Peoples Physics Concepts

http://www.ck12.org Chapter 4. Newton’s Laws

4.5 Friction

• Define friction and explain the difference between static and kinetic friction.


• Solve problems involving friction.

In this lesson students will learn about friction, the difference between static and kinetic friction and how to solve
problems involving friction.

Key Equations

Static and Kinetic Friction

{

fs≤μs|FN| Opposes potential motion of surfaces in contact

fk=μk|FN| Opposes motion of surfaces in contact

Guidance

• The force offrictioncan actually be described in terms of thecoefficient of friction, μ. It is determined
  experimentally and varies depending upon the two surfaces in contact.


• Static friction(μs) acts between two surfaces that are in contact but not in motion with respect to each other.
  This force prevents objects from sliding. It always opposes potential motion, and it rises in magnitude to a
  maximum value given by the formula below.^2


• Kinetic friction(μk) acts between two surfaces that are in contact and in motion with respect to each other.
  This force reduces the acceleration and it always opposes the direction of motion.^2

Example 1

Calculate the force necessary to slide a 4.7-kg chair across a room at a constant speed if the coefficient of kinetic
friction between the chair and the floor is 0.68.