MadSci Network: Physics

Re: Why must an object reach 'escape velocity' to get away from Earth?

Date: Mon Jun 7 12:37:41 1999
Posted By: Nick Steph, Faculty, Physics, Franklin College
Area of science: Physics
ID: 927937469.Ph

All such objects have mass.  According to Newton's Theory of Gravity, the 
earth exerts force on such objects.  The force is proportional to the 
object's mass, inversely proportional to the distance from the center of 
the earth to the object and directed toward the center of the earth.  
Thus, this force is never zero except in the limit of being infinitly far 
from the earth.  If an object is moving away from the earth, and the force 
of the earth on the object is the only force acting on the object, then, 
according to Newton's second law of forces, it will accelerate toward the 
earth.  This will reduce the speed of the object; and unless the original 
speed is equal to or greater than escape velocity, the object will 
eventually stop and begin moving toward the earth with increasing speed.

If an object is seen moving away from the earth at a constant speed, then 
we must conclude that the net force on the object is zero and therefore 
there are one or more forces acting to counter the force of the earth.

Toss a rock in the air.  Notice that the faster it is moving, the higher 
that it goes.  When the initial speed reaches escape velocity, then the 
rock goes away, always slowing down, but never stopping.

[Moderator note:  You may want to look at these articles in our archives which 
discuss escape velocity:
Article 1
Article 2
Article 3

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