Static friction

Definition

Static friction is the force that resists the initiation of sliding motion between two surfaces in contact. It acts parallel to the surface and opposite to the direction of potential movement.

5 Must Know Facts For Your Next Test

1. Static friction must be overcome for an object to start moving from rest.
2. The maximum static frictional force is proportional to the normal force and can be calculated using $f_s \leq \mu_s F_n$, where $\mu_s$ is the coefficient of static friction and $F_n$ is the normal force.
3. Static friction does not have a constant value but varies up to a maximum limit depending on the applied force.
4. If no external forces are applied, static friction can be zero even if there is contact between surfaces.
5. The coefficient of static friction ($\mu_s$) depends on the materials in contact and their surface conditions.

Review Questions

• What must be true for an object to overcome static friction and begin moving?
• How do you calculate the maximum static frictional force?
• What factors affect the coefficient of static friction?

Related terms

Kinetic Friction:

The resistive force that opposes relative motion between surfaces once they are already sliding past each other.

Normal Force:

The perpendicular contact force exerted by a surface on an object resting on it.

Coefficient of Friction:

A dimensionless scalar value that represents the ratio of the force of friction between two bodies and the normal force pressing them together.