Gravitational Potential Energy quiz Help Please!!!!!!!!!! (1 Viewer)

[carlytse621]

I am stucked on one of the dot point question and i need help ><'''

Satellite X has its orbit around Earth changed from an altitude of 10,000km to an altitude of 20,000km. Satellite Y has its orbit around Earth changed from an altitude of 20,000km to an altitude of 30,000km. Both satellites have a mass of 500kg.

a.) Predict the amount of work done on X compared to the amount done on Y.
I said the work done are same as they moved the same amount.

b.) calculate the amount of work done on each satellite to see if your prediction was correct.

I stucked on this.....i used the GPE formula to calculate their GPE changes (which is work done), and the 2 result are different, which means they didn't produce the same amount of "work done". But when i used the formula W=mgh and i got that X and Y produce the same amount of 'work done' and i am so confused!!!!

And i don't get the answer at all. It said "calculations show that the initial Ep of X is -3.119 X 10^10 J and final is -3.114 X 10^10 J for an increase of 5 X 10^7 J. The initial Ep of Y is -3.114 X 10^10 J and final is -3.109 X 10^10 J for an increase of 5 X 10^7 J."
so it means their work done are same. But i can't get any of the value the answer got (i.e. those -3.119 X 10^10 etc....)

I am sooooooooo lost!!!! so will it produce the same amount of "work done" actually???

Please help ><!!!!!!!!!!! Thanks!!!

 

[4theHSC]

I am stucked on one of the dot point question and i need help ><'''

Satellite X has its orbit around Earth changed from an altitude of 10,000km to an altitude of 20,000km. Satellite Y has its orbit around Earth changed from an altitude of 20,000km to an altitude of 30,000km. Both satellites have a mass of 500kg.

a.) Predict the amount of work done on X compared to the amount done on Y.
I said the work done are same as they moved the same amount.

b.) calculate the amount of work done on each satellite to see if your prediction was correct.

I stucked on this.....i used the GPE formula to calculate their GPE changes (which is work done), and the 2 result are different, which means they didn't produce the same amount of "work done". But when i used the formula W=mgh and i got that X and Y produce the same amount of 'work done' and i am so confused!!!!

And i don't get the answer at all. It said "calculations show that the initial Ep of X is -3.119 X 10^10 J and final is -3.114 X 10^10 J for an increase of 5 X 10^7 J. The initial Ep of Y is -3.114 X 10^10 J and final is -3.109 X 10^10 J for an increase of 5 X 10^7 J."
so it means their work done are same. But i can't get any of the value the answer got (i.e. those -3.119 X 10^10 etc....)

I am sooooooooo lost!!!! so will it produce the same amount of "work done" actually???

Please help ><!!!!!!!!!!! Thanks!!!

Ok I really didn't real most of your post (2.30 AM really tired) but its basically the GPE of the higher orbit- the GPE of the lower orbit... both have an increase of 10 000 and have the same masses so when applied into -Gm1m2/r u get the same answer,,,

 

[k02033]

a.) Predict the amount of work done on X compared to the amount done on Y.
I said the work done are same as they moved the same amount.

b.) calculate the amount of work done on each satellite to see if your prediction was correct.

I stucked on this.....i used the GPE formula to calculate their GPE changes (which is work done), and the 2 result are different, which means they didn't produce the same amount of "work done". But when i used the formula W=mgh and i got that X and Y produce the same amount of 'work done' and i am so confused!!!!

What you have said is absolutely true (except that change in GPE is -ve of workdone). The GPE change for the 2 objects should be differnet, as you have calculated using teh GPE formula. This is because change in GPE is a measure of the effect of a force on a system. The 2 objects have gone on a journey of equal distances, but the gravitational force from the earth is different for those 2 journeys. ie force is weaker for the object further out from earth. so the 2 force are different so obviously the effects of the forces on the system is going to be different.

The reason why mgh gives you the same workdone is because, if you use mgh, you are assuming that force of gravitity is just mg for both journeys, ie constant. so force are the same, effects on system are the same, GPE change is teh same. But in teh question given, the distances involved are huge, so we are not allow to just use mg as the force acting. We have to use F=-GmM/r^2

 

[carlytse621]

yea....i got different Ep for satellite X and Y
so it means the book is stupid!!! stupid book!!!!
anyway, thanks guys

 

[the-derivative]

yea....i got different Ep for satellite X and Y
so it means the book is stupid!!! stupid book!!!!
anyway, thanks guys

Yeah, there are a few flaws in Shadwick's book.