Many Forces
http://auto.howstuffworks.com/auto-parts/towing/towing-capacity/information/fpte3.htm"Let's look at a diagram of a car. When the car is sitting still, gravity exerts a downward force on the car...But the ground exerts an equal and opposite upward force on the tires, so the car does not move."
What Is Normal Force?
http://www.wisegeek.com/what-is-normal-force.htm"It is commonly said that there are four fundamental forces of nature**: the gravitational, electromagnetic, weak nuclear and strong nuclear forces. The normal force is actually a collection of electromagnetic forces. At the atomic level, two objects resist being smashed together because the electrons of one object resist those of the other. Electrons have a negative charge, so two electrons will tend to repel each other when they are near. When many atoms behave this way collectively, the result is the normal force.
This force is always directed perpendicular to the surface that causes it. When a car is on flat ground, the normal force is vertical and will exactly cancel out the gravitational force. This is why a car on level ground will accelerate neither upward nor downward"
** 3, if you count electromagnetic and weak as the electroweak, but it's the electromagnetic facet that's relevant here.
