[neilep (OP)]

reading Paul Fr question about spacecraft leaving the earth made me wonder why there needs to be an escape velocity of seven miles a second ? (unless I am mistaken)

Could not one just happily keep flying up at a good old 500 mph ?

After all...planes get off the ground quite easily !..Is there  some almighty barrier in the atmosphere that needs to be punched through at such a speed ?

Let me guess !..the atmosphere IS the barrier eh ?...but it's just air and stuff isn't it ?

Thanks

neil

[syhprum]

The escape velocity of about 7 miles per second has nothing to do with the atmosphere in fact the effect of the atmosphere is ignored in quoting this figure.
this is the velocity that a body would acquire if it fell to Earth from a quasi infinite distance sorely under the influence of the Earths gravity and conversely the velocity it would need to start off with to escape the Earth and not fall back.   

[daveshorts]

Yeah, the escape velocity from a planet is defined as the speed that you would need to be fired from a gun from the surface to keep on going to infinity.

If you had a big enough rocket you could escape the earth's gravity at a constant 500mph, but it would use loads of fuel as you would be fighting the earth's

[FreeThinker]

"If you had a big enough rocket you could escape the earth's gravity at a constant 500mph"

Uhh... but you'd fall right back to earth as soon as the rocket fuel ran out....  Escape velocity is the speed (relative to the earth itself) required to escape permanently.

The escape velocity can more accurately be described as just faster than the speed required to maintain orbit around the earth... go too slow, and you will fall down to earth...too fast, and you "escape" orbit, flying off into space.

This all has to do with how being in orbit works.  When you are in orbit, you are actually in freefall.  You just happen to be moving forward so fast that when you fall, you fall "around" the earth, instead of right into it.