Why don't racetracks hold their events in wet weather? Because the road is wet.

If you're content with that explanation, stop reading. You're done. Go fishing or take a nap. But if you don't mind a little deeper explanation of the physics involved in racing, read on.

Racing is all about pushing the limits of what is physically possible. Physically, as in physics. And so, just to forewarn you, we're going to touch on more than one physics concept, starting with the very basics.

You might think racecars move forward because their tires push against the road beneath them. And you're right to think that, but in an indirect way.

Isaac Newton's second law of motion states that the net force on an object is equal to the mass of that object multiplied by its acceleration. Mathematically, the equation is F=3DMA. This means that a force applied to a racecar of given mass will produce some definite acceleration, but the force is not applied to the racecar by its tires. The force comes from the Earth.

Newton's third law of motion states that any applied force will be accompanied by an equal force in the opposite direction, more commonly summarized as "every action has an equal and opposite reaction." When tires push against the Earth, the Earth pushes back, accelerating the car. But the Earth accelerates as well. It must, according to the aforementioned second law of motion.

A deeper look into F=3DMA reveals that larger masses experience smaller accelerations, assuming the magnitude of the force is constant like it is between a racecar and the road. But because the Earth is immensely more massive than the car, its acceleration is negligible. The car's acceleration, on the other hand, can be awe inspiring.

We're almost to the rain.

When gravity pulls you down against the carpet beneath size-seven Sketchers, your atoms don't pass through the carpet atoms. Gravity acts through you to create a downward force on the carpet, and Newton's third law creates an upward force on you through the carpet. But the forces between a car and road are not up and down. They are sideways, accelerating the two objects forward and back instead of up and down. Enter friction.

Friction is the force between two objects as they slide past and against each other. Push your foot straight down against the carpet, and it will come to rest. Keeping pushing it down, but now slide it forward or back.

Congratulations — you just experienced friction, a force which must obey Newton's laws just like all other forces. Racecars are friction's slave.

The "coefficient of friction" is a representative measure of how intensely two objects interact. The larger the coefficient, the larger the force of friction,

When tires push against the Earth, they push sideways. If friction exists between their treads and the road, a force will be applied to the Earth and, by Newton's third, to the car. If the coefficient of friction is zero, however, the tires will spin and spin and spin and the car will not accelerate.

Rain influences the interaction between tires and the road. To be precise, rain lowers the amount of friction created between tires and the road. So when a racecar tries to accelerate on a wet racetrack, the forces between the two are reduced.

This is a safety hazard. A racecar coming out of a turn needs to accelerate to the left in order to avoid pounding the wall. When rain lowers the coefficient, friction is reduced and drivers are at an increased risk of having a wreck. Or an injury.

Why don't racetracks hold their events in wet weather? Because the road is wet.

You probably knew that answer was correct, whether you realized it was incomplete or not, but now you know all the physical intricacies that a wet road entails.

Rocky Mountain Raceways will resume its racing schedule Saturday May 31 with the ASA Weekly Racing Series. Maverick modifieds, super stocks, mini cups, legends, and pro trucks will tear up the track for an awe-inspired crowd.

Unless Mother Nature again lowers the coefficient of friction.


E-mail: jwilliams@desnews.com