Can you Throw a Ball Out Of Orbit?

reply to post by ngchunter


eureka! The diagram really helps, thank you. I am a little slow at grasping the idea, but I am starting to see it. Many years ago I read a book on the special theory of relativity and it made me head hurt for a week. Seeing this diagram reminded me of something I read in the book. Einstein says that there is no absolute state of motion and that your movement is always relative to something else.

I find it hard to think of the space station as a platform that is moving relative to the earth. When I see video for some reason I imagine it is stationary with the Earth drifting along below it. Even though the arch of the orbit diminishes the effects of gravity the ball would still be traveling relative to earth. The escape velocity will always be the same relative to earth and NOT the astronaut throwing the ball.

Am I on the right track?

Originally posted by Setharoo

I find it hard to think of the space station as a platform that is moving relative to the earth. When I see video for some reason I imagine it is stationary with the Earth drifting along below it. Even though the arch of the orbit diminishes the effects of gravity the ball would still be traveling relative to earth. The escape velocity will always be the same relative to earth and NOT the astronaut throwing the ball.

Am I on the right track?

I think you're starting to get it. A simple analogy would be to imagine a hula hoop representing the orbit of the ball and when you throw the ball towards the earth, represented by the person holding the hoop around them, it pushes the hoop a little towards them on one side and away on the other side. Escape velocity would be if you could bend the hoop so hard it breaks and becomes a "U shape." Likewise, the best way to reach escape velocity in orbit is to accelerate your spacecraft along your velocity vector (the path that your spacecraft follows illustrated by the black circle), called prograde. That will change the shape of your orbit so that the point opposite of where you are goes further and further out from your parent body while the spot you are at does not change in altitude. At a certain velocity the circle "breaks" into a parabola shape and that is your escape trajectory.

[edit on 8-8-2008 by ngchunter]

reply to post by ngchunter


The broken hula hoop analogy is very helpful. Thank you. The problem is that with every answer you give me I think of 10 new questions. I am sure that I could research this all on the internet, but just don't have that kind of ambition anymore. I am new to the world of "feeds" and "threads" and I now totally see why so many people are using them. This is a great way to get targeted information!

I really am starting to understand escape velocity. It has always been a question in the back of my mind and now that I am thinking of orbits in a whole new way, my curiosity is growing. I envy you people that can understand this stuff so easily.

Now I have a new question: Lets say that NASA has already built the proposed space elevator (platform in space tethered to earth by dental floss) and the astronaut takes his ball to the top of this platform, but on this trip he brings his "magic ball". The magic ball is special because it has a magic rocket that keeps pushing in the direction the ball was thrown. The astronaut throws this ball from the top of the platform directly away from earth at 30mph and the rocket keeps it at this velocity. The way I see it we have just achieved escape velocity at 30mph!

Damn! After I typed this I realized the flaw in my thinking, but I will post it anyway. OBVIOUSLY NASA does not have "magic rockets" and as soon as the rocket runs out of the pixy dust is was burning for fuel, the ball would slow down and reverse direction and eventually crash back to Earth. I would assume that escape velocity is a relative term that assumes there is not an endless supply of fuel.

Thanks for all the info.

The best way to think about an orbit is to realize that things in Earth's orbit are still falling towards the earth due to gravity. But since these objects in orbit also have a "sideways" velocity, the falling object "misses" the ground becuase the spherical Earth's surface curves back under itself before the falling object actually reaches the ground.

For instance the space station is actually "falling" but it also has a "sideways" velocity of about 17,000 mph. This velocity is enough to keep the space station from hitting the Earth's surface, since the surface curves under itself.

If you go fast enough (roughly 25,000 mph) your sideways velocity would take you in a straight line out of the grasp of Earth's gravity rather than in the eliptical line of an orbit. This eliptica path of a orbit is actaully caused by gravity pulling an object back towards the earth.

Another key point is to realize that an orbit is defined by gravity; it is not defined by the lack of gravity (i.e. objects in orbit are subjected to 99% of the Eargh's gravity).

Here is a wikipedia article with a very good graphic that will explain it all:
en.wikipedia.org...

Originally posted by Setharoo
The astronaut throws this ball from the top of the platform directly away from earth at 30mph and the rocket keeps it at this velocity. The way I see it we have just achieved escape velocity at 30mph!

Basically, the energy needed to do this would be the same as if you expended it all at once at the initial throw to achieve escape velocity with no further thrusting, with just enough energy that you're traveling at 30mph relative to earth once you leave the earth's gravity well. In truth, with a real rocket expending fuel (and therefore losing mass over time as it thrusts), it's "cheaper" from an energy point of view to expend the fuel all at once rather than slowly. Why? Because if you expend it all at once then you don't have to carry that added mass with you as you climb your way out of the gravity well of earth. Imagine you're at the bottom of a hill and you have a backpack. If you take the backpack off before you start to climb you'll have an easier time than if you slowly drop items out of your backpack as you climb at a slower speed.

[edit on 12-8-2008 by ngchunter]

wait a second... That one comment about the astronaught weighing as much as the ball, that doesnt make any sense... I thought that in space it doesnt matter how much you weigh, what matters is the density. or what ever. (sorry im only a 13 year old) I'm just looking for some cool information... and I definatly got it! thanks for all the information! and by the way, it's way cooler to read the page al the way through! : )