Friction

What is friction?

A force that acts parallel to the interface of two surfaces that are in contact, opposing their relative motion.

What does the coefficient of friction measure?

The amount of friction existing between two surfaces.

What does the magnitude of the coefficient of friction depend on?

Materials in contact

Steel on ice (iceskating)

Low coefficient of friction

Rubber on the ground (running)

High coefficient of friction

What increases the size of the coefficient of friction?

Increased interactions between the molecules of the interacting surfaces

Coefficient of friction formula

Friction force = coefficient of friction x normal force

What is static friction?

The force acting between two surfaces that are not moving relative to each other (scalar)

What happens to static friction when a force is applied to a stationary object?

Static friction acts to resist the motion until the applied force overcomes it.

What is the formula for static friction?

Force of friction ≤ coefficient of static friction x normal force

What is dynamic friction?

The forces of two objects sliding against each other (the force it takes to keep an object moving)

What is the relationship between the coefficient of dynamic friction and the coefficient of static friction?

The coefficient of dynamic friction is usually lower than the coefficient of static friction.

What is the formula for dynamic friction?

Force of dynamic friction = coefficient of dynamic friction x normal force

Strategies to maximise friction

• playing surface

• footwear (studs)

• equipment (chalk rock climbing, wax on a surfboard)

• temperature (warming F1 tires)

Strategies to minimise friction

• playing surface

• footwear (ice skates)

• manipulation of equipment (lubricants)