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evilthom
04-28-2001, 03:04 PM
I know that when in space objects become 'weightless' but does that mean then you could pickup/push an object away from you with a mass of several tons i.e. another spaceship. If this is true why don't space craft get knocked off course when Astronauts bounce off(push against) the sides of their ship to move around.

Podkayne
04-28-2001, 03:19 PM
There's a difference between weight and mass.

To remove the effects of gravity, imagine that you have a box on a nearly frictionless surface--ice, or an airtrack, or something like that--and you're trying to push it horizontally. Even though the weight of the object is not an issue, it's harder to push a 200 kg box than a 50 kg box. The 200 kg box just has more mass. You'd have to go to just as much effort to push such a box on the Moon, or in space.

In space, you don't have to work against gravity, or (in most cases) against friction, so that means astronauts can manhandle more massive stuff, but if there's a little 'ol astronaut trying to push a spacecraft weighing (or rather massing) several tons, the astronaut loses.

This is something they frequently get wrong in the movies. Can't count the number of times an astronaut effortlessly stops an object that by all rights should have crushed them into paste.

Okay, that was rambly, but I hope it makes some sense. There's probably a better thread in the archives somewhere.

Squink
04-28-2001, 05:15 PM
Spacecraft DO get knocked off course by the astronauts bouncing around inside them. Fortunately the force of exerted by an astronaut bouncing off a wall is counteracted by the equal force that the astronaut must exert in an opposite direction if he or she wishes to stay within the spacecraft.
Angular momentum (spin) does not have this neat self canceling feature, so an astronaut could eventually build up a significant spin by climbing around and around the circumference of a spacecraft like a hamster on an exercise wheel.

Chronos
04-29-2001, 02:52 PM
Angular momentum (spin) does not have this neat self canceling feature, so an astronaut could eventually build up a significant spin by climbing around and around the circumference of a spacecraft like a hamster on an exercise wheel.If you mean what I think you mean, angular momentum does have that "self cancelling" feature. It's called consevation of angular momentum. An astronaut could set his ship spinning in the manner you describe, but he'd be spinning in the opposite direction. If he grabbed ahold of a handhold again and stopped, the ship would stop spinning, as well.

Nothing that an astronaut does entirely within the spacecraft can change its course, for the same reason that you can't pick yourself up by tugging on your bootstraps.

Drake Christensen
04-30-2001, 12:12 PM
Astronauts moving around, along with the minuscule drag at that altitude, and the occasional orientation and reboost firings all affect the spacecraft. That's one reason people often use the term "microgravity" over "weightlessness."

RickJay
04-30-2001, 01:19 PM
Originally posted by evilthom
I know that when in space objects become 'weightless' but does that mean then you could pickup/push an object away from you with a mass of several tons i.e. another spaceship. If this is true why don't space craft get knocked off course when Astronauts bounce off(push against) the sides of their ship to move around.

You're sort of asking two questions, both of which are easily answered:

1. As has been alluded to, objects become weightless but their mass is the same. Of COURSE you can push a big object around in space. However, as you know, for every action there is an equal and opposite reaction.

Imagine you are in space, on a spacewalk, and you try to push your spacecraft away from you. You are also, if you think about it, pushing yourself away from the spacecraft, right? Both you AND the spaceship will move away from the point at which you're pushing it. You will, in fact, successfully push the spacecraft away from you, but you are also pushing yourself away from it

However, you have much less MASS than the ship. Something that has more mass has more inertia; it will accelerate less with the same amount of force than will a smaller object. So while you will be pushing both the ship away from you and you away from the ship, YOU will move away from the point you did the pushing much faster than the ship will. The force of your push acts equally on both you and the ship, and that amount of force will propel you (with maybe 150kg of mass, including your spacesuit) much faster than it will the 10,000kg spacecraft.

2. As has been pointed out, unless you go flying out the door, you can't do anything inside the spacecraft you won't cancel out.

Think about it; you are inside the International Space Station. You start on one side by pushing away from the side of the station with your feet. As you have said, this will cause a very small correction in the space station's course, since you have pushed it. However, that change will be immediately cancelled by the exact opposite change when you crash headfirst against the other wall and break your skull, which will push the station in the other direction.