# Friction Facts

• Friction is the force that acts between two things rubbing together. It stops them sliding past each other.
• The friction that stops things starting to slide is called static friction. The friction that slows down sliding surfaces is called dynamic friction.
• The harder two surfaces are pressed together, the greater the force that is needed to overcome friction.
• The coefficient of friction (CF) is the ratio of the friction to the weight of the sliding object.
• This car is streamlined to reduce drag (friction with air). The air flows over and under its `wing, producing a downward force that presses the car to the ground, enabling it to go faster. 184
• Waxed skis on snow have a CF of just 0.14, allowing cross-country skiers to slide along the ground very easily.
• Metal sliding on metal has a CF of 0.74; ice sliding on ice has a CF of 0.1. This means it is over seven times harder to make metal slide on metal than ice on ice.
• Friction often makes things hot. As the sliding object is slowed down, much of the energy of its momentum is turned into heat.
• Fluid friction is the friction between moving fluids or between a fluid and a solid. It is what makes thick fluids viscous (less runny).
• Oil reduces friction by creating a film that keeps the solid surfaces apart.
• Brakes use dynamic friction to slow things down.
• Drag is friction between air and an object. It slows a fast car, or any aircraft moving through the air.