# Escape Velocity

1. Sep 8, 2015

Hi,

First FYI, I have no education in physics.
Anyway -

I know earth's escape velocity is about 41,000 kph. Anything less, and you'll eventually fall down back to earth.

Two points that seem to contradict each other -
1. Escape velocity gets decreased the farther away you are from the center of mass.
2. I'm pretty sure most rockets I've seen don't accelerate to 41,000 kph right off the bat. They take some time to reach full speed.

So - If you managed to launch the rocket from the ground, that's the hardest part by definition. Getting it from an altitude of 100m to 200m requires less force than from 0m to 100m. Yet the rocket doesn't reach 41,000 kph before the 100m mark. And it could be even slower the higher up it goes. So it seems you start off slow (not 41,000 kph) and get even slower until you're out of earth gravity well.

Where does the 41,000 kph thing come into play?

Thanks!

2. Sep 8, 2015

### Orodruin

Staff Emeritus
The rocket does not need to reach escape velocity right from the start because there is a force acting on it due to the engines. The escape velocity is the velocity an object needs to escape the gravitational field if there are no forces other than gravity acting on it.

3. Sep 8, 2015

### Staff: Mentor

To elaborate a little, if I shoot an object out of a cannon, it needs to be traveling at escape velocity by the time it leaves the cannon or it will fall back to Earth. This is because, as Orodruin said, there is no other force acting on the projectile other than gravity (we're ignoring air friction here).

However, a rocket has a constant force from the engines acting against gravity, so it doesn't need to reach escape velocity.

4. Sep 8, 2015