Final answer:
The skateboarder moving forward by pushing the ground backward is an example of Newton's 3rd Law of Motion.
Step-by-step explanation:
The skateboarder moving forward by pushing the ground backward is an example of Newton's 3rd Law of Motion, also known as the Law of Action and Reaction. According to this law, for every action, there is an equal and opposite reaction. When the skateboarder pushes the ground backward with their foot, the ground exerts an equal and opposite force on the skateboarder, propelling them forward.
Learn more about Newton's 3rd Law of Motion