Re: Rocket lift-off questions.
- From: "addams013" <mfunke@xxxxxxxxxxxxxxxxxxxxxxx>
- Date: 1 May 2006 05:55:56 -0700
Escape velocity 11 km/s. But certainly rocket does not blast off with
this speed.
Oh, there are a few things to keep in mind there:
First of all, 11 km/s is only the escape velocity at Earth's surface.
The escape velocity goes down as you get further away, because gravity
gets weaker as you get further away. Escape velocity is the minimum
speed that you need to go away *and never come back* (unless, of
course, you have rockets to turn around with).
Second, a lot of rockets don't achieve escape velocity. A lot of them
simply go into orbit, which requires less speed than escape velocity.
A satellite in orbit isn't in freefall because it's moved fast enough
to escape Earth's pull; in fact, it's Earth's pull that's holding it in
orbit! It's just that a satellite is moving fast enough *sideways*
that while it falls, it never actually intersects the Earth's surface
(the Earth's surface curves away too fast). As it turns out, for a
given altitude, the escape velocity is always the square root of 2
times the circular orbit velocity.
And is it possible to blast off rocket with acceleration much slower
than g? Will it fall or go upward?
Interesting question. Yes, you can accelerate much slower than one g
upward. The astronauts inside will still feel more than one g no
matter what, though, because any acceleration you generate during
launch will be felt in addition to Earth's pull. If you can keep doing
that for long enough, you'll eventually get to a point where Earth's
gravity has "died off" enough where even your meager velocity will
allow you to escape forever.
Can we propel rocket or any body in upward direction with 'constant
velocity', say just 5 centimeter per second?
You'll have to accelerate it from rest to 5 cm/s, but once you've done
that, there's no reason you couldn't eventually get to escape velocity
this way, too. (The escape velocity of Earth is 5 cm/s when you're
about 320 trillion km away from the Earth, if I've done my math right.
That's over thirty light-years -- by that time, other gravitational
fields will be much more predominant.)
Of course, in real life, this is somewhat impractical. Rocket engines
during launch tend to have a set mass flowing out the back at a set
velocity; the amount of *push* remains pretty constant, but the mass of
the rocket is constantly decreasing. Therefore, as the rocket engines
continue to fire, the rocket will accelerate more and more. But as a
thought exercise to see how rockets and physics work, your 5 cm/s
rocket is perfectly feasible.
.
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