The discussion centers on the mechanics of an acrobat landing on a trampoline with zero net angular momentum. It is established that while an acrobat cannot generate net angular momentum, they can manipulate their body position by swinging their arms, which can create minor adjustments in orientation. This technique is similar to how stuntmen control their falls to avoid injury, utilizing body movements to manage angular momentum. The conversation also draws parallels with the falling cat's ability to twist and land on its feet, emphasizing the importance of rotating body halves around non-parallel axes to maintain stability during free fall.
PREREQUISITESAcrobats, stunt performers, physics students, and anyone interested in the mechanics of body movement during free fall and landing techniques.
Small or not, this is exactly what stuntmen do when they play bad guys falling off buildings. Sometimes you can see it in the movies, especially in cheap ones: the guy who's just been kicked, stabbed and shot, falls off the roof and suddenly starts swinging his arms around like mad. He's doing it to avoid landing upside-down and breaking his neck.rcgldr said:The acrobat can't generate any net angualr momentum, but an acrobat could move his body a small amount by swinging his arms around in one direction, which would move his body in the other direction. This would be a very small effect.
Normally the flailing of the arms is just for show. The intial fall motion determine the amount of angular momentum, and after that, the stunt person can pike or tuck to speed things up, or extend his arms to slow things down a bit. Stuntmen have enough experience to initiate the fall correctly, rather than trying to compensate for a mistake on the way down.Delta Kilo said:Small or not, this is exactly what stuntmen do when they play bad guys falling off buildings. Sometimes you can see it in the movies, especially in cheap ones: the guy who's just been kicked, stabbed and shot, falls off the roof and suddenly starts swinging his arms around like mad. He's doing it to avoid landing upside-down and breaking his neck.
Probably mostly so. It is also sometimes said that cats use tail rotation in free fall in order to land on their feet. While it might be a factor, the contribution would be too small to turn the cat around 180° within a few feet falling distance.rcgldr said:Normally the flailing of the arms is just for show.
Delta Kilo said:I was referring to something like this:
This guy jumps into the water which is a lot less forgiving than airbags. See how he purposely rotates his arms to prevent landing on his back.
A falling cat twists up to 180° by circling the front and rear halves of it's body while bent at the middle, it doesn't significantly change it's end over end rate of rotation. High divers normally do a double flip using the amount of pike to control the rate of rotation, although the woman does a back gainer from 120 feet in this video:A.T. said:A trampoline jumper would have use something similar to the falling cat, in order to turn 90° with zero net angular momentum, within just a few meters.
rcgldr said:A falling cat twists up to 180° by circling the front and rear halves of it's body while bent at the middle, it doesn't significantly change it's end over end rate of rotation.
By "end over end" I mean flipping like a front or back flip as opposed to twisting which is what the cat does in the video. Angular inertia for twisting is much less than for flipping (end over end). A person whirling arms won't generate a lot of flipping motion, but can generate some twisting motion. By bending at the waste and trying to whirl the legs and upper body, similar to the cat's movement, a person can twist while in mid air more effectively than trying to whirl arms.A.T. said:Not sure what you mean by "end over end rate of rotation" but here is a nice video to demonstrate what I mean.
What you say is that: The cat's moment of inertia around it's long body axis is less then around it's lateral axis.rcgldr said:By "end over end" I mean flipping like a front or back flip as opposed to twisting which is what the cat does in the video. Angular inertia for twisting is much less than for flipping (end over end).