Help!!! HSC 2006 Q.18 ... Solution need it . plzzz (1 Viewer)

[GG-Legit]

Can some one plzz do this ..

http://img175.imageshack.us/img175/4403/18fx6.jpg

 

[Forbidden.]

Skip straight to Part 2 if you don't need to see how the change in gravitational potential energy was derived.
All units are SI units i.e. distance is all in metres and energy is in joules.


Part 1 - The derivation for the gravitational potential energy.

F - final
I - initial

ΔE_P = - GmM/r_F - (- GmM/r_I)
ΔE_P = - GmM/r_F + GmM/r_I
ΔE_P = GmM/r_I + GmM/r_F (Switch signs)
ΔE_P = GmM (1/r_I - 1/r_F) (Factorisation)



Part 2 - Finding GmM from the work done to move an object from 10,000,000 m to 20,000,000 m given the work required is 1.0 MJ

NOTE:
Avoid substituting 6 x 10²⁴ kg into M for the planet as the question did not state the planet was Earth.

ΔE_P = GmM (1/r_I - 1/r_F)
10,000,000 = GmM (1/10,000,000 - 1/20,000,000)
GmM = 10,000,000 / (1/10,000,000 - 1/20,000,000)
GmM = 2 x 10¹³



Part 3 - Using GmM to calculate work done to move object from 20,000,000 m to 80,000,000 m

ΔE_P = GmM (1/r_I - 1/r_F)
ΔE_P = 2 x 10¹³ (1/20,000,000 - 1/80,000,000)

ΔE_P = 750,000 J

 

[GG-Legit]

Thanx for the solution mate:

but ,

i think you misread the question:

For Part 2:its suppose to be
Finding GmM from the work done to move an object from "centre of the planet" m to 20,000,000 m given the work required is 1.0 MJ


For Part 3 :its suppose to be
Using GmM to calculate work done to move object from "Centre of the planet"m to 80,000,000 m

and btw: MG = 10^6 J

You should check it ..

 

[Forbidden.]

Thanx for the solution mate:

but ,

i think you misread the question:

For Part 2:its suppose to be
Finding GmM from the work done to move an object from "centre of the planet" m to 20,000,000 m given the work required is 1.0 MJ


For Part 3 :its suppose to be
Using GmM to calculate work done to move object from "Centre of the planet"m to 80,000,000 m

and btw: MG = 10^6 J

You should check it ..

Yeah, just modify my working by replacing my values with your own.
It happens to me quite often.

 

[GG-Legit]

Yeah, just modify my working by replacing my values with your own.
It happens to me quite often.

But doesnt the vaue going to be zero when you are calculating it from planet.

thanx .... that will do ..

just wanted to check answer with you

 

[kooltrainer]

Skip straight to Part 2 if you don't need to see how the change in gravitational potential energy was derived.
All units are SI units i.e. distance is all in metres and energy is in joules.


Part 1 - The derivation for the gravitational potential energy.

F - final
I - initial

ΔE_P = - GmM/r_F - (- GmM/r_I)
ΔE_P = - GmM/r_F + GmM/r_I
ΔE_P = GmM/r_I + GmM/r_F (Switch signs)
ΔE_P = GmM (1/r_I - 1/r_F) (Factorisation)

how did the plus sign change to minus sign from ur last step?

 

[Mark576]

It was just a mistake.

 

[xiao1985]

3rd last step -> 2nd last step was the mistake.... last step was correct.

 

[Forbidden.]

I hope you all do/did pay the same attention in class when your teachers make stupid mistakes.

 

[Mark576]

3rd last step -> 2nd last step was the mistake.... last step was correct.

Yeah xiao I knew that, perhaps I should of said 'it was due to a mistake'.