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The normal force is responsible for keeping two objects physically separated in space. Without it, solid objects would merge together without any resistance. It is fundamentally caused by the electrical forces of many atoms repelling each other. Objects are typically brought into contact by either the gravitational force or by collision. The normal force is an important concept in classical mechanics for calculating friction between solid objects.
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. The normal force on a car going down a hill, however, forms a right angle with the road—even though the gravitational force is still directed vertically. Since these forces no longer will exactly cancel out, a car will accelerate downhill even if no gas is used.
Another possible situation involving the force is when two objects are colliding. When this happens, it is not gravity, but momentum that is responsible for the normal force. Newton’s first law states that objects in motion will remain in motion unless acted on by an external force. So colliding objects must exert forces on each other in order for them to change velocities. In fact, this is exactly what they do—the normal forces they exert on each other are just enough to send them moving with their final velocities.
In classical mechanics, the normal force is important for determining static and kinetic friction. These refer to the friction between solid objects, such as a box sliding across the floor. This kind of friction is proportional to normal forces—meaning that doubling the normal force will double the force of friction. In everyday cases, the weight of an object is what is responsible for its normal force. This is why it is more difficult to push a heavy box than a lighter one.