**Ask a monkey a physics question thread**

[Keith]

The singularity is pretty well defined, as far as I'm aware. It's a region of mass with zero volume in space. If it has volume, it ceases to be a singularity. And if it has no mass, then it has zero density, not infinity.

Can it have density if it has no volume?

[OngBonga]

Can it have density if it has no volume?

idk, ask mojo, I can only answer questions directed at mojo.

As for the falling thing, well I remember as a kid watching a tv programme where they threw eggs out of a plane in an effort to not break one, I think it was probably Record Breakers with that dude who died of lung cancer thanks to passive smoking. Roy Castle, that's his name. I digress. They eventually succeeded from what one would consider a ridiculous height considering it's an egg, the egg that made it hit a hill at exactly the right angle to basically continue its fall coming to a gradual halt, rather than hitting the ground perpendicular. So yeah I reckon it's doable if all factors are precisely correct, based on eggs.

edit - I just remembered they were putting a lot spin on the egg as they threw it, so if you ever find yourself falling out of a plane with no parachute, aim for a hill and attempt to get yourself spinning in the air in the direction you'll go down the hill.

[MadMojoMonkey]

idk, ask mojo, I can only answer questions directed at mojo.

lol

So yeah I reckon it's doable if all factors are precisely correct, based on eggs.

-.-
Not sure if it's physics or if it just sounds like physics.

Anyway, this throws a kink in wuf's question, because I thought it was implied that the jumper hit the ground perpendicular to their velocity... I.e. a square hit. If they're allowed to land on a curved slope, then that changes everything. The only thing to protect against is friction burns, then.

edit - I just remembered they were putting a lot spin on the egg as they threw it, so if you ever find yourself falling out of a plane with no parachute, aim for a hill and attempt to get yourself spinning in the air in the direction you'll go down the hill.

This is what a baseball pitcher does to throw a curve ball. The pitcher puts forward spin on the ball, causing the Magnus effect to pull the ball downward by deflecting the air flowing under the ball such that the air goes upward. Newton's 3rd says that if the ball pushes the air upward, then the air pushes the ball downward, making it curve toward the ground faster than gravity would normally pull it down.

You'd want to spin the other way to exploit the Magnus effect. This would send the air going above you toward the ground. Which would exchange your forward velocity for lift, reducing your rate of falling.

Unfortunately, the Magnus Effect tends to "spin up" the object. Once you start the rotation, it gets amplified. You might have more trouble keeping your limbs in close and not flailing wildly. The flailing would probably not be best for an injury free landing.

[OngBonga]

This is what a baseball pitcher does to throw a curve ball. The pitcher puts forward spin on the ball, causing the Magnus effect to pull the ball downward by deflecting the air flowing under the ball such that the air goes upward. Newton's 3rd says that if the ball pushes the air upward, then the air pushes the ball downward, making it curve toward the ground faster than gravity would normally pull it down.

You'd want to spin the other way to exploit the Magnus effect. This would send the air going above you toward the ground. Which would exchange your forward velocity for lift, reducing your rate of falling.

Unfortunately, the Magnus Effect tends to "spin up" the object. Once you start the rotation, it gets amplified. You might have more trouble keeping your limbs in close and not flailing wildly. The flailing would probably not be best for an injury free landing.

I think it was back spin, so when it hit the ground, the spin attempts to cause it to roll uphill. Of course, the spin is utterly overwhelmed by gravity, but the tiny effect it has could be the difference between it breaking and not.

Or maybe it was top spin so it hit the ground and immediately rolled quickly down the hill.

I'm pretty sure the spin is intended to help with the roll though, and not reduce its velocity.

[MadMojoMonkey]

Can it have density if it has no volume?

No and yes. This question is a bit trickier than it seems at first glance.


No because you're talking about mass density and implicitly talking about mass per unit volume. I'm not entirely sure if there are any substances which would allow us to talk about mass per unit area as such. Maybe the electrons in graphene?


Yes because mass density isn't the only kind of density worth discussing.

For that matter, electrons have measurable mass and charge, but no measurable volume. We have put limits on the biggest an electron can be, given our subtle observations, but we have yet to prove either that they have small size or that they have 0 size. Turns out that proving a measurement is exactly 0 is easier said than done.

If you're talking about a conductor with a net charge, then talking about the charge density of the conductor still doesn't really make sense, because all the net charge is located at the surface of the conductor. So when we talk about the charge density in that case, we're really talking about charge per unit area.

The same could be said for charge on a line, but in that case, we're making an idealization since there is no truly 1D conductor.