Not a force, but the space curvature. What goes up it usually goes down again, not because it's not the moving in a straight line, nor that the planet is moving, it's because the planet is curving the space around it. That's the side effect of the mass, and some people are saying that movement is also curving the space - the relativity theory is explaining that effects.
Anyway, take care when you fly to your destination (taking a plane, a helicopter or a flying car) - you will need to return to the ground one point or another, one moment or another, so return safely.
In the following episode of Perspectives, four experts discuss what we know about gravity as well as some of the things we’re still trying to figure out. Listen them carefully.