Acceleration

If its velocity stays constant, its acceleration is zero. Acceleration is the rate of change of velocity: it describes how much velocity changes each second, rather than how fast an object is moving.

Acceleration is the rate of change of velocity and also has a direction. Momentum depends on an object's mass and velocity, and its direction is the direction of the velocity. Energy, by contrast, has a measurable amount but no specific direction, so it is a scalar.