Friction and normal force

Contact forces in everyday life

Many forces we experience come from direct contact. A chair pushes up on you. Your shoes push against the ground. A table supports a book. A rough floor slows a sliding box. These are contact forces, and two of the most important are the normal force and friction.

The normal force is the support force a surface exerts on an object touching it. The word normal means perpendicular. So the normal force always acts perpendicular to the surface. If a book rests on a flat table, the table pushes upward on the book. That upward support force is the normal force.

Why the normal force exists

At first, it may seem strange that a table can push on a book. The book is not visibly bouncing. But at the microscopic level, the atoms in the book and table resist being squeezed into each other. The table deforms a tiny amount, often too small to see, and pushes back.

If the book is at rest on a horizontal table, gravity pulls downward and the normal force pushes upward. The forces are equal in size only because the book is not accelerating vertically. In that simple case, $F_N = mg$, where $F_N$ is the normal force, $m$ is mass, and $g$ is the gravitational field strength near Earth.

But the normal force is not always equal to weight. On a ramp, the normal force is smaller than the object's weight because it acts perpendicular to the slanted surface. In an elevator accelerating upward or downward, the normal force can be larger or smaller than weight.

Friction resists relative motion

Friction is a force that resists sliding or the tendency to slide between surfaces. If you push gently on a heavy box and it does not move, static friction is matching your push. Static friction acts when surfaces are not sliding past each other. It can adjust up to a maximum value.

Once the box starts sliding, kinetic friction acts. Kinetic friction usually has a roughly constant size for a given pair of surfaces and normal force. It acts opposite the direction of sliding motion.

A simple model for kinetic friction is:

$F_f = mu F_N$

Here $F_f$ is friction force, $F_N$ is normal force, and $mu$ is the coefficient of friction. The coefficient depends on the surfaces. Rubber on dry pavement has a larger coefficient than ice on metal.

Friction can help motion

Friction is often described as a force that slows things down, but it is also essential for movement. When you walk, your foot pushes backward on the ground. Static friction from the ground pushes your foot forward. Without friction, walking would be like trying to move on smooth ice.

Car tires also rely on friction. The tire pushes backward on the road, and the road pushes the tire forward. Brakes use friction to convert kinetic energy into thermal energy, slowing the car.

Drawing force diagrams

A force diagram helps organize contact forces. For a book on a table, draw gravity downward and the normal force upward. If a horizontal push is applied, draw the push in that direction and friction opposite the possible or actual sliding.

The most important step is to focus on one object at a time. Ask: What forces act on this object? Which are contact forces? Which act from a distance? Then determine whether the forces balance or produce acceleration.

The big idea

The normal force is a surface's support force, perpendicular to the surface. Friction is a surface force that resists sliding or helps prevent slipping. Together, they explain much of the everyday motion around us, from standing still to driving, walking, dragging furniture, and stopping safely.