Physics

Momentum is just mass times velocity—a vector that tells you how much "oomph" an object has in each direction. Since velocity already has components in both $\hat i$ and $\hat j$ directions, you'll multiply the mass by each component separately to get the momentum vector. Once you have that vector, finding its magnitude uses the Pythagorean theorem in 2D, just like finding the length of any vector from its components.

Momentum is just mass times velocity—a vector that tells you how much "oomph" an object has in each direction. Since velocity already has components in both $\hat i$ and $\hat j$ directions, you'll multiply the mass by each component separately to get the momentum vector. Once you have that vector, finding its magnitude uses the Pythagorean theorem in 2D, just like finding the length of any vector from its components.