Question

what forces make a car decrease its speed

Answer 1

Ok,see....
Longitudinal forces operate in the direction of the car body (or in the exact opposite direction). These are wheel force, braking force, rolling resistance and drag (= air resistance). Together these forces control the acceleration or deceleration of the car and therefore the speed of the car. Lateral forces allow the car to turn. These forces are caused by sideways friction on the wheels
................Hope it helps................

Answer 2

Force is applied to the car as breaking the car to reduce the speed of a car is the force as the newtons second law of motion is negative or retarding force