# Friction

Friction can be defined as the resistance to motion of two moving objects or surfaces that touch. Friction plays a very important role in many sports, such as bowling and curling.

There is both Static Friction and Kinetic Friction. Static friction is the friction before an object starts to slide, while Kinetic friction is the friction when the object is actually moving or sliding. The formula for both is the same, except they have different coefficient of friction values.

## Friction Equation

When a force is applied to an object, the resistive force of friction acts in the opposite direction, parallel to the surfaces. The standard friction equation for determining the resistive force of friction when trying to slide two solid objects together is written as Fr = μN, where Fr is the resistive force of friction and N is the perpendicular force pushing the two objects together (both in units of force, pounds or newtons), and μ is the coefficient of friction for the two surfaces. The coefficient of friction varies for each situation, and is related to the two specific surfaces that are in contact with each other.

## Friction in Sports

Friction plays a big role in rolling sports such as tenpin bowling and curling. In tenpin, the friction resistance on the ball makes it slow down and also enables the spin on the ball to make it roll in an arc. The mass and the surface composition of the ball and the amount of oil on the lane will affect the magnitude of the friction between the bowling ball and bowling lane. The more oil on the lane means the ball is slow down less and the harder it is for the bowler to send the ball in a curved path. (see more about Bowling Physics)

## Air Resistance

Air resistance is also a form of friction, as it describes the resistance between the surface of an object or person and the air. Air resistance plays a role in many sports in which balls or other objects are thrown, and in sports in which the person moves through the air such as running and cycling (see more about Cycling Physics). Swimmers have to contend with both air and water resistance. See also Aerodynamics.