Expert: Jayendra Upadhye - 4/27/2007

Question
QUESTION: how move possible in space?
ANSWER: Hi,
there are two aspects to the answer.
1 - newton's first law..inertia..bodies in motion continue to be in motion.
a) - Any body entering space (say like a rocket that has been launched from earth..) will continue with its last "final velocity" into space.
b) - Gravity will start working on the body and begin reducing that speed.
c) - But if the body enters a stable orbit before its velocity becomes too low, then the body can continue to go around earth with "same tangential" velocity for a long time.
d) - the very thin atmosphere does slow down things in the end and all satellites "near earth" do fall down later.

This is "inherited motion".

2 - Newton's second law. (action and reaction).
when in space (say in stable orbit..) one is doing a space walk.
a) - Then to manouver (change ones orientation) or move slowly around the exterior (exra vehicular activity or EVA) one normally uses small controlled bursts of gas to get "pushes" in the right direction.
b) - The lunar module (apollo series) while descending to and ascending from the moon, used such thrusters situated on all its 4 sides, to control its 3 axial stability.
c) - The apollo command module also had the same on its outer peripheri.you can see these as small "cross" like sets or cluster of nozzles on the cylindrical outer peripheri of the command module in photographs.

That is "acquired" motion at "expense" of some hot gas.
(like in rockets)

Hope you have your answer.
jayen

---------- FOLLOW-UP ----------

QUESTION: if there is no friction in space how movement is possible?

Answer
Kunu..
All i can say is "oh dear"!!

Kunu, newton's law of action and reaction precludes friction completely from the picture.

Like one earth, pushing against something is understood to be "pushing against a water body with a paddle" or "pushing against a sold road using a rotating tyre surface" or "pushing against a solid ground using ones feet, in a walk"....

Rockets..and those thrusters are indeed controlled rockets..push on the "insides of their combustion chambers".

For simplicity's sake think how a baloon "rockets" through the air when you let go its nozzle. it is "moved" by the conservation of momentum principle.

the air exiting the baloon has a certain momentum. the remaining mass of "air in baloon + mass of rubber in baloon" gets equal and opposite momentum, keeping "system momentum zero. [conserving it to its original zero value].

In rockets of fixed size combustion chambers, the pushing force equals "cross-sectional area of nozzle * pressure in the chamber".

If pressure in the chamber is constant, gas exit velocity will be constant, and thus push pressure * nozzle cross-section will be constant. knowing the product, and the instantaneous mass of the rocket (mass depletes with depleting fuel) ..instantanous accleration can be found.

the instantanous velocity is a historical result of "initial mass" and "current mass".

It is also true that instantaneous momentum of rocket is a function of momentum of exiting gas from its nozzle.

The famed tsiolkovsky's rocket equation says it all.
[No i dont remember it ..except that it contains a log term in it].

Friction can be exploited as a driving mechanism (it is but one force linkage in the drive, as long as there is something to push against.

In absolute space, one "pushes against" something expendable ..inshort it boils down to

"there is no such thing as a free lunch in absolute space"!
It is always a diminishing returns game ...throw something permanently away (hot gas) and get movement in return.
And all the time the supply of throwable "fuel + oxidizer" is diminishing on the rocket.

The ultimate such diminishing returns game is that of entropy! No matter what one does it only increases, with no known way of reversing the process.
In the end it is all chaos and no cosmos when entropy wins and takes all.

Hope that suffices.
Jayen