GPE q. (1 Viewer)

[namburger]

Previous hsc question

http://community.boredofstudies.org/attachment.php?attachmentid=15860&d=1199167761

using ΔEP = - GmM/rF - (- GmM/rI)

1000000 = - GmM/20000 + GmM/0
= undefined

Am i using the wrong formula?
This was one of the previous posts, but i couldn't get the answer using the working out on that post.
Need help =D

 

[kooltrainer]

too harrdd, undo-able

 

[undalay]

read the question properly nam
AND OMG ITS JAAKKIE

edit:
You don't use the change in GPE formula for the first part basically.y.

 

[Mark576]

ΔGPE = -GMm/r₁ + GMm/r₂ = -GMm[r₂ - r₁]/r₁r₂
Let r₂ = 20000km = 2*10⁷m and r₁ = 1*10⁷m.
Then GMm = (1*10⁶*2*10¹⁴)/1*10⁷ = 2*10¹³
Now the change in energy moving from 20000km to 80000km is given by:
ΔGPE = GMm[8*10⁷ - 2*10⁷]/1.6*10¹⁵
Using GMm = 2*10¹³, this comes out to be equal to: 750000 J = 0.75 MJ. Notice at points I omitted the negative sign in front of GMm.

EDIT: Not sure about this but the answer comes out quite nice and seems correct.

 

[twilight1412]

youre using the formula wrong
the definition of the equation is
-the work required to move an object from infinity to a particular point in space

 

[me121]

I have provided an answer to this question in my HSC Past Paper Solutions document, available here.

 

[fedexpress]

hey...
too hard...lol typical jackie

 

[namburger]

thx =D

gay q., thought it said it takes 1MJ to reach from centre of planet to 20 000 km

 

[kooltrainer]

lala

 

[me121]

[noparse]

[/noparse]

 

[Mark576]

Thanks for that me121