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There are always forces acting on the air, but most of the time, the forces almost exactly cancel each other, and the acceleration is very weak.
Most moving things require a balance of forces, since most moving things are affected by friction. For example, in a car, to travel at a steady speed, the engine and drive train must supply a forward force. If this forward force is stronger than the combined effects of wind resistance and friction with the road, you speed up; if this forward force is weaker, you slow down. In fact, you speed up or slow down until the resistance and friction are as large (or small) as the force supplied by the engine, at which point your speed becomes steady. Most of the time, the forces do balance each other, and you travel along at a constant speed.
The atmosphere is similar; most of the time, air is in motion because two or more forces balance each other.
Forces have both magnitude (i.e. strength) and direction. If someone pushes you, they can push you strongly or weakly, and they can push you forward or backward (or to the left, etc.). When two forces balance, that means that one force is pushing in one direction, and the other force is just as strong but is pushing in exactly the opposite direction. So the direction of a force is just as important as its strength.
In the rest of this module, we'll find out about each of the important forces in the atmosphere, and about how some of them always just about balance each other.
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