As Galileo quoted "All objects fall at the same rate in a vacuum".

So to answer your confusion (half of which you probably know already)...

An object that falls through a vacuum is subjected to only one external force, the gravitational force, expressed as the weight of the object (i think this is where your confusion lies within).

weight = mass X gravity

Now the value of gravity is 9.8 meters per square second on the surface of the earth. The gravitational acceleration g decreases with the square of the distance from the center of the earth.

The 2nd part that need to be understood is Netwon's second law of motion, force equals mass times acceleration

Force = mass X acceleration

But we also know that accelaration equals force divided by mass.

acceleration = Force / mass

AND with a free falling object, the net external force is just the weight of the object:

Force = Weight

Going back to what i started with on the weight, when someone pushes someone else in a vacuum, their own body mass is taken account to create acceleration (so yes you are right in saying they are weightless but the mass still exist).

Hope that roughly answers your 1st part (i know the explanation isn't great but my mind is kind of congested with 100s of others things at the moment lol).


Assuming i understood you correctly, you are asking why we can't use standard aircraft technology to go past the atmosphere and into space.

Well the key way the aircraft create lift is due to the atmosphere and the shape of the aerofoil. The fluid dynamics goes into all of the calculations but simply said the air below the wing itself has a slower flow of air with higher pressure and above the wing it has a higher flow of air with lower pressure, utimately holding them up in the air (this is how gliders work).

But to climb upwards into the atmospere, you need a form of acceleration i.e. like the engines. This creates the correct thrust to go up against any other force acting on the aircraft itself such as gravity (from the earth). It then uses the wings (or better said the shape of the aerofoil) to create furthur lift or drag (drag to go downwards back to earth). Hence the quickest way to climb up against gravity is still rockets (unless someone created antigravity propulsion systems lol)

Hope i understood your confusion and hope you understood my reply, although it's not very clear, sorry about that lol



[edit on 18-5-2008 by coldhand]

Ok, sorry if I'm sounding stupid but once you get to a very high altitude, wouldn't the bottom pressure of a plane just propel it further? I mean, much less atmosphere=less resistance via gravity.

I guess in the end, I am just so unsure of what a vacuum really is. It makes no sense in my head. When you get into dark matter and plasma, then you're talking a whole new encyclopedia of knowledge, I understand the concept of the string theory more than I understand a vacuum.

Lets think about this (PS I have no idea what I'm talking about, please correct me with evidence that my typically-conspiratorial mind can comprehend).


Can and why would a vacuum hold temperature?

Princeton Def. of Temp. -

the degree of hotness or coldness of a body or environment (corresponding to its molecular activity)

Perhaps I am confusing a lack of life with a lack of molecules. If so, what molecules are in a vacuum like outer-space?

Are there elements in space?

Originally posted by SantaClaus

So I have a theory that will probably be debunked by the first person that responds to my thread. Instead of using rockets to go STRAIGHT up into space, against ALL that atmosphere and inertia, why not use a plane's technology to get some of that atmosphere off your back? In other words, get at a cruising altitude, and then go further. Like a shuttle, except going UP.

Well there is a small company that did that. But they used a plane to fly another craft so high, then they separate and the rocket blasts it into space. The main raseon for them doing so was saying, nasa etc arnt working on getting people into space anytime soon. Its small compaines like them who will do it for us.

It was really good, the plane had a weird tale that could tilt for re-entry into the atmosphere so it didnt burn up.

Originally posted by SantaClaus

Ok, I understand the stones and the astronaut acting on each other, but they are both weightless. How do they act on each other with no gravitational pull?
I'm still not sure how things BEGIN movement in a vacuum.
So while we're at it, can anyone help me understand initial movement in a vacuum.

The answer to this is pretty simple. Just completely ignore gravity. Gravity does not matter in this context. All you're doing when you jump up is using the earth to push you up. And in that second that you are accelerating upwards, gravity doesn't matter (except to slow you down, which is irrelevant to this).

Initial motion in space is the same. Instead of the rocket pushing off the planet, it is pushing off many many small particles.
An astronaut throwing stones is exactly the same as standing up close to a wall and pushing it.

So the question is, by using a plane's natural affinity to get high in our atmosphere, couldn't we then have a much more fuel-efficient way of getting into space?

The key mistake there is believing that aircraft have a 'natural affinity' to get high. They don't. They want to sit on the ground, and will attempt to do so the entire time they are in the air. That's why planes have large, powerful engines that must run at all times while they're flying.

Getting a plane into the air requires of LOT of fuel. A 747 can hold about 100 tonnes. And the higher a plane goes, the thinner the atmosphere is, which means the engines have to work a lot harder.
Passenger jets usually cruise at about 40, 000 feet. The highest a manned aircraft ever got was about 120, 000 feet, if memory serves. Sounds really high, doesn't it? But in reality, 120, 000 feet is just 1/3 of the distance from the surface of the earth to when space officially starts, which is 100 kilometres high.

Aircraft, going by the name itself, need air to fly. 50% of the air is within 5 kilometres of the surface, with 80% being within the first 20 kilometres. To go any higher than 20km is really, really hard. The planes have to be specifically designed for the purpose, and can only hold the bare minimum of weight. Strapping a 130 tonne Space Shuttle to the thing could introduce a few problems.
And even if you did do all of that, all you've done is lifted the shuttle to about 1/10 of the total height it needs to get to (ISS orbits at 320+ km).


Hope this has gone some way to answering your questions.

There is no need to push off of anything. Any reaction causes an equal and opposite reaction. It's Newton's Third Law of motion. If you are floating in space and you throw an object in front of you, you will move backward with equal force. As to WHY that happens, is it really important?

[edit on 5/19/2008 by StarryEyed]