The cat flip

[crwper 7]

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This is a little bit off topic (related to physics, but not BASE), but a friend and I were arguing the other day about whether or not there's a linear analog to this. Do you think, if you were floating around in zero g, you could get from point a to point b by small linear "scoots", the same way you can spin around while sitting on a barstool by several "flail" type motions that each rotate you a little ways?

I don't think so. You can actually generate a continuous rotation of your body if your swing your arms. Imagine that you are floating in the middle of space. Then begin windmilling your arms. This introduces angular momentum to your arms, which must be countered by an opposing momentum in your body, i.e. your body will rotate in the opposite direction. The reason this can work is because angular motions can eventually return to the starting point without changing the direction of motion, so you can do it indefinitely. With a linear motion, the only way to move forward would be to shoot something backward. But you can't get back to the "starting point" without reversing your hard work.

Michael

[crwper 7]

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I thought the law of conservation of angular momentum would prevent the body from doing 180 in dead air, unless you have a tail, which you can rotate to make your body rotate in opposite direction, like cats do.

For the physics-inclined... Imagine you're looking down on someone's head. They have just launched on their back and are now in freefall, but not yet with any appreciable windspeed. They are going to roll over onto their left side.

First, they cross the right arm and right leg over the left, and possibly also the left arm and leg under the right, but keeping them close to the axis of the body. This generates a small amount of angular momentum in the counter-clockwise direction (mass of arms and legs, but with a very small radius of motion). The body will respond by moving very slightly in a clockwise direction. Thus, total angular momentum is zero.

Next, the arms and legs are brought back to the starting position but around a wide arc. This means arms and legs as far as possible from the body. This is the same mass as the first motion, but over a much larger radius. Thus, more angular momentum in a clockwise direction. The body responds with a significant movement in the counter-clockwise direction.

When you have completed the cycle, you have at no point created anguular momentum (the total was zero all the time), but the net effect is that your body has rotated in a clockwise direction.

As I said in another post, part of the reason this seems wierd is because it's possible to carry out a rotational motion and wind up back where you started. The classic linear momentum experiment, where you throw a ball away from you in freefall, would look a little different if you could throw the ball away from you, then catch it again without ever actually reversing the momentum of the ball.

Michael