|Friction ( or )||A contact force that resists sliding between surfaces.|
|Kinetic friction ( or )||Friction when an object slides along a surface. Direction is opposite the object’s sliding direction and is parallel to the contact surface.|
|Static friction ( or )||Friction that prevents an object from sliding along a surface. Direction stops the object from sliding against another surface and is parallel to the contact surface.|
|Coefficient of friction ()||A number typically between and that describes the roughness between two surfaces, where is slippery and is very rough. A unit-less ratio of the frictional force to the normal force. The static friction coefficient is for surfaces that are not sliding, while kinetic is for sliding surfaces.|
|Equation||Symbol breakdown||Meaning in words|
|is kinetic friction, is coefficient of kinetic friction, is normal force||Kinetic friction magnitude is directly proportional to the normal force magnitude and the roughness between the sliding surfaces.|
|is static friction, is coefficient of static friction, is normal force||Static friction magnitude is directly proportional to the normal force magnitude and the roughness between the sliding surfaces.|
|is friction, is coefficient of friction, is normal force||The coefficient of friction is the ratio of the magnitude of frictional force divided by the normal force magnitude.|
How to find direction of friction force
Static friction prevents sliding
Kinetic friction acts opposite the object’s sliding direction
How to determine magnitude of friction force
- Coefficient of friction (): this describes the roughness between two surfaces. A high coefficient of friction produces more friction.
- Normal force (): squeezing surfaces together more tightly increases the friction. This is one reason why heavy objects are harder to slide across the ground.