You can get a good feel for gravity and inertia playing this game! Try to land on the moon, on the Earth and on Jupiter. The moon will be the easiest since the gravity is the weakest there... You won't get pulled down as fast. Jupiter's gravity is really strong, so it's going to be really hard to land there. As for inertia, you'll see how your ship kind of drifts in whatever direction to aim it. It won't drift forever, of course, since there is an outside force of gravity effecting it.

Lunar Lander was created by John Donohue

Use the space bar for the landing thruster and the left/right arrow keys to spin the lander. Try to land on the blue landing pads. Land successfully and you get half your fuel back. Change the gravity to make it harder.