Escape velocity is defined as the velocity with which a body is thrown so that it escapes away from the pull of earth’s gravity and never fall back.
For a body thrown from the surface of the earth the value of escape velocity is about 11 km/sec. Escape velocity for a body is independent of mass of the body and even angle at which it is thrown. It only depends upon the mass of the planet and the distance from the center of the planet to the point from where the body is thrown. Now getting back to our question, is it necessary for a body to be thrown with escape velocity to escape the planet? Or even is it necessary that a body is thrown so that it escapes? Suppose an aircraft is moving vertically upwards from the surface of earth with a velocity far less than escape velocity, let say 2 km/sec. continuously with the help of a rocket engine. Will it not be able to escape then ? Yes it will! Then why do we require escape velocity? Because it will take a very long time for that aircraft to escape away with that velocity and we can’t afford to burn the rocket engine for so long as it will require a large amount of energy. Besides this procedure will require very large rocket engines. For instance, lets assume you have to take a small stone from the ground to the rooftop of a five storey building. There are two possible ways to do so (there can be many more though).
First, you hold up the stone from the ground and walk up through the stairs to the rooftop (no, you are not allowed to use the elevator ). Second, you hold up the stone from the ground and throw it up to the rooftop. Which one would you use ? If you are smart (just like me) and if you have healthy muscles (just like me) and if you are lazy (just like me) then you will go for the second one. Why ? Because second method requires less energy and less time. Though in second case you will have to provide certain minimum velocity to the stone while throwing it but in first case there is no such restriction. First one is just an analogy of the body moving with the help of rocket engine and the second one is the analogy of the body being thrown with escape velocity. In the space launches (may it be from Kennedy Space Center or from Satish Dhawan space centre) we have seen that a spacecraft is not thrown with some kind of super gigantic arms. Rather a rocket engine first takes it to a certain height so that at that height escape velocity required is less than that required at the surface and when it acquires the escape velocity it releases the space probe which then escapes away from earth’s gravity.
So now we know that actual space launch is a non-chemical mixture of these two methods. And now we also know how to escape without even acquiring escape velocity. But please dont try acquiring smartness just like me or healthiness just like me or laziness just like me by throwing stones to the rooftop of building because possibility is that you will end up breaking your skull (if not window panes) just like me.
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