# Oliver Sacks & pushing the shuttle off course

1. Mar 3, 2013

### atyy

“Imagine Shane in space,” Oliver said, watching him with pride. “One violent movement and he would push the shuttle off course.”
- from Paul Theroux's My friend, the doctor

Is this possible, assuming Shane does not leave the shutte?

2. Mar 3, 2013

### Staff: Mentor

No, the shuttle is far too big to be pushed severely off course by a person, and the person splatting against the opposite wall would put it right back on course. But I expect that the intent of the sentence is about small things upsetting larger things. The very next sentence after your quote is:

"Several times he mentioned, with genuine amusement, the possibility of highly sophisticated technology upset or destroyed by such tics or flailings."

3. Mar 4, 2013

### atyy

So if the shuttle were smaller, he could push it off course? Or would the correcting splat on the opposite wall come faster, and so correct it faster?

4. Mar 4, 2013

### Staff: Mentor

No, you could not ever push it off course. This is a common perpetual motion fallacy. No internal action can ever change the external motion of an object.

Not even the initial push: while to an external observer the initial push may appear to move the shuttle, what they won't know is that all it did was shift the center of mass of shuttle and occupant, leaving the path of the center of mass unchanged.

That's a demonstration of conservation of momentum, similar to what is done in high school physics labs with kids on skateboards.

5. Mar 4, 2013

### Staff: Mentor

Re: Oliver Sacks &amp; pushing the shuttle off course

No, as he is inside the shuttle. As soon as he hits the other side both he and the shuttle stop moving and the course remains unchanged.

6. Mar 4, 2013

### Staff: Mentor

Note that because the outside observer can't follow the changing center of mass, he may THINK the TRACK was permanently altered.

7. Mar 4, 2013

### Staff: Mentor

More complete explanation, starting with some navigation definitions:

Course is the direction of travel.
Track is the path in 3d spacel; the line along which you travel.

If a person pushes off the right side wall of the shuttle, near the COM (so there is no rotation), a person on the outside would see the shuttle's course change slightly to the right. When the person hits the other side wall and stops, the observer sees the shuttle resume its original course, along a new track, just slightly to the right of its original track.

What the observer doesn't know is that the only thing that actually changed was a shift in the location of the center of gravity on the shuttle, with the center of gravity never actually deviating from the original track or course.

8. Mar 5, 2013

### atyy

Drakktth & russ watters, thanks! I will tell Oliver Sacks to read this thread:)

9. Mar 6, 2013

### CWatters

If the shuttle had a circular running track in the payload bay then an astronaught running one way round it would cause the shuttle to rotate in the opposite direction. However on it's own that wouldn't change it's course either.

Quite a lot of people have wasted time trying to invent a "reactionless rocket" but that would violate conservation of momentum.

http://en.wikipedia.org/wiki/Reactionless_drive

Edit: if the shuttle had it's engines or thrusters running then changing the direction they point by running around the track would have the desired effect of altering the course... but that's not in the spirit of the question.