- Direction: Opposes relative motion between two surfaces
- Kinetic friction
- Static friction, matches external force up to maximum
- Concept: Friction points opposite to relative velocity
- Problem: A thin slab of mass rests centered on two cylinders of radius rotating with angular velocity in opposite directions. What force is required for the slab travel with velocity across the cylinders? The coefficient of friction is .
- Concept: Reaction force angle
- The reaction force is the net force of friction and the normal force . The reaction angle is the angle between the reaction force and normal force. Once the interaction between two surfaces is determined, the reaction angle does not change.
- Problem: A force is applied at an angle from the hortizontal to move a block across the ground wih constant velocity. At what angle is the force minimized? The coefficient of friction is
- Concept: Self-locking
- Occurs when the applied force to displace an object contributes to the normal force and friction . There is a critical angle beyond which the object will not move no matter how much force is applied.
- Problem: A rod of length rests vertically on the ground. A rope is attached to the top end and the ground, making an angle with the verticle. If a force is applied horizontally on the rod, how far from the bottom does it need applied to be for the rod to self-lock? The coefficient of friction is .