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NickW
2009-Oct-12, 04:57 AM
Say I was in a perfect orbit around earth (say the altitude is 300km) and i threw something in the direction of travel, would that object just change orbit change in apogee and perigee or would it slowly spiral outward?

I ask because I am having a hard time understanding orbital mechanics.

01101001
2009-Oct-12, 05:08 AM
Say I was in a perfect orbit around earth (say the altitude is 300km) and i threw something in the direction of travel, would that object just change orbit change in apogee and perigee or would it slowly spiral outward?

It wouldn't spiral outward without constant acceleration.

Perfect circular orbit? Your briefly accelerated object would just go into a different orbit, a slightly less circular one.

As would you and your craft, for the equal and opposite force you experience as the thrower.

Ken G
2009-Oct-12, 05:12 AM
That is true, indeed both the object you threw, and yourself, would return over and over to the exact same spot where you launched the object (though not at the same time, the object you threw forward would take a little longer to return than you would). That's because of Kepler's laws, which say that if the only force acting is gravity, then no matter what the velocity is at some point (assuming it is not enough to escape altogether), the orbit must be an ellipse. That's a closed curve that returns over and over to the same point.

Tobin Dax
2009-Oct-12, 05:14 AM
Additionally, the position where you throw the ball would be perigee for the ball's new orbit (since it speeds up) and apogee for your new orbit (since you slow down).

Jeff Root
2009-Oct-12, 05:19 AM
This was asked in a thread at the end of August, just over a month ago.
My detailed answer was in post #14:

http://www.bautforum.com/1563559-post14.html

-- Jeff, in Minneapolis

NickW
2009-Oct-12, 05:28 AM
Thank you, Jeff. Your link really helped me visualize what is happening.

ETA: Sorry for not seeing the past conversation which you explained this.

mugaliens
2009-Oct-13, 12:27 AM
Say I was in a perfect orbit around earth (say the altitude is 300km) and i threw something in the direction of travel, would that object just change orbit change in apogee and perigee or would it slowly spiral outward?

Yes, it would result in a change in perigee and apogee, regardless of the original orbit.

2552
2009-Oct-13, 01:00 PM
Actually, only the apogee would change, the perigee would stay the same. The animation on wikipedia's Orbit page may help you understand:

http://en.wikipedia.org/wiki/Orbit#Understanding_orbits

zenbudda
2009-Oct-13, 04:53 PM
2552,

thanks for that link. i have actually played those gravity games like http://www.bigideafun.com/penguins/arcade/spaced_penguin/ where you launch objects through space filled with large objects (planets, moons, suns etc). you have to pick the correct velocity and trajectory to land your object (in the case of space penguin, it's a penguin) in a target. interesting/fun stuff.

cjameshuff
2009-Oct-13, 10:14 PM
Actually, only the apogee would change, the perigee would stay the same. The animation on wikipedia's Orbit page may help you understand:

This is true if it is thrown in the direction of motion relative to the planet, and if it starts out in a circular orbit or at perigee of an elliptical one. Throwing an object in other directions, from other orbits or from other positions in an elliptical orbit, will lead to variations in perigee, apogee, and/or inclination.

Tobin Dax
2009-Oct-13, 11:47 PM
This is true if it is thrown in the direction of motion relative to the planet, and if it starts out in a circular orbit or at perigee of an elliptical one.
[Bold mine.]
That does seem to be the case that the OP is asking about.