# Friction

• When one object slides over another, its motion is always opposed by a resisting force called friction.
• Static friction causes resistance when an object at rest is pulled or pushed.
• Keeps objects at rest (relative to what it could slide over).
• It’s as big as it needs to be, until it fails.
• Ranges from 0 (when there’s no applied force) to fsmax, the maximum force before the object starts to slip.
• When pulling something at a constant velocity, its net force is 0, so its kinetic friction is equal to the pulling force.
• Kinetic frictional force is smaller than static frictional force.
• Graph of an object being dragged across a table: • The force of friction depends on the normal force and the materials.
• When the object pushes down on the surface harder, there’s more normal force. This increases the frictional force.
• The material of the surface that the object is being dragged across also affects its maximum static friction force (before it moves) and kinetic friction force (after it starts moving).

## Things to know about friction

1. Friction always acts in the opposite direction of the motion.
2. Friction is proportional to normal force between two surfaces.
3. Friction depends on materials that make up surfaces.
4. For solid objects, friction is independent of area of contact.

## Calculating friction

• The coefficient of friction (mu, or μ) is how we measure the “stickiness” between two surfaces.
• It cannot be measured theoretically, it must be measured by actually dragging objects at a constant speed.
• μ = (frictional force)/(normal force)
• Basically, it measures how much of the normal force is translated into frictional force.
• frictional force = μ * normal force