Why does most of the work done in launching a rocket take place when the rocket is close to the Earth's surface?

Answer 1

Inertia can be thought of figuratively as a mass's laziness. Think of when you are lazy. When you're lazy and doing nothing, you tend to stay that way. You continue to do nothing unless prodded to do something. (Perversely, in people, laziness causes idleness and idleness causes laziness.) Well, masses are the same way. When they're inert -- meaning not moving -- they tend to stay that way because of their laziness. That laziness is called inertia. When a huge rocket laden with tons of fuel is sitting on the launch pad, it has tremendous inertia because it is massive. To get that thing moving takes a huge amount of power. But once it's moving, the rocket's inertia, instead of being a handicap, becomes an asset, because any mass, once it's moving, likes to stay moving. Moreover, as the rocket burns off fuel, its mass decreases, so less energy is required to propel it.