Here's why:

* Momentum is a measure of an object's mass in motion. It's calculated by multiplying the object's mass (m) by its velocity (v): Momentum (p) = m * v

* Velocity is a vector quantity that describes both the speed and direction of an object.

* Speed is a scalar quantity that only describes how fast an object is moving.

Therefore, if an object has zero speed, it also has zero velocity. And if it has zero velocity, it also has zero momentum.

Think of it this way: An object at rest isn't moving, so it has no momentum. To have momentum, an object needs to be in motion, which means it needs to have a non-zero speed.