On: Japan's zero-gravity space drone sends first pictu...

[_hmpc]

how does conservation of linear momentum apply? edit: this question was asked when the parent comment claimed that this machine could "generate linear momentum" from the wheels.

[venning → _hmpc]

Imagine holding up a bicycle in the air from the midpoint. Now spin both wheels in opposite directions. No movement. Now apply one brake. Now you have movement. Now do that in three dimensions with precision and you have this thing.

Conservation of momentum applies when there are "no external forces", which wouldn't be the case here when you apply the brake.

EDIT: Yeah, my apologies. I didn't mean to write "momentum".

[averagewall → venning]

The brake is an internal force though (the force and its reaction both act on the bicycle). The whole unit's only external forces are from the fans so that's all it can use to translate. That and bumping into things.

[TeMPOraL → averagewall]

> That and bumping into things.

Come to think of it - in air you're constantly bumping into things - air molecules. I think it could technically be possible to steer in zero-g within an atmosphere by pure rotation (e.g. consider that a fast-spinning object is kind of a (crappy) fan).

[LeifCarrotson → TeMPOraL]

And, in fact, that's what this device does.

It uses an array of 12 small rotating fans placed around its surface to push jets of air and move itself from place to place.

[DonHopkins → LeifCarrotson]

Can it face you with one of its fans and run it at the same high speed as the opposite fan, to keep you cool, and take photos of you with your hair blowing in the wind?

[TeMPOraL → LeifCarrotson]

Yes, I know.

My point was that in principle, you could abuse reaction wheels to give you translation capability if you're moving through air.

[_hmpc → venning]

though less technical, "movement" implies "momentum", no?

In the person + bike system, the brake action would result in internal forces. I don't see how this system could translate.

[venning → _hmpc]

Not sure about the movement-momentum question.

Thinking about it, the bicycle would only rotate about the center unless it had something to react against, as was pointed out by others.

[_hmpc → venning]

well, momentum is defined as the product of mass and velocity. movement implies velocity, and we are not talking about massless things here, so yes, movement implies momentum.