Friction and Driving Fast

This is not an astronomy post, but one about physics. In general, it's about driving safely for conditions on the road.


Every winter and during rain storms, I see people driving like the roads are completely dry. The worst offenders are SUV drivers. The reality is that driving fast in wet conditions on the road is not only stupid, but extremely dangerous.

In this picture, Fg is the force due to gravity on the car. This is the same as the weight of the car. N is the normal force to the surface of the road. You can think of the normal force as the force that pushes up on the car by the road. In this case, the normal force equals the weight of the car. As the car travels at a constant velocity, there is no force acting along the road itself, so v is constant.


Let's change things up and say the car is accelerating. There is a frictional force, Ff acting opposite to the acceleration. In this case, to accelerate the car, the car has to overcome a static frictional force to begin to speed up. Once it overcomes the static frictional force, the frictional force becomes kinetic and is generally smaller than the static frictional force.

Now, suppose you want to slow down. You apply the brakes to the car and your velocity decreases. The acceleration of the car is pointed in the opposite direction of travel. (Vector physics). The acceleration of the car is dependent on two things: gravity and the coefficient of friction between the road and the tires. Now, if you assume that all tires are the same (and I know they are not), this is independent of the mass of your vehicle! For dry conditions, μ is less than one, but larger than for dry conditions. As the pavement becomes wetter and slicker due to rain, snow, or ice, μ decreases. So if you are travelling at 50 mph in wet conditions, it takes you longer (both in time and distance) than if the roads are dry.

So, please, slow down when the roads are wet, or please make sure there is more space between you and any vehicles in front of (or behind) you.