MCAT Content / Force / Friction Static And Kinetic

Friction, Static, and Kinetic

Topic: Force

Friction is a force that opposes relative motion between systems in contact and its magnitude has two forms: static friction and kinetic friction.

Friction is a force that is around us all the time that opposes relative motion between systems in contact but also allows us to move. One of the simpler characteristics of friction is that it is parallel to the contact surface between systems and always in a direction that opposes motion or attempted motion of the systems relative to each other. If two systems are in contact and moving relative to one another, then the friction between them is called kinetic friction. But when objects are stationary, static friction can act between them; the static friction is usually greater than the kinetic friction between the objects.

Frictional forces, such as f, always oppose motion or attempted motion between objects in contact. Much of the friction is actually due to attractive forces between molecules making up the two objects (even perfectly smooth surfaces are not friction-free). The direction of friction is always opposite that of motion, parallel to the surface between objects, and perpendicular to the normal force. The magnitude of the frictional force has two forms: one for static situations (static friction), the other for when there is motion (kinetic friction).

When there is no motion between the objects, the magnitude of static friction fs is:

 fs ≤ μsN

where μs is the coefficient of static friction and N is the magnitude of the normal force (the force perpendicular to the surface).

Once an object is moving, the magnitude of kinetic friction (fk) is:

fk = μkN

where μk is the coefficient of kinetic friction. A system in which fk = μkN is described as a system in which friction behaves simply.

For both static and kinetic friction, the coefficient of friction depends on the two surfaces that are in contact.

Key Points

• Friction is a contact force between systems that opposes the motion or attempted motion between them. Simple friction is proportional to the normal force N pushing the systems together. (A normal force is always perpendicular to the contact surface between systems.) Friction depends on both of the materials involved. The magnitude of static friction fs between systems stationary relative to one another is given by  fs ≤ μswhere μs is the coefficient of static friction, which depends on both of the materials.

• The kinetic friction force fk between systems moving relative to one another is given by fk = μkwhere μk is the coefficient of kinetic friction, which depends on both of the materials.

Key Terms

Friction: The resistance to motion of one object moving relative to another.

Kinetic friction: Also known as sliding friction or moving friction, is the amount of retarding force between two objects that are moving relative to each other.

Static friction: The friction that exists between a stationary object and the surface on which it’s resting.

Normal force: Contact or support force exerted upon an object that is in contact with another stable object. The component of a contact force that is perpendicular to the surface that an object contacts.

Magnitude of static friction fs:  fs ≤ μsN

Magnitude of kinetic friction fk: fk = μkN

Coefficient of friction: A value that shows the relationship between two objects and the normal reaction between the objects that are involved. Its value is dependent on both of the materials involved.

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