resistiveforce

Resistive force is a general term for any force that opposes the motion of an object through a medium or across a boundary. It acts opposite to the direction of motion and tends to reduce kinetic energy, often converting it into heat, deformation, or other forms of energy.

Frictional resistive forces arise from contact between surfaces. Static friction prevents motion up to a threshold, while kinetic (sliding) friction resists ongoing motion with a magnitude that is commonly modeled as F_f = μ N, where μ is the coefficient of friction and N is the normal force. In many practical problems μ depends on surface chemistry, temperature, and speed.

Fluid resistance, or drag, occurs when an object moves through a fluid. Drag depends on velocity, fluid density, object size and shape, and flow regime. In turbulent regimes, drag is often approximated by F_d = (1/2) ρ C_d A v^2, where ρ is fluid density, C_d is the drag coefficient, A is cross-sectional area, and v is speed. In laminar, low-Reynolds-number conditions, Stokes law gives F_d = 6π η R v for a sphere of radius R in a viscous fluid, with η the dynamic viscosity.

Viscous resistance also appears inside fluids as internal friction, leading to linear or nonlinear drag proportional to velocity in some models. Collectively, resistive forces influence acceleration, terminal velocity, energy dissipation, and the design of vehicles, athletic equipment, and industrial systems.