Author Topic: Population decline  (Read 2109 times)

Offline JoshW

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Re: Population decline
« Reply #75 on: July 20, 2020, 05:45:49 pm »
Quote
It constantly refers to escape speed as escape velocity, but nobody's perfect.
I always thought it was escape velocity. I'm fairly sure that was what it was called when I did physics A level
I dare say it's still called that by some physics teachers and in some physics text books but:
1 if you call something a velocity you have to include a direction (they never do)
2 the direction doesn't matter - ignoring air resistance, or on a planet with no atmosphere, you could "throw" it at escape speed in any direction from vertical to tangential to the surface and it would escape.

In fact if you ignore the resistance of the planet (!) you could throw it downwards.


Wait, mind blown.

So if I threw something at the escape velocity/speed at just above horizontal, it would escape the gravitational pull just as successfully as if I threw it perpendicular to the ground? Surely the speed required would be greater, as the vertical component would be lower otherwise.

ahh wait it's different to me throwing something because it'd have a constant force to keep it's speed up.

Hence the need for more propellant to keep the speed up.

Shit me, no wonder I flunked A level Physics.

Offline andrewmc

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Re: Population decline
« Reply #76 on: July 21, 2020, 08:14:10 am »
So if I threw something at the escape velocity/speed at just above horizontal, it would escape the gravitational pull just as successfully as if I threw it perpendicular to the ground? Surely the speed required would be greater, as the vertical component would be lower otherwise.

It makes more sense in energy terms.

Something in a fixed gravitational field has a certain quantity of gravitational potential energy. To 'escape' the gravitational field (i.e. to be able to travel away from it forever) you need to have more kinetic energy than the gravitational potential energy. Kinetic energy is just 1/2 m v^2, so is related to the speed only and not the velocity component (yes I know I used a v for velocity, but squaring it means the direction is irrelevant).

If you threw it straight down and it travelled through the planet, it would accelerate and then decelerate as it passed through, and would then be travelling at the original speed when it left on the other side, so no difference.

Offline andrewmc

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Re: Population decline
« Reply #77 on: July 21, 2020, 08:24:14 am »
Isn't that just to compensate for air resistance normally? Why space rockets launch straight up instead of tangentially like planes, which need air resistance to fly? Or at least controlled resistance. If there's no atmosphere there's no friction, and so I don't think it matters what angle it goes off at as long as it can overcome gravity. I suspect the lunar modules blasted off back to the orbiter vertically simply to minimise the amount of propellant required.

Rockets launch straight up - *initially* - because they need to get out of the atmosphere quickly. Once they are say 50 miles up though, which takes not very long at all, there is a lot less air. If you watch a rocket launch, they go straight up to start with, but very very rapidly turn over and start firing sideways. In the case of a rocket launched from the Earth, they use a less efficient but very powerful first stage to get up quickly out of the atmosphere and avoid 'gravity drag'. Basically all the effort you spend going up is wasted; what you want to do is go sideways. Once out of the atmosphere, the more efficient but usually lower thrust second stage has the job of adding orbital velocity - normally it needs to get to 17 thousand mph or so (the first stage will have helped here as well) before the thing crashes back down. By reaching orbital velocity (purely firing sideways) the thing keeps falling around, rather than down on, the Earth.

As for the lunar lander, I suspect it took off vertically rather than sideways because the rocket was on the bottom. Facetious answer I know...
But more practically, you can't just launch sideways as until you have enough horizontal speed you aren't in orbit, so you have to go up a bit first before you go sideways, and the lunar ascent module will have had to go sideways as quickly as possible. It can't just go vertically up to the command module as a) it would whizz past at 3,600 mph and b) the lunar ascent module would then fall back down. So the lunar ascent module must have also gone 3,600 mph sideways to match its speed (and then actually meeting up just involving tweaking your orbit slightly so you go slightly faster/slower).

Incidentally, the command module was orbiting at only 69 miles up, which presumably gives a bit of room for error but would be an unstable orbit around the Earth due to air resistance.

Offline mikem

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Re: Population decline
« Reply #78 on: July 21, 2020, 08:32:48 am »
Presumably landing lots of heavy equipment on the moon also has the potential to affect its orbit, as there's no atmosphere to absorb the forces involved.

Offline JoshW

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Re: Population decline
« Reply #79 on: July 21, 2020, 09:20:48 am »

As for the lunar lander, I suspect it took off vertically rather than sideways because the rocket was on the bottom. Facetious answer I know...

reminds me of the joke, why do scuba divers fall backwards off of the boat?

because if they fell forwards they'd still be in the boat

Offline mikem

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Re: Population decline
« Reply #80 on: July 21, 2020, 10:54:52 am »
Also, without a runway, a sideways rocket would tend to rotate it around its base, so it might just end up lying on the surface.

Offline Duck ditch

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Re: Population decline
« Reply #81 on: July 22, 2020, 04:01:42 pm »
And there you have it.  A report not released because MPs have dirty money with the Russian dirty London laundry property system.  Greedy greedy billionaires feeding off greedy Greedy millionaires, thus corrupting and manipulation politics.  Greed is winning over any political values.  Tax em to the hilt or kick em out.  These guys are rich from oil in a power grab not through any cleverness. let’s plough there money into green issues. Save the planet.
It shouldn’t be a left right thing.  but it will be. 
Oh and close tax havens too.  Sure the  uk can’t do this bit alone, but our oversees territories are all about greed.


 

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