“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 this Law works once 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, say from a rocket motor) comes along to change
that condition.
There are some residual air molecules 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 this decay from time to time.
The 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, you need to apply a continuous forward force. This
is why automobiles must constantly run their engines. Your car has
additional friction from the wheels, wheel bearings, and from the
road. Therefore, the sole purpose of running the engine continuously
is to overcome all the different friction sources. Otherwise, a
small push would keep your car going forever. (Hills and valleys
present special situations…but that’s another story.)
