Stack two books and starting tilting the bottom book - how far can you tilt it before the top book slides off? What's going on between the books up until that point, and what changes when the book starts sliding?

One of the most important forces we deal with every day but don't really think about is friction. Friction is the force between two surfaces that are in contact, and it always tries to prevent or oppose the motion between the surfaces. Friction is the reason that moving things eventually stop moving.

Why is friction so important? Sure, friction prevents your stuff from sliding off your desk or your chair from moving around. But friction is also what allows you to hold things and walk forwards, it allows cars to drive on a road, and pretty much everything that we're used to in everyday life.

If it's so important, why do we often see "ignore friction" in physics problems? We usually ignore friction for the same reason that we ignore air resistance (which is really just a type of friction) - it's an extra force that complicates things, so we study the motion caused by the other forces acting on an object first. But compared to air resistance, friction plays a bigger role in most physics problems and the equation for friction is much easier to work with.

In this lesson we'll learn about three different types of friction: static friction, kinetic friction and rolling friction. We'll also learn how the friction force depends on the coefficient of friction and the normal force.

Multiple-Choice Questions

Answers

• Professor Dave - Frictional Forces: Static and Kinetic
• Khan Academy - Intuition on static and kinetic friction comparisons
• Michel van Biezen - What is the Friction Force
• Dynamics Demo: Inclined Plane
• Michel van Biezen - Friction Force Graphical Representation

• Khan Academy - Static and kinetic friction example
• Michel van Biezen - Friction Force on a Slanted Surface

• Flipping Physics - Do Anti-lock Brakes use Static or Kinetic Friction?
• Smarter Every Day - The Controversial Physics of Curling

• Pulling a box
• Static and kinetic friction on an incline
• Sliding and rolling friction

Other Resources

Stack two books and starting tilting the bottom book - how far can you tilt it before the top book slides off? What's going on between the books up until that point, and what changes when the book starts sliding?

One of the most important forces we deal with every day but don't really think about is friction. Friction is the force between two surfaces that are in contact, and it always tries to prevent or oppose the motion between the surfaces. Friction is the reason that moving things eventually stop moving.

Why is friction so important? Sure, friction prevents your stuff from sliding off your desk or your chair from moving around. But friction is also what allows you to hold things and walk forwards, it allows cars to drive on a road, and pretty much everything that we're used to in everyday life.

If it's so important, why do we often see "ignore friction" in physics problems? We usually ignore friction for the same reason that we ignore air resistance (which is really just a type of friction) - it's an extra force that complicates things, so we study the motion caused by the other forces acting on an object first. But compared to air resistance, friction plays a bigger role in most physics problems and the equation for friction is much easier to work with.

In this lesson we'll learn about three different types of friction: static friction, kinetic friction and rolling friction. We'll also learn how the friction force depends on the coefficient of friction and the normal force.