* Gravity: Gravity pulls the skateboard down towards the ground. This force is what causes the skateboard to accelerate downward when it is not being pushed.

* Friction: Friction is the force that opposes the motion of the skateboard. This force is caused by the contact between the skateboard and the ground. Friction slows the skateboard down when it is being pushed.

* Air resistance: Air resistance is the force that opposes the motion of the skateboard through the air. This force is caused by the collision of the skateboard with air molecules. Air resistance slows the skateboard down when it is being pushed.

* Propulsion: Propulsion is the force that moves the skateboard forward. This force is usually generated by the rider pushing off with their feet. Propulsion can also be generated by the wind, if the skateboard is being sailed or towed.

The relative strength of these forces determines how the skateboard will move. For example, if gravity is greater than propulsion, the skateboard will slow down and eventually stop. If friction is greater than propulsion, the skateboard will not move at all. If air resistance is greater than propulsion, the skateboard will slow down and eventually stop.