PDA

View Full Version : Explain physics to a spastic


Lazarus and the Gimp
10-11-2005, 19:13:07
Would it be possible to travel into space slowly (ie- leaving Earth's gravity at a speed slower than what's supposed to be escape velocity) just to prove a point?

Answers must contain no equations or funny foreign letters. Only good peasant English words.

Dyl Ulenspiegel
10-11-2005, 19:18:26
Problems with your DIY spaceship?

Spartak@CPH
10-11-2005, 19:18:32
Dunno. Does that make me a spastic?

paiktis22
10-11-2005, 19:21:52
= , .

paiktis22
10-11-2005, 19:22:26

Spartak@CPH
10-11-2005, 19:22:50
eh?

paiktis22
10-11-2005, 19:23:31
It was just the secret of the universe.

Japher
10-11-2005, 19:30:31
of course it would be possible... space elevators would do just that.

Dyl Ulenspiegel
10-11-2005, 19:37:41
Originally posted by paiktis22
���=��� ��� ��� �������, ���� ������������� ��� ��� ��� ������������ ��� �������� �������� ���������.

Sounds greek to me.

HelloKitty
10-11-2005, 19:54:12
Originally posted by Dyl Ulenspiegel
Sounds greek to me. Thats the sound little boys make when molested by hairy men?

Lurker the Second
10-11-2005, 19:59:42
Originally posted by Japher
of course it would be possible... space elevators would do just that.

Or really long stairs. Or that beanstalk thing that Jack used.

Drekkus
10-11-2005, 20:06:39
spaceships take off really slowly, don't they? At least at first.

Lazarus and the Gimp
10-11-2005, 20:21:09
Originally posted by Japher
of course it would be possible... space elevators would do just that.

So all this "escape velocity" stuff is actually a load of crap, right?

Japher
10-11-2005, 20:26:20
no idea

mr.G
10-11-2005, 20:38:36
i have a idea

KrazyHorse
10-11-2005, 20:43:15
Originally posted by Lazarus and the Gimp
Would it be possible to travel into space slowly (ie- leaving Earth's gravity at a speed slower than what's supposed to be escape velocity) just to prove a point?

Answers must contain no equations or funny foreign letters. Only good peasant English words.

Yes. In fact, all the shuttle missions, satellite launches etc. do just that. Even the original moon landings, which involved completely escaping the Earth's gravitational well at no point travelled faster than the escape velocity at the Earth's surface.

The escape velocity from a planet's gravitational field goes down the further out you go. What the lunar landings did was to launch into an Earth orbit (still bound by the gravitational well of the Earth), meet up with a previously-launched fuel container, then launch again.

Escape velocity is indicative of the minimum energy requirement to get a mass out of the Earth's gravitational well. Think about the difference between throwing a ball 20 feet into the air and hauling it twenty feet into the air on a rope. The ball was never travelling fast enough to go twenty feet straight up on its own, but the energy you put into lifting it with the rope was exactly the same as the energy you would need to put into it to throw it fast enough to go twenty feet up.

Rockets are actually less efficient (energetically speaking) than would be either a giant launch cannon (throwing the ball up) or a space elevator (hauling the ball up on a rope). The reason for this is slightly more complicated. I can give it to you if you like.

Lurker the Second
10-11-2005, 20:46:42
I'd like to know how that beanstalk grew so high, please.

Japher
10-11-2005, 20:56:14
anabolic steroids

Gary
10-11-2005, 21:49:44
100-0 protection.

If you reach escape velocity, you need not expend any further energy to get away. You're already on your way. But if you want to leave slower, you can, but you have to keep that 'engine' going.

Lazarus and the Gimp
10-11-2005, 22:16:31
So how many other laws of physics are actually guidelines?

Beta1
10-11-2005, 22:31:57
All you ever wanted to know about gravity

http://www.acme.com/jef/science_songs/gravity-32.mp3

from

http://www.acme.com/jef/science_songs/

Lazarus and the Gimp
10-11-2005, 23:02:56
Downloading MP3's make the baby Jesus cry.

Immortal Wombat
10-11-2005, 23:49:34
Downloading that mp3 would make anyone cry.

The Mad Monk
11-11-2005, 00:16:19
Originally posted by Lazarus and the Gimp
So how many other laws of physics are actually guidelines?

It's not a guideline. If you want to toss a dead weight out of Earth's gravity well, it must be going at least that fast.

Objects that can propel themselves aren't covered to the extent of the effectiveness of their propulsion.

MOBIUS
11-11-2005, 01:28:16
I thought it was a spastic explaining physics to the rest of us...:cute:

KrazyHorse@home
11-11-2005, 07:01:41
Originally posted by Lazarus and the Gimp
So how many other laws of physics are actually guidelines?

Escape velocity is not a "law" of physics. It's a consequence which is fully derivable from things which are actually laws (or, more precisely, for things which are really good approximations) like conservation of energy and Newton's Universal Gravitation
for a specific situation.

Escape velocity is simply defined as the velocity with which an object would have to be fired from a planet's surface for it to completely escape the planet's gravitational field. Nothing more and nothing less.

Just becuase people have used it wrong or you interpreted it wrong, don't blame us.

People usually don't get the right answers from us because it's hard to properly explain most complicated situations without referring to all sort of complicated mathematics.

We don't just use them for the hell of it, you know.

Gary
11-11-2005, 08:26:55
Ah you say that, but we all know they're used to keep the priesthood free from the heretics and other unworthys.

KrazyHorse@home
11-11-2005, 08:38:29
I thought that was the reason we didn't shower.

MoSe
11-11-2005, 08:49:56
as they'd say in dutch:

jets, no propellers

MoSe
11-11-2005, 09:00:51
Originally posted by KrazyHorse
What the lunar landings did was to launch into an Earth orbit (still bound by the gravitational well of the Earth), meet up with a previously-launched fuel container, then launch again.

:eek:
WHA....???
:confused:

It's the first time I hear something like that!

Wait... are you talking of MANNED lunar landings (i.e. Apollos)???

1. I didn't know that Apollos took a breath in earth orbit before starting again towards the moon, but that's a detail a layman can easily have overlooked
2. I though Apollos brought all their fuel on temselves, never heard about pre-launching fuel tanks in orbit and never heard about Apollos rendez-vous with those tanks on theirway to the moon.

If instead you're talking about the several moon probe, robots etc missions, well, I knew no technical detail about them, so that would be interesting news to me
:)

Nills Lagerbaak
11-11-2005, 09:33:09
It's funny but I was thinking about this very same problem the other day. The old model they gave you at school was that a rocket is in effect a projectile (with only one moment of force, great enough to launch it into orbit), but of course a rocket is constantly burning fuel on its ascent.

MoSe
11-11-2005, 09:51:48
in theme with the thread title, you could say that in the simple model the missile gets fired up with a single initial spasm...

Kitsuki
11-11-2005, 09:53:08
I can give it to you if you like.

Lucky Laz!

(surprised nobody else made this joke...)

KrazyHorse@home
11-11-2005, 10:00:35
Originally posted by MoSe
:eek:
WHA....???
:confused:

It's the first time I hear something like that!

Wait... are you talking of MANNED lunar landings (i.e. Apollos)???

1. I didn't know that Apollos took a breath in earth orbit before starting again towards the moon, but that's a detail a layman can easily have overlooked
2. I though Apollos brought all their fuel on temselves, never heard about pre-launching fuel tanks in orbit and never heard about Apollos rendez-vous with those tanks on theirway to the moon.

If instead you're talking about the several moon probe, robots etc missions, well, I knew no technical detail about them, so that would be interesting news to me
:)

I fucked up. I forgot that the Apollo missions went with a Lunar orbit rendezvous, not an Earth orbit rendezvous. They did not dock with a fuel depot in Earth orbit.

They did, though, take a breather in Earth orbit. And at no point, as I said before, did they travel faster than the escape velocity at the Earth's surface.

KrazyHorse@home
11-11-2005, 10:08:26
Originally posted by Nills Lagerbaak
It's funny but I was thinking about this very same problem the other day. The old model they gave you at school was that a rocket is in effect a projectile (with only one moment of force, great enough to launch it into orbit), but of course a rocket is constantly burning fuel on its ascent.

Not just that; also, most launches put the rocket into an Earth orbit (i.e. they haven't actually escaped the Earth's gravitational field). Those that are actually interplanetary tend to stay in Earth orbit for one or two orbits in order to adjust their trajectory (it tends to be a bit off after the bumpy ride through the atmosphere). I don't know if any spacecraft we've ever launched has at any point gone faster than the escape velocity at Earth's surface. My feeling is that some of the interplanetary probes probably have.

KrazyHorse@home
11-11-2005, 10:12:24
Of course, you must bear in mind that at some point during its flight anything which escapes Earth orbit has travelled faster than the escape velocity at that point

If it only crosses over to escape velocity 10000 km above the Earth's surface then that escape velocity is significantly lower than it would have been on Earth's surface...

MoSe
11-11-2005, 10:21:09
Originally posted by KrazyHorse@home
They did, though, take a breather in Earth orbit.
yep, I had forgot/overlooked that http://www.nasm.si.edu/collections/imagery/apollo/FIGURES/Fig1a.jpg
Originally posted by KrazyHorse@home
And at no point, as I said before, did they travel faster than the escape velocity at the Earth's surface.
I had no obection on that part :)

actually, I can appreciate better now all the engineering issues and design decisions involved in the project...
but I don't want to ruin the thread :p

Funko
11-11-2005, 10:26:03
It's not rocket science.

Nills Lagerbaak
11-11-2005, 10:48:35
But surely at any point during a rockets flight it will never reach the escape velocity (at the earth's surface), otherwise no need for the fuel (wind resistance aside).

I mean anyone can see that, as Funko said, it's not rocket science.

MoSe
11-11-2005, 11:37:23
you forgot gravity
that's a bitch, man

MoSe
11-11-2005, 11:40:00
to you physicists...
we're talking about RADIAL velocity, do we?

IIRC we had a thread months ago wondering what the TANGENTIAL escape velocity would be on Earth...

Nills Lagerbaak
11-11-2005, 11:42:23
What? The whole point of he escape velocity is the speed required such that the deceleration due to gravity gives it a zero end speed, outside the influence of gravity.

MoSe
11-11-2005, 11:46:58
to Nills:

say, at Earth's surface, the escape velocity is V
at some height H the escape velocity would be V(H)

Imagine that at H you have V = V(H) = 0.4 V

if you lift off at V1 < V and apply no further energy (thrust), gravity will exert a force slowing your missile, and BY DEFINITION
IF V1 < V
THEN V1(H) < V

BUT if you can go on thrusting your missile after liftoff, then you can start at 0.6 or 0.3V, it doesn't matter, as long as you manage to have a a V > 0.4V at height H, that is AT THAT HEIGHT you are faster than the escape velcity required AT THAT HEIGHT. By definition, from that point on you need no more thrust, you're on your way out, starbound.

Funko
11-11-2005, 11:48:32
Yes, that's exactly right.

Nills Lagerbaak
11-11-2005, 12:02:18
Agreed. and well put!

Gary
11-11-2005, 12:02:44
Much the same thing from different viewpoints isn't it ?
Except, do you ever get outside the influence of gravity ?

Nills Lagerbaak
11-11-2005, 12:07:57
Well the theoretical escape velocity is calculated by integrating the force multiplied by the tiny increments in distance, and as the force is an inverse square relationship you can integrate to infinity and come up with a finite number.

KrazyHorse@home
11-11-2005, 12:12:05
Originally posted by Nills Lagerbaak
Well the theoretical escape velocity is calculated by integrating the force multiplied by the tiny increments in distance, and as the force is an inverse square relationship you can integrate to infinity and come up with a finite number.

I tend to think of things in terms of potentials.

Forces are just so...physics 101

Be glad I'm not solving everything with Lagrangians.

KrazyHorse@home
11-11-2005, 12:13:18
So to get a force I usually take the potential and then take the negative gradient...

KrazyHorse@home
11-11-2005, 12:15:58
Originally posted by MoSe
to you physicists...
we're talking about RADIAL velocity, do we?

IIRC we had a thread months ago wondering what the TANGENTIAL escape velocity would be on Earth...

The radial and tangential escape velocities are one and the same.

http://scienceworld.wolfram.com/physics/Eccentricity.html

KrazyHorse@home
11-11-2005, 12:16:56
as long as E > 0, e > 1 therefore the orbit is hyperbolic i.e. the mass escapes.

MoSe
11-11-2005, 12:16:58
OK, embarassingly delayed X-post
:o
______________
I'm moved :blush:

so, my previous question stands, I wasn't kidding:

DOES the bearing of the escape velocity matter, or is only its module important???

that is, say you need to go up at 10km/s from a point to escape.
Is it the same if you go 10km/s SIDEWAYS in that gravitational field?.....


hmmm....
I know that by falling in a force field, a body transforms its potential energy in kinetic energy but for the conservation principle the sum does not vary
the same applies in reverse for a body going up...
but then my physics is too rusty and my neurons too depleted to go on from there....
:)

KrazyHorse@home
11-11-2005, 12:19:03
See my previous post.

The escape velocity (more properly the escape speed) is independent of the direction in which the vector points.

Any mass with a positive total energy will follow a hyperbolic path. Any mass with a negative total energy will follow an elliptical path.

MoSe
11-11-2005, 12:23:06
Originally posted by KrazyHorse@home
as long as E > 0, e > 1 therefore the orbit is hyperbolic i.e. the mass escapes.

WHAT?
won't it come back in from the other end of the asymptote?????

;) ;) ;)

MoSe
11-11-2005, 12:31:07
Interesting
http://en.wikipedia.org/wiki/Escape_velocity

so at the Earth's surface (ignoring air drag/friction) an object without propulsion needs to go at

11.2 km/s to escape the Earth (ignoring air drag/friction)
42.1 km/s to escape the Solar System
~1000 km/s to escape the Milky Way
:)

KrazyHorse@home
11-11-2005, 12:31:14
The gravitational potential energy of a body is -GMm/R where M is the mass of the Earth, m is the mass of the body, R is the distance separating the center of the Earth and the center of mass of the body (radius of the Earth, in the case of surface escape velocity) and G is the gravitational constant 6.67 * 10^-11 Nm^2/kg^2

Kinetic energy is mv^2/2. Setting kinetic energy + gravitational potential energy = 0 gets you escape velocity.

Theoretically I should note that you must take the velocity of the earth's rotation into account when calculating v. The velocity you solve for will be the escape velocity of the mass relative to the centre of the Earth. If you launch going East you will gain ~1000 km/h boost from the Earth's rotation (depends on latitude). Also, the earth is slightly pancake shaped (oblate spheroid, e = 0.08) so we should take that into account too...

KrazyHorse@home
11-11-2005, 12:31:54
Originally posted by MoSe
WHAT?
won't it come back in from the other side of the asymptote?????

;) ;) ;)

:lol:

"Other side" of a hyperbolic asymptote indeed...

Dyl Ulenspiegel
11-11-2005, 13:18:32
:brwncard:

Fergus & The Brazen Car
11-11-2005, 13:23:06
Originally posted by KrazyHorse@home


"Other side" of a hyperbolic asymptote indeed...

>snap!<

With cutting comebacks like that, it's a wonder you aren't in training to be a drag queen...

>snap!<

MoSe
11-11-2005, 13:31:22
please, now, don't swing as low as to ask the escape velocity of a mass from Uranus.....

Fergus & The Brazen Car
11-11-2005, 13:46:43
Originally posted by MoSe
please, now, don't swing as low as to ask the escape velocity of a mass from Uranus.....


I had the equations already, but I rectum.


A fundamental mistake. Where shall I caecum ?

It's all alimentary.

Japher
11-11-2005, 13:48:39
don't use big words, it doesn't suite you

Fergus & The Brazen Car
11-11-2005, 14:00:09
Originally posted by Japher
don't use big words, it doesn't suite you


Is that a three piece suite you had in mind, covered in Dralon ?

Japher
11-11-2005, 14:01:17
that makes know since

Fergus & The Brazen Car
11-11-2005, 14:16:12
Originally posted by Japher
that makes know since


And so should prove familiar to you.

Lurker the Second
11-11-2005, 14:38:28
goddammit laz this is your fault

Japher
11-11-2005, 15:01:57
jes jes

Venom
11-11-2005, 15:11:28
Damn this thread. Damn it straight to hell.

MoSe
11-11-2005, 16:46:03
no escape from hell, at any speed

except for bats

Lazarus and the Gimp
11-11-2005, 19:54:28
Originally posted by KrazyHorse@home
The gravitational potential energy of a body is -GMm/R where M is the mass of the Earth, m is the mass of the body, R is the distance separating the center of the Earth and the center of mass of the body (radius of the Earth, in the case of surface escape velocity) and G is the gravitational constant 6.67 * 10^-11 Nm^2/kg^2



What?

Lazarus and the Gimp
11-11-2005, 19:55:32
I gave up Maths lessons at 16 (and gave up paying attention in them at 14). Seeing letters in sums makes my brain bleed.

Japher
11-11-2005, 19:56:45
x+y=2z


Graph it!

Lazarus and the Gimp
11-11-2005, 20:04:53
Why? Does it make a rude shape?

Japher
11-11-2005, 20:07:02
I don't know. My maths is pretty pathetic. Especially for an engineer.

KrazyHorse@home
12-11-2005, 05:57:01
It makes a plane passing through the origin, with normal (1, 1, -2)

Lazarus and the Gimp
12-11-2005, 08:30:54
What?

Immortal Wombat
12-11-2005, 09:09:19
That means it doesn't make a rude shape.

Fergus & The Brazen Car
12-11-2005, 09:55:58
Originally posted by KrazyHorse@home
It makes a plane passing through the origin, with normal (1, 1, -2)

>snap!<

Hot damn!


You'll be voguing like a natural next.


>snap!<

KrazyHorse@home
12-11-2005, 10:17:27
I have no sense of humour when it comes to math or physics. :(

Fergus & The Brazen Car
12-11-2005, 11:02:08
Originally posted by KrazyHorse@home
I have no sense of humour when it comes to math or physics. :(


No, but great hair humour.

Dyl Ulenspiegel
12-11-2005, 12:26:55
He's greek?

Fergus & The Brazen Car
12-11-2005, 14:33:38
Originally posted by Dyl Ulenspiegel
He's greek?


No, he had a mohawk for Hallowe'en.


It was great, went very well with his Bowie/Blair eyes.

KrazyHorse@home
12-11-2005, 20:36:25
Still have it...

Fergus & The Brazen Car
14-11-2005, 09:59:23
Originally posted by KrazyHorse@home
Still have it...


We all know you do, dahlingggg...

But have you tried passing it on to someone else ?


There's the Princeton rub....

Funko
14-11-2005, 10:28:33
Originally posted by KrazyHorse@home
I have no sense of humour when it comes to math or physics. :(

Shame, they are both very funny.

Nills Lagerbaak
14-11-2005, 11:23:51
They certainly are! Did you hear the one about the constipated mathematician? He sat down and worked it out with a pencil!

Funko
14-11-2005, 11:58:48
:lol:

MoSe
14-11-2005, 12:01:00
are you laughin at the joke or at the jest?
:lol:

Funko
14-11-2005, 12:37:22
Both.

paiktis22
14-11-2005, 19:42:09
Originally posted by HelloKitty
Thats the sound little boys make when molested by hairy men?


Venom
14-11-2005, 19:55:12
Why won't this thread die?

Japher
14-11-2005, 19:58:43
synergy

Colon
14-11-2005, 20:32:07
It's a perpetual motion thread.

Japher
14-11-2005, 20:37:20
perpetual motion exists to piss off Venom

MoSe
15-11-2005, 14:00:21
perpetual spasms

Sir Tristram
15-11-2005, 16:56:48
Originally posted by Nills Lagerbaak
They certainly are! Did you hear the one about the constipated mathematician? He sat down and worked it out with a pencil!

That was because he could not do logs?

Funko
15-11-2005, 16:58:39
:lol: :beer:

Nills Lagerbaak
15-11-2005, 17:28:32
Yay!! Quality.

MoSe
15-11-2005, 17:30:29
no, not off the top...