Velocity Folding
conceptual physics question no math please help!?

Suppose a driver springs into the air with no initial angular velocity. Can the driver begin to rotate by folding into a tucked position?

Thank you!

Yes.

Remember that one always has to have something to push against in order to produce or change motion.

(That's why I hate it when the Hulk lifts an entire skyscraper and throws it: when he tries, he is pushing against the ground. Which seems more likely: that the ground is firm enough for him to even hold the incredible weight of the skyscraper not on its city block base, but on the area of his, admittedly large, feet OR that instead of lifting the skyscraper, he'd either drive himself into the far less resilient ground (in other words, instead of lifting the skyscraper by pushing against the ground, he'd be driving himself into the ground by pushing against the skyscraper) or he'd simply manage to rip off a chunk of the skyscraper's wall or facade? Yeah, that last one. On the other hand, Superman could do so because he can fly (somehow without disturbing anything around him) and so his flying power could be what he pushes against, not the ground. Except of course for when he grabs shoddy old skyscrapers that must not have been made with old-style, slip in during the night and help the cobbler, elven quality resulting in Supes also just ripping off a piece of the skyscraper...)

But... consider yourself in the air, with no anything to push against. You'd be able to push against the air, of course, but we'll consider that rather insignificant if you please (I know, it's not: consider how a tennis ball with spin imparted to it pushes against the air and drops like a rock or stays up amazingly far, or how a baseball curves in just 60 feet, both by pushing against the air). So when you fold yourself toward the tuck position, what would the dynamic for the pushing be? You'd be pushing/pulling against somewhere in your middle. The upper body portion of the tucking folding would, viewed from the side, be "forward" and down while the lower body portion would be backward and up which would net in a rotating motion. Just how much rotation would depend on your skill and body, but the more force you can achieve in the movement to the tuck position, the faster it will occur so your angular acceleration would be greater. The greater that angular acceleration, the more rotating you could do before plowing back into the ground. (Actually, as I'm sure you mean "diver" and not "driver" I suppose it should be more like "before slamming into the water.")

Simple as that.

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physics angular velocity question..help please!?

someone sits on a rotating stool and rotates at 30 rev/min while holding 10kg mass in each outstretched arm. The moment of inertia of the person plus the stool is 5 kg.m² and is considered constant. the two masses initially rotate at a radius of 1.0m but then the arms are folded to .15m. what is the new angular velocity of the system (in rev/min)??

In the beginning, was the total moment of inertia, MI1; and I saw MI1, and it was good. Not only was it good, but it was given by:

MI1 = 5.0kg m^2 + 2 (10)(1)^2 kg m^2 = (5 + 20) kg m^2
= 25 kg m^2.

But then arms were folded, and lo: there was a lower moment of inertia. And amid the gnashing of teeth and the renting of garments, the moment of inertia was found to be:

MI2 = 5.0kg m^2 + 2 (10) (0.15)^2kg m^2 = (5 + 0.45)
or 5.45kg m^2.

But the angular momentum was conserved, i.e. MI1 w1
= MI2 w2;

thus 25.0 w1 = 5.45 w2, so that w2 = (25.0/5.45) 30 rev/min

= 137.6... rev/min.

A dizzying conclusion !

Live long and prosper.

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