Put your math or physics textbook on the table, and just leave it there. Notice how it doesn’t move. Newton’s first law states simply that an object at rest will stay at rest, and an object in motion will stay in motion *when acted on by an outside force*. To see this in action, apply a force (a push) to the textbook and notice how (unsurprisingly) it starts moving! When you stop pushing, the textbook stops, too. This seems to violate Newton’s first law, but it’s actually because there’s another force at play pushing back: friction.
In the case of the textbook, the force you applied with the push was necessary to overcome the force of friction to get it moving, but if you were to give a textbook that same push in space, it would just drift on in a straight line at the same speed for as long as it took for something else to push on or pull at it.
