Sliding Friction: Definition, Formula & Examples

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  • 0:00 Definition of Sliding Friction
  • 1:06 Sliding Friction Formula
  • 1:32 Increasing Sliding Friction
  • 2:15 Example Problems
  • 3:26 Lesson Summary
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Lesson Transcript
Instructor: Sharon Linde

Sharon has a Masters of Science in Mathematics

Sliding friction is different than static friction for most material combinations. Find out how it differs, when to apply this concept, and how to calculate the forces involved in this lesson.

Definition of Sliding Friction

Hans is really into robot battles. He spends all of his spare time researching how to make his robots better and today is no exception. His robot lost the last battle because it got pushed around by the other robot, even though they were in the same weight class. He wants to know why, so he is researching friction. He finds that there are many different kinds of friction, but he starts reading about sliding friction first.

Sliding friction is also known as kinetic friction, or moving friction, and is defined as the force that is required to keep a surface sliding along another surface. Hans learns a couple of things about sliding friction that may help him with his robot battles:

  • Sliding friction depends only on two variables: the materials in question and the weight of the object. Changing the surface area in contact does not change the sliding friction.
  • Sliding friction for most materials is less than the static friction. Exceptions include metals, which have static and sliding friction coefficients that are essentially the same, and very small surfaces, where molecular attraction forces take over.

Sliding Friction Formula

The equation for sliding friction on a flat surface is pretty simple - it's the coefficient of sliding friction times the normal force.

Formula for Sliding Friction
sliding friction

The normal force confuses Hans until he realizes that 'normal' is a scientific term that means perpendicular to the surface, or at a right angle. Most of the books give the equation on a tilted plane. However, Hans ignores these since all of the battles happen on a flat surface.

Increasing Sliding Friction

Hans thinks about all the facts he's learned about sliding friction and what they mean for his robot battles. Since there are only a couple of ways to increase sliding friction, there are only a couple of ways Hans can make his robot better: making it heavier or changing the materials.

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