Explain why an object when thrown in the air or slid across the ground doesn't travel forever

Gravity and friction inhibit the motion of the object, respectively

We often think of the air as "nothing" but in reality, there is always something there. It is so light that we often don't notice it, but the reason it still slows things down is the same as why something slid across the ground doesn't go forever - friction. You can feel how much the air pulls and pushes if you stick your hand out the window of a car or ride on a roller coaster. In each of these cases you will feel a force pushing you back. In reality, you are the one pushing through the air.