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# orbital mechanics question - can anyone help me ?

## orbital mechanics question - can anyone help me ? - Physics Forum

### orbital mechanics question - can anyone help me ? - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.

#1
11-09-2003, 07:59 PM
 steve mew Guest Posts: n/a
orbital mechanics question - can anyone help me ?

If :

1) A body at rest on the ground experiences a force due to gravity of 1g
and

2) A body traveling at 29,739,569.6 ms-1 @ altitude 150,000m experiences an
effective downward force of 0g due to escape velocity

then what effective gravitional force exists for a body at say 100,000 m
altitude and 2000 ms-1 ?

What is the formula to work this out ?

Can anyone point me in the right direction ? Any help greatly appreciated.

v = 29739569.6 ms-1

#2
11-10-2003, 02:30 AM
orbital mechanics question - can anyone help me ?

"steve mew" <[Only registered users see links. ]> wrote in message
news:sUxrb.16625\$[Only registered users see links. ]...
an

Wrong. The body at 150 km altitude experiences a downward "force" due to
gravity ( personally prefer to think of it as curvature of space). Because
of its velocity, it is moving away from the earth even as the earth's
gravity moves it towards the earth. When these effects are in balance, we
say the object is "in orbit." The *net* force may be 0, but thet does not
mean that the individual forces are zero.

To solve the problem in old-fashioned Newtonian mechanics, the "force" of
gravity varies as 1/r^2 (from Newton's Law F = -G*M1*M2/r^2), so for two
given objects at two different distances (m1 and m2 are the same r changes)

F1 = -G*M1*M2/r1^2
F2 = -G*M1*M2/r2^2

so

F2/F1 = r1^2/r2^2

OTOH, the "force" with which the circular motion is throwing the smaller
object outwards is given by

F = m*v^2/r

where v is the velocity perpendicular to the radius.

#3
11-10-2003, 04:36 AM
 steve mew Guest Posts: n/a
orbital mechanics question - can anyone help me ?

Thanks for the clarification and I am sure what you have given me is very
useful but I am not sure what to do with it. (been about 9 years since I was
ina physics class)
I need a rearranged equation to give me the net effect of force in the y
plane as a projectile is travelling in a decaying orbit.
How do I calculate y force given speed and altitude ?

Thanks

s

news:[Only registered users see links. ]...
Because
changes)

#4
11-10-2003, 08:58 AM
orbital mechanics question - can anyone help me ?

"steve mew" <[Only registered users see links. ]> wrote in message
news:gtFrb.17928\$[Only registered users see links. ]...
was

It's been about 20 years for me.

If the orbit is decaying, then there is something stealing angular momentum
and energy from the motion. This is called "friction" or "drag."

You need to know more, such as the drag coefficient. Given speed and
altitude you can calculate the energy and the angular momentum. Without
drag, the orbit will remain stable because angular momentum and energy are
conserved - i.e. they will not change. The motion and gravitation will stay
in balance.

To calculate the rate at which the object is "falling" - presumably the rate
at which the orbit is decaying, you need to know something else about what
is going on, such as what fraction of the energy or angular momentum is lost
in a given unit of time.

Tom Davidson
Richmond, VA

 Tags mechanics , orbital , question

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