question 16 ? (1 Viewer)

[danchurchill]

i think that was it. this was the only question i was unable to answer.
it was a 3 marker question and asked you to work out the amount of energy required to move an object 80000km away from a planet knowing that it takes 1MJ of energy to move it from 10000 to 20000.
i was starting to use the gravitational formula to work it out but then realized u dont have the mass of the object. then i tried to work out the mass of the object and realized that the planet ISNT earth so we dont know the mass (double doh!)

so how did u do it! lol
im thinkin now about having to use a ratio sort of thing ? ?

 

[AngelButterfly]

i think i did it wrong lol
i couldnt think of how to do it so i just made something up, i cant even remember what i wrote lol

 

[shinji]

i just used the graviational potential energy formulae assuming tht work done to move an object a certain distance is stored in the objects potential energy.

hence i just assumed M1 x m2 is a konstant, worked it out. and had 250,000 joules to move the object 80,000 km. .. which DOESN"T make sense!!!!

wah.

 

[Shady01]

Idk if I'm right becuase i neglected the fact that as you get further away the grav field of a planet lessens, but i did:
i said if it takes 1.0Mj to move 20,000 kms i think it was?
then to move the other 60 to the point 80,000kms it must have to use 6.0Mj of eneery? I seriosuly doubt that is right but.

 

[brendanm88]

C = Change
Ep = Grav. potential energy
r = radius

C.Ep = -GMm / C.r

1x10^6 = -GMm/(1x10^4)

-GMm = 1x10^10
...........................

C.Ep = -GMm / C.r

= (1x10^4)/(6x10^5)

=166.67 KJ

Im pretty sure this is how its done

 

[Ioup]

Yeah, I did much the same as Shinji to calculate the gpe knowing m1m2 is constant. Then I just used the change in gpe equation.

 

[mlinger]

I had no idea with this question aye.. so i jus wrote the Ep formula down.. might gain a mark for identifying the right formula... ohh and isnt ur answer above being in KJ to small considering it took 1.sumthin MJ to move it just 10000km???

 

[Riviet]

I wasn't sure either, but I did remember to convert km into metres.

 

[brendanm88]

Gravitational potential energy decreases the further you get away from Earth, so it should be smaller, but im not too sure about that answer

 

[Tbomb2k]

C = Change
Ep = Grav. potential energy
r = radius

C.Ep = -GMm / C.r

1x10^6 = -GMm/(1x10^4)

-GMm = 1x10^10
...........................

C.Ep = -GMm / C.r

= (1x10^4)/(6x10^5)

=166.67 KJ

Im pretty sure this is how its done

Yeh i think your wrong cuz it should be grater than 1MJ to lift it to a height like greater than before.

I got 6MJ too and so did a few ppl

it's just W=fs
and u end up getting 7MJ to take it to 80 000km so since it's from 20 000km then it's 6MJ i.e. 1MJ per 10 000km lifted.

Well that's what i think is the way. fuk i'm not real confident though.

 

[Sgs2006]

C = Change
Ep = Grav. potential energy
r = radius

C.Ep = -GMm / C.r

1x10^6 = -GMm/(1x10^4)

-GMm = 1x10^10
...........................

C.Ep = -GMm / C.r

= (1x10^4)/(6x10^5)

=166.67 KJ

Im pretty sure this is how its done

Yea that is correct

 

[danio33]

yeah I think that I'm way wrong. I used Ratios but ended up just confusing myself. I got like 1.75 X 10^10 Joules which cant be right. I'm just hoing i get marks for working

 

[Lockhart]

Unless i've remembered the question horribly wrong, or I made a stupid mistake. Wasn't the quetsion asking for us to move something from the centre of the earth to 80000km away. In which case what puzzels me is how we are meant to calculate the poteintual energy of the obect is when it is in the centre as the distance in the eqausion is equal to zero the potientual energy become an negitive infinate number as it is -MmG/r.
If that follows it requires an infinate amount of energy to move the object away from the centre. That really confuses me. In thye end I think I said something like its 8 times the distance therefore 8 times the Ep because of a linear relationship.

I'm hoping for 1 mark?

 

[Mumma]

I got exactly 750,000J or 750KJ.

I have attached my solution, I hope it makes sense (and that its right)...

 

[MrMe]

HAha i didn't have the foggiest... but the only one so i was happy!

 

[Shady01]

i think 6Mj is wrong, but people it wasnt earth it was a random planet, random planet haha

 

[Jameswilmot2000]

just used the graviational potential energy formulae assuming tht work done to move an object a certain distance is stored in the objects potential energy.

hence i just assumed M1 x m2 is a konstant, worked it out. and had 250,000 joules to move the object 80,000 km. .. which DOESN"T make sense!!!!

wah.

yeah i did what you did and it certainly didn't make any sense whatsoever ...

 

[aunshi]

Yeah, I got 0.75MJ too

 

[brendanm88]

The value of GMm is worked out from the first part of the question and then substituted into the second, you dont need the mass of Earth anywhere in the question. And Mumma, the grav potential energy equation is not linear, so you cant use your method.

 

[Roobs]

i back the 0.75MJ answe--did it a diferent way to the pdf posted, but got the same asnwer