# Earth's escape velocity

1. Sep 24, 2015

### Neon

The earth's escape velocity is in km per s. But i can jump off the ground! i know but the estimate for the earth's escape velocity in for an object of what mass? Defintely more than a ton.

2. Sep 24, 2015

### Staff: Mentor

But you're not escaping. You'll come right back down.

The escape velocity is independent of mass.

3. Sep 24, 2015

### Neon

if i jumped and the earth gravity disappear or i have that force constantly pushing me up with a typical jump force, i will be in space right?Technically I used enough force to oppose the earth's gravity and go the opposite direction. So... any referances?

4. Sep 24, 2015

### Staff: Mentor

The only thing pulling you back down is the earth's gravity. So if that gravity "disappeared" you would keep going. (Not a particularly realistic scenario. What's your point?)

To jump off the ground, you just need a net upward force. Easily achievable. So what?

References for what?

5. Sep 24, 2015

### Staff: Mentor

"Escape velocity" doesn't mean "the speed you need to be able to jump up from the ground". It means "the speed you need to be able to escape to infinity, starting from the Earth's surface, with no further force applied". ("Escape to infinity" means "get so far away from the Earth that its gravity would be negligible". The Earth's gravity is certainly not negligible a few feet off the ground--as Doc Al pointed out, it will pull you right back down when you jump.

The Earth's gravity can't disappear; that would violate the laws of physics. It's pointless to ask what would happen if the laws of physics were violated; there's no way to answer the question since the only means we have for predicting what would happen is to apply the laws of physics.

If you have a force constantly pushing on you, yes, you can get to space; that's what rockets do (their engines are constantly pushing on them until they get into orbit). But the concept of escape velocity doesn't apply in that case, because there's a continuous force being applied; as I said above, escape velocity is the speed you would need to move to escape to infinity with no further force applied.

6. Sep 24, 2015

### Staff: Mentor

Shoot a gun straight up. If the bullet will eventually fall back to the earth, then the muzzle velocity of the bullet was less than escape velocity. If the bullet never falls back, then the muzzle velocity was greater than or equal to escape velocity. The mass of the bullet doesn't matter, just the speed (although it will of course take a more powerful to gun to fire a heavier bullet at the same speed).

In practice, no traditional gun is capable of firing bullets at speeds greater than escape velocity, which is why we use rockets to launch spacecraft instead.

7. Sep 24, 2015

### QuantumPion

If you were on the surface of a small asteroid or comet you could escape by jumping. For example, comet 67P/Churyumov has an escape velocity of 1 m/s.

8. Sep 24, 2015

### Neon

Ok i get the escape to infinity part so instantly accelerate to the speed and stop WILL escape earth right?

9. Sep 24, 2015

### Neon

What about the area where the gravity reaches out that is significant enough to be measure? like the hill sphere of earth and black hole are vastly different. So outside the event horizon would be and low in escape velocity as I need to move through less of its gravity.

10. Sep 24, 2015

Staff Emeritus
What event horizon?

I think you should take a moment to collect your thoughts, because your question seems to be shifting as people try and answer it.

11. Sep 24, 2015

### Neon

I am asking if the earth's gravity gets weaker faster, would escape be easier?

12. Sep 24, 2015

### Staff: Mentor

Are you familiar with the equation for g (the acceleration due to Earth's gravity) as a function of radius from the center of the Earth? You should be able to calculate this for yourself if you've seen the equation. If you haven't seen it, you can use wikipedia to find it. Let us know if you need help finding it.

13. Sep 24, 2015

### Staff: Mentor

And BTW, we do not address questions that propose breaking the rules for physics here. We address questions about how the real physics works. Does that make sense?

14. Sep 25, 2015

### Neon

Ok i get it now thanks everyone!