Science Quiz
- Thread starter #1 Person
- Start date
The Last Conformist
Irresistibly Attractive
The moment of inertia for the body-half with extended legs is much higher than for the other one because the centre of mass is farther from the axis of rotation. That's why the later gets most of the rotation.
betazed
Seeking...
- Joined
- May 9, 2003
- Messages
- 5,224
Exactly.
Your go! So here are the steps it follows. We have to understand that there is already some initial angular momentum of the whole cat as it starts falling. If the initial angular momentum is absolutely zero then I am not sure how the cat will behave. Anyway,
step 1 - the cat rotates its two halves in opposite directions. the front legs towards the ground and the hind legs away from it.
step 2 - now it knows how much it is rotating as a whole from the initial angular momentum. it has to control this rate of rotation so that its forward legs point towards the ground when it reaches the ground. It controls this angular velocity by changing its moment of inertia by splaying its hind legs and with its tail.
step 3 - it hits the ground with its front legs and then rotates the hind legs towards the ground.
The Last Conformist
Irresistibly Attractive
As suggested by my title, a black hole continuously loses mass by Hawking radiation. However, if we placed a stellar-mass black hole in a corner of our universe that is totally devoid of matter it could suck in to make up for the loss, it would still experience net growth. Why?
Pirate
Emperor
Does it have to do with the pairs of particles that are constantly popping into existence from the vacuum? If the pair forms close to the event horizon then instead of quickly anhilliating each other, one particle is sucked into the black hole, the other is jettisoned out. Just a guess, but I've heard of this phenomenon somwhere before.
The Last Conformist
Irresistibly Attractive
That's part of the mechanism behind Hawking radiation (basically, the energy needed to give the escaping particle permanent existence is sucked from the black hole). Doesn't answer the question - it makes the black hole shrink.
Even though it is totally devoid of matter, it would still have light from all the other stars in the universe. This would be captured by the black hole which would thefore grow.
We could even add in the captured neutrinos now that we know they have some mass too - but they now qualify as 'matter' I guess.
The Last Conformist
Irresistibly Attractive
col's right.
It would actually suffice with the 2.7K background, because the effective radiation temperature of a stellar-mass black hole is alot less than 2.7K (millionths of a kelvin, IIRC).
Your go!
It would actually suffice with the 2.7K background, because the effective radiation temperature of a stellar-mass black hole is alot less than 2.7K (millionths of a kelvin, IIRC).
Your go!
The Last Conformist
Irresistibly Attractive
They've got energy, which the black hole converts to mass.
How can friction make an object increase in speed?
The Last Conformist
Irresistibly Attractive
Wind blowing on an object. Friction acts to pass some momentum fromt the air to the object, which, at least initially, accelerates in the direction of the wind.
Hmm - I see your argument and the way I phrased it doesnt exclude it but I'm looking for a situation where we have friction between a non moving object and a moving object and the moving object speeds up..
The Last Conformist
Irresistibly Attractive
In other words, we've got two objects in relative motion, and friction causes the relative motion to speed up? That's tougher ...
A wheel which is spinning too fast in relation to how fast it's going forward relative to the ground to fulfill the "rolling condition" (no idea what the correct English term is; angular frequency times radius equals centre of mass speed).
A wheel which is spinning too fast in relation to how fast it's going forward relative to the ground to fulfill the "rolling condition" (no idea what the correct English term is; angular frequency times radius equals centre of mass speed).
betazed
Seeking...
- Joined
- May 9, 2003
- Messages
- 5,224
Firction between a rotating body on a plane surface can make it move faster increasing the body's linear velocity. You can think of it this way. If friction was zero then linear velocity would be zero too!
Remember tire on ice! Putting spikes not only adds to grip but may add to speed.
EDIT: I see that LC and me posted together.
Not a spinning object. The object gains kinetic energy.
The Last Conformist
Irresistibly Attractive
Why does the question get redefined each time I answer?
Well, can't think of anything ATM then.
Well, can't think of anything ATM then.
I guess my object is to make people think and discuss rather than have a simple yes/no right/wrong answer.
betazed
Seeking...
- Joined
- May 9, 2003
- Messages
- 5,224
I can appreciate that.
Yes, or no is boring. 
To expand on LC's original approach, aa object moving thru air can benefit from friction in another way. Friction creates turbulence and some turbulence can lessen overall drag.
An example, is the flying formation of migratory birds. The typical V formation is because of turbulence created in the wake of the brids in the lead reduce drag for the birds in the rear. And there would be no turbulence without friction.
But I am also sure you had something else on mind.
Similar threads
