There are several types of friction, including static, sliding, rolling and fluid. Static friction occurs when a force is exerted on an object, but the object does not move. When you push a box across a floor, sliding friction occurs in the opposite direction. Rolling friction, seen in wheels and balls, is easier to overcome than sliding friction in similar materials. Fluid friction occurs when an object moves through a fluid, such as when a boat moves through water or when an airplane flies through the air.