Before Freddy lands on the skateboard it has a certain momentum. After landing, the skateboards momentum

remains the same

Explanation:

Momentum refers to the quantity of motion of a body. When any body of mass moves, it possess momentum. Numerically,

Momentum =  mass x velocity

i.e. momentum is the product of the mass x velocity

Momentum of a body is always conserved.

In the context, the skateboard has certain momentum before Freddy lands on it. After Freddy lands, the momentum of skateboard remains the same, there is no change in the momentum.

This is because, here the momentum is conserved. After Freddy lands on the skateboard, the total mass on the skateboard increases and so the velocity decreases making the momentum same before the landing.

scientific laws are the result of testing theories and proving them to be facts.