Re: Rocket lift-off questions.

neo wrote:
Escape velocity 11 km/s. But certainly rocket does not blast off with
this speed. On tv, i see rocket slowly lifting upward. How much
distance it covers in initial 4-5 seconds?

And is it possible to blast off rocket with acceleration much slower
than g? Will it fall or go upward?

Can we propel rocket or any body in upward direction with 'constant
velocity', say just 5 centimeter per second?

Ignoring the significant practical matter of propelling a rocket at constant speed up from the Earth into space, yes a rocket could "escape" Earth's gravity going a constant 5 cm/sec.

Escape velocity is defined relative to a particular place in the gravitational field--typically the surface of the gravitational source. So 11 km/sec is the escape velocity from the Earth's surface. At a point further from the Earth the escape velocity is lower. At many thousands of km from Earth the escape velocity will be less than the 5 cm/sec of this example.

Escape velocity is also defined for an object under the influence of only gravity, no propulsion and no drag or other forces. In other words, an object is given enough kinetic energy to bring it up to the escape velocity and then left alone. The object in that case will slow down, coasting to rest at infinity (theoretically speaking). The rocket in your example is NOT left alone, but is continually propelled--with diminishing impulse--to keep it at the constant speed of 5 cm/sec.

It is better to think of "escape energy" rather than escape velocity. It sounds so easy to consider we could escape Earth's gravity just ambling along at 5 cm/sec. But the energy required is at least M*V*V/2,
(V is the escape velocity, M is initial mass) regardless of how that energy is expended.

http://en.wikipedia.org/wiki/Escape_velocity

--
Steve Humphrey
(replace "spambait" with "merlinus" to respond directly to me)
.

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