Question

Why does the skateboarder continue to move forward when he hits the trash?

Answer 1

An object at rest stays at rest and an object in motion stays in motion. Or Newtons first law of motion.

Answer 2

Answer:

According to Newton's first law:  An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

There is a force that moves the skater and his skate forward, after the hit, his speed is reduced, but this force is not completely counteracted, and that's why he continues moving forward.