The principal force operating on the swing when it goes from its lowest position to either peak is momentum, and when it falls from either peak to its lowest point is gravity. Air resistance and gravity will hold you down if you do not pump your legs quickly enough. These are the only two forces involved in swing action. In order for the swing to remain in motion there must be an external force acting on it. This force can be called "friction" but it is really the result of two different types of friction: air resistance and ground friction. As the swinging limb approaches the top of its arc or cycle, it comes into contact with less and less air, which is why you can feel the effect of air resistance more as you near the top of your swing. As it approaches the bottom of its arc or cycle, it contacts the ground and begins to move again, so ground friction becomes important as well. These are the two types of friction that affect the swing. The more air resistance you can overcome by using large swings with short pauses between them, the faster you will go. The more ground friction you can overcome by using low swings with long pauses between them, the farther you will go. Both types of friction can be reduced by making sure the surface you're swinging on is smooth and by wearing appropriate footwear.