Intermediate · 11 min
Friction and contact forces
Model sliding friction and distinguish it from static friction.
A force along the surface
Friction acts along a contact surface and opposes relative sliding or its tendency. For a sliding object, a useful model is F = μ × N, where μ is the coefficient of kinetic friction and N is the normal contact force. The coefficient has no unit. On a horizontal surface with no vertical acceleration or additional vertical forces, N = m × g. That equality does not apply to every surface or force arrangement.
Worked example
A sliding box has N = 50 N and μ = 0.2. Kinetic friction = 0.2 × 50 = 10 N
Static friction adjusts
Static friction can keep two surfaces from sliding. Its magnitude adjusts to what is needed, up to a maximum μₛ × N; it is not always at that maximum. Once sliding begins, the kinetic-friction model applies. To find acceleration, first subtract opposing forces to get the net force, then use a = Fnet ÷ m. State which friction coefficient your model uses.
Worked example
A 4 kg sliding box is pulled right with 18 N while friction acts left with 10 N. Net force = 8 N right; acceleration = 8 ÷ 4 = 2 m/s²
Draw a sliding box
- Draw the forces on a 2 kg box sliding on a horizontal surface. Use g = 10 N/kg and μ = 0.3 to calculate friction.
- Add a horizontal pull of 10 N. Find the net horizontal force and acceleration.