# Phsyics graph

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Thread starter 3 years ago
#1
Can anyone take a look at my graph for question 6c and let me know if it satisifies the mark scheme.

Also can anyone explain what the graph means. I thought that it wanted to see how the grav potential of moon varied with distance to earths surface so i assumed that it would start at -2.9MJ/kg and decrease in a 1/r fashion.

However i think the graph shows the potential of the earth-moon system.

I dont know if that even amde any sense any input is appreciated as always.
0
3 years ago
#2
Graph looks good. V = - GM/r so it starts off at earth's surface with large neg V as r is small (and M is large), and attractive force by earth is strong. It needs a lot of work done on it to escape earth's pull / potential well.

It then makes its way to the moon's surface, and you're right it's affected by both bodies. Their forces on it are opposite in direction so negate eachother. At one point the net force is zero, tho total V =/= 0 as its scalar.

It gets past the instantaneous point F = 0 due to momentum, then attracted to moon's surface as moon's pull starts to dominate. |V| is smaller here than on earth as moon's mass is much less than the earth's (tho r is smaller than on the earth's surface).

Worth noting all of this happens travelling straight from the Earth to the Moon. Would be a more complicated picture otherwise.

Also note grav potential is gpe per unit mass at a given point in a grav field, think of a test mass here. There are 'potential rings' of these points of a given r, in each of the earth's and moon's fields. The values in the graph are total V (earth & moon) at each point.
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Thread starter 3 years ago
#3
Much appreciated, when i first read the question i thought it was the gravitational potential due to the moon not focusing on that of the earth.

Thanks for clarifying
1
3 years ago
#4
(Original post by Shaanv)
Much appreciated, when i first read the question i thought it was the gravitational potential due to the moon not focusing on that of the earth.

Thanks for clarifying
Yes its total V. Also when I discuss V at each of their surfaces, I'm only focusing on V due to that body as it dominates over the body far away (as does the force).

Once rocket leaves either far enough, both forces and potentials are signif to the totals. The -63 potential at earth's surface is much larger (mag) than moon's, as M/r (earth) > M/r (moon), with M (earth) >> M (moon) and r (earth) > r (moon); mass difference dominates.
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