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Homework Help: Escape velocity huh?

  1. Jun 7, 2007 #1
    Though my question is stupid, please help me out.........

    Why can't I get out of the gravitational potential field without the escape velocity?
  2. jcsd
  3. Jun 7, 2007 #2
    Escape velocity is not a thing. It is soft of a barrier...you need to cross it escape to "infinity." You wouldn't have sufficient kinetic energy to do that if your velocity were any less.
  4. Jun 7, 2007 #3

    Doc Al

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    Staff: Mentor

    Do you understand what escape velocity means? Perhaps you can rephrase your question.
  5. Jun 7, 2007 #4
    Ok then ......

    If I travel at a constant velocity of 300 km/hr I am just going to keep on moving up.
    When we throw a ball, I don't think we are imparting escape velocity to it. But still it goes up and reaches 0 velocity and comes back again, and thats because theres no constant thrust.
    So when I am giving a constant thrust of 300 km/hr, even if its going to take a hell of days for me, won't I be out of the earth's gravitational field.
  6. Jun 7, 2007 #5
  7. Jun 7, 2007 #6
    No, you will not, at least not from the Earth. The escape velocity for the Earth is around 11km/s.

    You can't impart escape velocity.

    Thrust is force
    300km/hr is a speed.
  8. Jun 7, 2007 #7

    Doc Al

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    "Escape velocity" is the speed you have to give to a projectile (no power source or thrust) so that it keeps going and doesn't fall back to earth. If you have a power source--riding a rocket, for example--then of course you can escape at a lower speed.
    Something must be applying an upward thrust on you to maintain that speed--otherwise you'll slow down and fall back to earth.
    I certainly hope not!
    Exactly. But if you threw it fast enough (ignoring air resistance for the moment) it wouldn't reach zero until it got infinity far away. That's the escape velocity.
    Sure. You can get as far away as you like.
  9. Jun 7, 2007 #8
    thank you fellas
    My doubt is cleared

    and neutrino check out the last line of the mentors post.......
    Last edited: Jun 7, 2007
  10. Jun 7, 2007 #9
    one more thing,
    even if you fire yourself with the escape velocity,
    (in real life) you will get affected by gravitional fields of other masses close by, such as moon, other planets, stars...
    therefore, (in reality) you cannot "go to" infinity even if you have that calculated escape velocity from the world
  11. Jun 7, 2007 #10


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    The thing is, escape velocity deceases with altitude. The escape velocity at sea level is greater than that at say 200 miles up. As you continue your climb upward at 300 km/hr, eventually you will reach an distance from the Earth where the escape velocity will be less than 300 km/hr.
  12. Jun 7, 2007 #11
    you cannot really escape from earth's gravitional field unless you reach infinity which you cannot. there will always be a distance between you and the earth which will make the gravitional field go smaller and smaller as you go away. but it will never actually be zero (it is the basic idea of a mathematical limit)
    also, even giving an object enough trhust to keep it moving at 0,000000001 m/s
    constant speed will keep it moving to infinity.

    the escape speed is the minimum speed you give an object right at the surface of another big object (in this case it is the earth) to send it infinitely away from the other object. But with this concept, you fire the thing and let it go. If you apply a thrust, you need to take that energy you are putting into the system into consideration.
    as I said if you apply a constant thrust that is able to lift it at the surface with a slight speed, it can "make it to infinity" :smile:
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