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Author Topic: What is Escape Velocity and why can't I climb a ladder to the moon?  (Read 13392 times)

Offline John Chapman

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Physicists and astronomers are always banging on about 'escape velocity'. Only yesterday I heard one moaning that carrying satelites was so expensive because of the incredible velocity needed to escape the Earth's clutches. 7 miles per second, and all that. Why?

Surely if a rocket is travelling in the right direction, regardless of how slow, eventually it will reach it's destination. In fact, why can't I build a gigantic pair of stepladders, tall enough to reach the moon, put a goldfish bowl on my head and spend the next ten years climbing to the moon? Once I got to the top I just need to wait for midnight and step off onto the moon's surface! This is a flippant, I know, but it illustrates my confusion about escape velocity.

I can understand the relevance of escape velocity in the context of throwing something into space by using a giant catapult, for instance, but not when that object has it's own means of propulsion.

So come on NASA. You don't need rocket propulsion. Just a large pair of stepladders!


 

Offline lightarrow

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Physicists and astronomers are always banging on about 'escape velocity'. Only yesterday I heard one moaning that carrying satelites was so expensive because of the incredible velocity needed to escape the Earth's clutches. 7 miles per second, and all that. Why?

Surely if a rocket is travelling in the right direction, regardless of how slow, eventually it will reach it's destination. In fact, why can't I build a gigantic pair of stepladders, tall enough to reach the moon, put a goldfish bowl on my head and spend the next ten years climbing to the moon? Once I got to the top I just need to wait for midnight and step off onto the moon's surface! This is a flippant, I know, but it illustrates my confusion about escape velocity.

I can understand the relevance of escape velocity in the context of throwing something into space by using a giant catapult, for instance, but not when that object has it's own means of propulsion.

So come on NASA. You don't need rocket propulsion. Just a large pair of stepladders!
Escape velocity: if you want to send an object out of the planet's grav. field *shooting* it out (rifle, cannon, ecc.), you have to give it the escape velocity. That's all. Of course, if the body has an engine (as a rocket) the concept doesn't apply.
 

Offline John Chapman

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Aha! So why does NASA talk about Escape Velocity in respect of rockets? That's the source of my confusion.
 

lyner

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Yes, you could climb a ladder to the moon - but someone would have to put it there (it would have to fitted to a rail going all round the equator - could be a problem!!)

It is (very remotely) feasible to use a cable, tethered to a Geosynchronous satellite, though.
See http://www.thenakedscientists.com/forum/index.php?topic=17911.0
and other "Space Elevator" links.
Once such a system was erected, the energy cost of launching into deep space would be  much reduced - rockets are hideously inefficient, particularly at low speed.

The term 'escape velocity' is used because people are familiar with it. It represents a measure of the energy needed and allows comparisons to be made. It's not very satisfactory but it works - once you know what they mean.
 

Offline Soul Surfer

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Once you get outside the atmosphere the only way you can support and accelerate an object against gravity is by throwing stuff away as fast as possible using a rocket motor.  This means that you are using up your fuel at a prodigious rate.  If you are only accelerating slowly it takes you longer to get to the place and speed you need to achieve and uses more fuel.   To go fast enough to achieve an orbit around the earth you need to get up to a horizontal velocity of about 5 miles per second The most efficient way is to do it as quickly as possible with an impulse.   To leave the earth entirely  you need to start with a velocity upwards of 7 miles per second but that leaves you with no speed at all once you have got a significant distance away from the earth.

It also matters in which direction you go and from where on the Earth's surface because the velocity at the earth's surface at the equator is about 1000 miles per hour or 1/3 mile a second
 

lyner

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Compare using a ladder to climb a skyscraper and using a rocket engine. The ladder wins on energy cost every time. But you have to build the ladder.
 

Offline Don_1

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Compare using a ladder to climb a skyscraper and using a rocket engine. The ladder wins on energy cost every time. But you have to build the ladder.

And you have to climb the bloody thing!!!
 

lyner

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Two ham sandwiches is all it would take.
 

Offline yor_on

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You are perfectly right about that ladder John, but even Mr Chem would have stop to rest at times while climbing it. If we say that you weigh about 80 Kg then the energy you would spend climbing that ladder should be approximately equivalent to what it should take a rocket of the same weight (the same velocity?) to get to the moon, I think?

Ah, it may depend on the rockets velocity? I probably should leave that calculation to our friendly mathematicians, thinking about it again:) but there ..must.. be a 'relation' somewhere between the both concepts.
And that ladder should work very well in space too as long as you follow the climbing rules, always have two grips on that ladder, one foot and one hand at least, perhaps one should do this barefoot?
And we will definitely need a really big fishbowl:)
 

Offline Bored chemist

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Don't forget that if you fall off the ladder from the top you will (neglecting air resistance) land at roughly the escape velocity for someone on the other side of the planet.
Incidenmtally,
"If we say that you weigh about 80 Kg "
then we are using the wrong units.
 

Offline Chemistry4me

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but even Mr Chem would have stop to rest at times while climbing it.
::)::)
Yeah whatever  ::)
 

Offline syhprum

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If you fell off the ladder at the top you would not fall anywhere you would be at geosynchronous height and float around the Earth once every 24 hours.
 

Offline Chemistry4me

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Really?
Now that is interesting.
 

lyner

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yor_on
The Gravitational Potential Energy is proportional to 1/r where r is the distance from the Earth's centre.
As it happens, the Geosynchronous orbit is in the order of ten times the radius of the Earth and Moon's distance is, again, about ten times that.
(I know that's a vast approx but it make the sums easier to appreciate)

This means that getting up into Geosynchronous orbit would take
1-1/10 = 0.9units of energy and getting from GEO to the Moon's orbit would need 1/10 - 1/100 = 0.09units.
That implies that you need about ten times as much energy to get to GEO as you need to get to the Moon from that distance.
The saving in energy needed to raise a payload to GEO height using the elevator would be enormous, compared with using a rocket (they have to use multistages for Earth launches, so that takes loads of extra fuel). With the elevator, you have only friction to provide you with losses.
Your estimate of equal energies is very pessimistic; I think we are talking hundreds of times the energy. I guess the Apollo mission stats would reveal the answer. I must have a look.
 

Offline Bored chemist

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If you fell off the ladder at the top you would not fall anywhere you would be at geosynchronous height and float around the Earth once every 24 hours.
Not unless you were at the right height when you let go.
I suspect that most people would try to cling on to the ladder and would therefore fall "down "it.
Excuse me while I run and hide behind coriolis forces.
« Last Edit: 19/02/2009 20:43:48 by Bored chemist »
 

Offline Chemistry4me

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Do they need a space suit? Or else they might get cold feet.
 

Offline Vern

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Do they need a space suit? Or else they might get cold feet.
If you fell off the ladder at the top you would not fall anywhere you would be at geosynchronous height and float around the Earth once every 24 hours.
Not unless you were at the right height when you let go.
I suspect that most people would try to cling on to the ladder and would therefore fall "down "it.
Excuse me while I run and hide behind coriolis forces.
Yep; a geosynchronous orbit is pretty high. The climber will get mightily tired getting up to around 26,000 miles. :)
Quote from: the Wiki Link
All geosynchronous orbits have a semi-major axis of 42,164 km (26,199 mi).[3] In fact, orbits [/url]
 

Offline yor_on

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Wouldn't the real problem be to get of that topmost rung at the correct time.
"A small step for man but a rather big one if the moon ain't there?"
 

Offline John Chapman

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Yeah, that's why I said wait until midnight. See? No-one can accuse me of not being scientific!
« Last Edit: 21/02/2009 23:26:13 by John Chapman »
 

Offline yor_on

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Thank you Sir.
But...

Isn't midnight always there.
Somewhere?
 

Offline Chemistry4me

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Yes, like here :)
 

Offline Chemistry4me

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That last sentence was false ;D
 

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