“An object at rest tends to stay
at rest and an object in motion tends to stay in motion.”
This choice is not the
right one for the example.
However, it still applies to rocket flight! It’s easiest to
see how Newton's First Law works when the rocket gets above the
Earth’s atmosphere (to avoid air resistance).
Because there is very little friction in orbit,
when a satellite is “in motion” it will stay “in
motion,” orbiting the Earth continually just as Newton predicted…until
something (a force, such as a rocket motor) comes along to change
There are some residual air molecules up where the
satellites orbit, so they do encounter very slight air resistance,
which can eventually slow them down (it takes many years). We call
this “orbital decay.” Rocket boosters
on a satellite can correct the decay from time to time. Otherwise,
the satellite must be repositioned.
Newton's First Law is also valid on Earth. However, on the surface
of the planet, air resistance is present and slows a moving object.
To counter air resistance, a continuous forward force must be applied.
For example, automobiles must constantly run their
engines in order to maintain a forward movement. An automobile has
additional friction from the wheels, wheel bearings, and the road.
The sole purpose of running the automobile engine continuously is
to overcome all the different friction sources. Otherwise, a small
push would keep an automobile moving forever. (Hills and valleys
present special situations…but that’s another story.)