You are used to hearing that what comes up must come down. In fact this is not quite true. If something moves fast enough upwards
or in fact in any direction on earth except down then it can escape
the force of gravity which pulls it back towards earth and it will
not come back down. The condition which determines the escape velocity is that the total mechanical energy must be greater than or equal to zero because then the object will be able to reach
infinite distance from earth where the potential energy finally reaches zero.
