Friction & the Contact Force
One contact force, two parts: the push straight out and the drag along the surface.
Lesson at a glanceShow the key points
- Two bodies in contact exert equal and opposite contact forces on each other (Newton's third law).
- The contact force splits into a normal component N (perpendicular to the surface) and friction f (parallel to it).
- Its magnitude is √(N² + f²); its angle θ from the normal satisfies tan θ = f/N.
- Kinetic friction acts when surfaces slip over each other; static friction acts when they do not.
When a surface pushes on you, it doesn’t have to push straight out. Tilt that push even slightly and you have discovered friction.
The contact force
Wherever two bodies touch, electromagnetic forces act between the charged particles at their surfaces. The upshot is that each body exerts a contact force on the other. By Newton’s third law the two contact forces are equal in size and opposite in direction.
The contact force on a body need not be perpendicular to the surface. So we split it into two perpendicular pieces:
A 500 g block slides across a rough horizontal floor, and the frictional force on it is 2.0 N. Find (a) the angle the contact force makes with the vertical and (b) its magnitude. Take g = 10 m/s².
Kinds of friction
Friction acts between two solids, between a solid and a fluid, and between two fluids. Friction in fluids is called viscous force and gets its own chapter later. Here we stay with solid surfaces, where two cases matter:
- Kinetic friction: the surfaces slip over each other.
- Static friction: the surfaces are in contact but do not slip.
Building a reliable theory of friction from atomic forces is hard. Fortunately a wide range of experiments can be summarised by a few simple laws, which the next lessons develop.
- 1Touching bodies exert equal and opposite contact forces on each other.
- 2Contact force = normal component N (perpendicular) + friction f (parallel to the surface).
- 3, at angle from the normal.
- 4Kinetic friction: surfaces slipping. Static friction: no slipping.
Check yourself
Keep exploring
Hand-picked videos, simulations and readings that explain this idea from a different angle.