The magnitude of static friction f s between systems stationary relative to one another is given by f s ≤ μ s N , where μ s is the coefficient of static friction, which depends on both of the materials 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 kinetic friction force f k between systems moving relative to one another is given by f k = μ k N , where μ k is the coefficient of kinetic friction, which also depends on both materials

friction — a force that opposes relative motion or attempts at motion between systems in contact. drag force — F D , found to be proportional to the square of the speed of the object; mathematically F D ∝ v 2 F D = 1 2 Cρ Av 2 , where C is the drag coefficient, A is the area of the object…. Stokes’ law — F s = 6 πrηv , where r is the radius of the object, η is the viscosity of the fluid, and v is the object’s velocity. static friction — a force that opposes the motion of two systems that are in contact and are not moving relative to one another.