A Level Physics gravitational forces
- Does having excess empathy often blow up in your face?
- How far from the Earth do you have to be to be at true 0 gravity?
- PAT space question
- Physics
- Help with dimensional analysis....
- Issac Physics Problem that I am stuck with
- AQA GCSE Combined Science: Trilogy - Physics Paper 1 (Higher) 8464/P/1H [Exam Chat]
- Maths - mechanics
- Career opportunities in psychology after masters
- Dr Kathy Romer answers your questions about Physics!
