Friction and Normal Friction

Friction and Normal Friction

Friction

  • Friction is a force that opposes motion. It happens when two surfaces slide over or attempt to slide over each other.

  • Friction depends on the materials from which the two surfaces are made.

  • The force of friction can be lower if the surfaces are smooth, but can be greater if the surfaces are rough.

  • Friction is needed for many everyday activities, such as walking and driving. Friction between our shoes and the ground, or between car tyres and the road, stops us from slipping.

  • Friction also produces heat. When you rub your hands together quickly, the friction between your hands converts the kinetic energy into thermal energy, and you can feel your hands get warm.

Normal Force

  • The normal force is the support force exerted upon an object that is in contact with another stable object.

  • It acts perpendicular to the surface the object is on.

  • If an object is resting on a flat surface, the normal force acting on the object is equal but opposite to the gravitational force acting on it (meaning the forces cancel out and it doesn’t move).

  • The normal force can change if the weight of the object changes.

Friction and Normal Force Relationship

  • The most important thing to understand between normal force and friction is that the frictional force on an object is directly proportional to the normal force.

  • This means that if the normal force increases (like if you add more weight onto the object), then the friction will also increase.

  • The proportionality of normal force and friction is described by the coefficient of friction. The coefficient of friction is a value that shows the relationship between the force of friction between two objects and the normal force. Different surfaces have different coefficients of friction.

  • When no motion is occurring, the calculated static friction (friction when an object is stationary) is a range up to the maximum amount of friction that can occur before the object begins moving. This friction is often modeled as μN, where μ is the coefficient of friction and N is the normal force.

  • Once motion has started, the friction decreases somewhat and becomes kinetic friction. This is often modeled as μkN where μk is the coefficient of kinetic friction.