What happens when A skateboarder pushes on the ground with her foot. She and the skateboard accelerate down the sidewalk due to the force?

When the skateboarder pushes on the ground with her foot, she exerts a force on the ground in one direction, causing the ground to exert an equal and opposite reaction force on her in the opposite direction. This reaction force propels the skateboard and the skateboarder forward, resulting in acceleration down the sidewalk.