Refer to the diagrams shown below.
The normal reaction of the road on the automobile is equal to the weight of the automobile (Newtons 3rd Law).
That is,
N = mg (g = 9.8 m/s²)
When the road surface is dry, the resisting frictional force at the wheel is
F₁ = μ₁N
where
μ₁ = the kinetic coefficient of friction.
The value of μ₁ is about 0.7 - 0.9 on a dry road.
When the road is wet, the resisting frictional force is
F = μ₂N
where
μ₂ = the kinetic coefficient of friction on a wet road.
The value of μ₂ is about 0.1 - 0.4 on a wet road.
Because μ₂ < μ₁, the automobile tends to slip on a wet road, especially if the tires are worn.
