Energy & Momentum
Two complementary bookkeeping languages for mechanics. Energy tracks scalar transfer and storage; momentum tracks directional motion through interactions.
The same interaction can be simple in one accounting system and awkward in the other.
Energy is a scalar, so direction disappears and transfer becomes easy to total. Momentum is a vector, so direction stays visible and collisions become easier to reason about. Neither replaces the other.
Energy
Work transfers energy. Motion and interactions store it in different forms. Conservation lets us compare states without reconstructing every instant between them.
Momentum
Momentum keeps direction. Impulse changes momentum, and total momentum becomes especially powerful when several objects exchange pushes during a collision.
A force through distance transfers energy. A force through time transfers momentum.
Conservation statements only become clear after deciding what belongs inside the system boundary.
Use the representation that preserves the information you need, then cross-check with the other when useful.