If a car slows down due to friction but its speed remains constant, its momentum will not change. Momentum is the product of an object's mass and its velocity, and in this scenario, the car's velocity (speed) is remaining constant.
Friction can act in the opposite direction of the car's motion, exerting a force that opposes the car's forward movement. This force of friction can reduce the car's speed, but as long as the car's velocity remains constant, its momentum will also remain constant.
Mathematically, momentum (p) is defined as:
p = m * v
Where: p = momentum m = mass of the object v = velocity of the object
As long as the mass of the car remains constant and its velocity remains constant, the car's momentum will remain constant, even if it slows down due to friction.