Here's why:

* Momentum (p) is directly proportional to both mass (m) and velocity (v):

p = m * v

* If momentum decreases, and the mass stays the same, then the velocity must decrease to maintain the relationship.

Think of it this way:

* Momentum represents the "amount of motion" an object has.

* Velocity is the speed and direction of that motion.

If an object has less momentum, it means it's moving slower or changing direction. This means its velocity has decreased.