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AP®PhysicsCollege BoardPhysics 1: Algebra-BasedStudy GuidesUnit 2: Force & Translational DynamicsKinetic & Static FrictionStatic Friction

Static Friction (College Board AP® Physics 1: Algebra-Based)

Study Guide

Ann Howell

Written by: Ann Howell

Reviewed by: Caroline Carroll

Static friction

  • Static friction may occur between the contact surfaces of two objects that are not moving relative to each other

    • If an object is static, then it is not moving; it is stationary

  • Static friction is present up until the point at which the object begins to move

  • For example, when applying a forward force to try to move a heavy object, static friction:

    • acts in the opposite direction to the applied force

    • increases to match the magnitude of the applied force

    • prevents the object from moving

    • continues to increase until it is finally overcome by the magnitude of the applied force, which results in the object beginning to move

  • Once the object is moving, then kinetic friction is present

Static friction

A person pushing a cupboard with arrows indicating forces: friction left, applied force right, normal force up, and gravitational force down.
When a force is applied to an object it will not move unless the applied force is greater than the static frictional force

  • When an object moves initially, it seems to slip suddenly

    • This sudden slip indicates the point at which kinetic friction takes over from static friction

  • Less force is required to keep an object moving than to initially make it move

    • A smaller frictional force is required to keep an object in motion than to keep it stationary

    • The force of kinetic friction is smaller than the force of static friction

open vertical bar F subscript f comma k end subscript close vertical bar space less-than or slanted equal to space open vertical bar F subscript f comma s end subscript close vertical bar

  • Where:

    • open vertical bar F subscript f comma k end subscript close vertical bar space equals spacemagnitude of kinetic friction, measured in straight N

    • open vertical bar F subscript f comma s end subscript close vertical bar space equals spacemagnitude of static friction, measured in straight N

Kinetic vs static friction

Four diagrams of forces on a box: normal and gravitational force; static friction and applied force; max static friction and applied force; kinetic friction and applied force.
When friction is applied to an object initially at rest the magnitude and type changes depending on whether the object is stationary, on the point of moving or moving.

  • Making an object heavier will increase the magnitude of the static friction and increase the magnitude of the forward force required to get it moving

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    Author
    Reviewer
    Ann Howell

    Author: Ann Howell

    Expertise: Physics Content Creator

    Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students, no matter their schooling or background.

    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.