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RE: Apollo Data and Energy



Hi all-
Ken Fox asks for insight, as follows:
******************************************************
Friends, I need some new insight.

I came across some data, presumably taken from NASA and printed in the
British Nuffield Physics program. It details the altitude and velocity of
the space craft as it travelled to the Moon. I expected to use this data to
show that while coasting , the sum of Potential energies ( Moon and Earth)
along with Kinetic would remain constant. In fact for much of the trip, it
does. However, for some of the early points there is a discrepency. I will
give the data: (hope the format comes through)

R from Earth(10^6m) : 11.054 26.306 54. 356 95.743
Speed (m/s): 8406 5374 3633 2619

My values of energy
in 10^5 J/kg: -8.87 -7.85 -7.79 -7.68
( and then it was constant at 7.68 +/- .01 til
the next burn)

I can make it work if v = 8420 5378 3636

I am I dealing with error in data or concept? Interestingly the data for the
return trip did the same thing making the energy lower for the distances near
the Earth with the same "discrepencies" in the speeds.
***********************************
Ken, you did not show your potential energy calculation, but I suspect
that is where the difficulty is. The rocket coasts in a straight line in
an inertial frame. It is not, however, following a straight line trajectory
from the earth to the moon in such a frame, because the moon is also moving.
Thus, the distance of the rocket from the moon is not just the earth-moon
distance less the earth-rocket distance. I think that things should work
out ok if you take the moon's motion into account.
Regards,
Jack