Falling freely

Natural motion: no force

Where no forces at all are acting on something, it's moving naturally. A spaceship a long way from anywhere is easy to imagine, but hard to do.

The lived-in world is full of useful things like air, roads, gravity, tables, grass and water.

Try and push something through water: it's a lot harder than pushing the same things through air (runners are faster than swimmers!).

Try cycling across rough ground: it's a lot harder than cycling on the road.

Put a glass of water on a table and it just sits there. Miss the table, and it falls until it meets the floor.

Motion is affected by any force exerted by the environment. These are forced motions. Things move naturally only when no forces are acting on them.