Friction The force between two surfaces in contact that causes a resistance of the motion or pending motion between the surfaces.

Friction always opposes the direction of motion or pending motion between the two surfaces in contact. Applied force friction Motion direction Friction is always parallel to the two surfaces in contact.

Friction always opposes the direction of motion or pending motion. Applied force friction Motion Direction

What Causes Friction? Irregularities between two surfaces in contact. A microscopic view of the two surfaces in contact.

Friction Measured in Newtons (N) because it is a type of force. Frictionless objects are fictitious. Static friction – the type of friction present when two surfaces are at rest with respect to each other Kinetic (Sliding) friction – the type of friction present when one surface slides across another surface. Static friction is usually greater than kinetic friction

Friction is only affected by: The texture or surface properties between the two surfaces in contact. (Coefficient of friction) The amount of contact between the two surfaces. (Normal force) f = frictional force μ = coefficient of friction η = normal force Friction equation: Friction is fun

Coefficient of friction Surface properties between the two surfaces in contact that affects pending motion or motion The larger the value the more resistance between the two surfaces. The coefficient of static friction only applies when an object is not sliding over another surface The coefficient of sliding (kinetic friction) only applies when one object slides over the surface of another surface. The coefficient of static friction is usually greater than the coefficient of sliding friction. Dimensionless quantity μ = The Greek Letter “mu” The ratio to the frictional force to the normal force μ = 0 for a frictionless surface. Value range typically between 0.03 (smooth ice on smooth ice)  1.20 (rubber on concrete) The value for coefficient can be interpreted as a percent. The force needed to move ice on ice is only 3 % of the normal force. Table

Static Friction increases to a maximum level Static friction matches the applied force to a certain level. After the maximum static friction value is obtained the object moves and kinetic friction is active. Applied force Frictional force Static friction Kinetic friction

Normal force Represents the amount of contact between two surfaces Measured in Newtons. Always directed away from the surface and perpendicular to the two surfaces in contact. η η η

Two types of friction Since there are two types of coefficient of frictions (static and kinetic), there are two types of friction f s =static friction (the friction present when one object does not slide over the surface of another surface f k = kinetic friction (the friction present when an object slides over the surface of another surface.) f s =μ s η f k =μ k η

If the applied force and frictional force are equal, then the object will move at constant velocity Applied force = F A Friction =f Motion direction F A =f s for an object not in motion F A =f k for an object in motion that slides over the surface of anther surface