Why escape velocity?

[tracer]

*Originally posted by The Bad Astronomer *
escape velocity of the solar system starting at the distance of the Earth = 1.4 x 30 = 42 km/sec. So you’d need to add over 30 km/sec to your rocket to get it to escape the solar system.

But since you’re launching from the Earth, which is already moving around the sun at 30 km/sec, wouldn’t your rocket already have a 30 km/sec “head start”?

[Chronos]

Quoth Whack-a-Mole:

Now you’ve gone and confused me again so let me restate what I think you are saying and then let me know if I’m right or wrong.

In order to move to a higher orbit you need to expend energy (ala fire your rocket engines). However, when you get to your higher orbit you will be moving slower than you were at the lower orbit. In effect, you weren’t speeding up so much as you were using energy to climb farther out of a gravity well. So, it is still proper to say a higher orbit is a slower orbit but you need to use more energy to get there (odd thinking you expend energy to achieve a lower speed).

What you have there is correct: You expend energy to reach a higher, slower orbit. The only part that you left out is that you expend that energy by (momentarily) increasing your speed. First, you increase your speed suddenly, using your engines. Then, your speed gradually decreases, in the process of climbing to the higher orbit. Then, you increase it again at the top, again using your engines. Your engines don’t do anything but increase your speed, but gravity ends up decreasing your speed even more.

[sailor]

>> You expend energy to reach a higher, slower orbit

In space you expend energy to do anything. You wanna get farther away from earth? You need to expend energy. You wanna get closer to earth? You need to expend energy. You wanna go faster? You need to expend energy. You wanna go slower? You need to expend energy. You wanna get farther away from the Sun? You need to expend energy. You wanna fall towards the Sun? You need to expend a lot of energy.

That’s why I hardly ever go into outer space. I feel tired just thinking about it.